A

A bandUnit 4: The dark band of a sarcomere that corresponds to the length of the thick (myosin) filaments and does not change during contraction.
abaxialUnit 3: Describing the lower surface of a leaf, the side facing away from the stem.
abiotic factorUnit 5: A non-living component of an organism’s environment that influences it, such as temperature, light intensity, pH or water availability.
ABO blood groupUnit 4: A blood grouping system based on the presence or absence of A and B antigens on the surface of red blood cells.
abscisic acid (ABA)Unit 4: A plant growth regulator that inhibits growth and promotes responses such as the closure of stomata and dormancy.
absolute refractory periodUnit 3: The short period immediately after an action potential during which a neurone cannot be stimulated to fire again, no matter how strong the stimulus.
absorbanceUnit 1: A measure of how much light of a given wavelength a solution absorbs as it passes through, read on a colorimeter and used to estimate the concentration of a coloured substance.
absorptionUnit 3: The uptake of the soluble products of digestion across the gut wall (e.g. across the small intestine epithelium) into the blood or lymph.
absorption spectrumUnit 3: A graph showing how much light of each wavelength is absorbed by a photosynthetic pigment.
absorption spectrum peakUnit 3: A wavelength of light at which a photosynthetic pigment absorbs maximally, shown as a peak on its absorption spectrum.
abundanceUnit 5: A measure of how many individuals of a species are present in a given area or volume.
accessory pigmentUnit 3: A photosynthetic pigment, such as a carotenoid, that absorbs light at wavelengths other than those absorbed by chlorophyll a and passes the energy to it.
acclimatisationCore terms: The gradual, reversible adjustment an organism makes to a change in its environmental conditions over time.
accuracyUnit 1: How close a measured value is to the true or accepted value of the quantity being measured.
acetyl CoAUnit 3: A two-carbon acetyl group joined to coenzyme A, formed in the link reaction, that delivers the acetyl group into the Krebs cycle.
acetyl groupUnit 4: A two-carbon group that is transferred to coenzyme A to form acetyl coenzyme A, linking the link reaction to the Krebs cycle.
acetylcholineUnit 4: A neurotransmitter released at many synapses and at neuromuscular junctions, where it triggers a response in the postsynaptic cell.
acetylcholinesteraseUnit 3: An enzyme in the synaptic cleft that breaks down the neurotransmitter acetylcholine, ending transmission across the synapse.
acid rainUnit 1: Precipitation made unusually acidic by dissolved pollutant gases such as sulfur dioxide and nitrogen oxides, which lowers the pH of soils and water bodies.
acidificationUnit 1: A fall in the pH of an environment such as soil or water, often caused by pollutants or dissolved carbon dioxide.
acrosomeUnit 4: The cap at the front of a sperm cell that contains enzymes used to digest a path through the layers around the egg at fertilisation.
acrosome reactionUnit 4: The release of digestive enzymes from the acrosome at the tip of a sperm, which break down the layers around the egg so the sperm can reach and fuse with the egg membrane.
ACTHUnit 4: A hormone from the anterior pituitary gland that stimulates the adrenal cortex to release its hormones.
actinUnit 4: A protein that forms the thin filaments of a myofibril and interacts with myosin to bring about muscle contraction.
action potentialUnit 3: A rapid, transient reversal of the membrane potential of a neurone (depolarisation to about +40 mV) caused by the inward movement of sodium ions when the threshold is reached, which propagates as a nerve impulse.
action spectrumUnit 3: A graph showing the rate of photosynthesis achieved at each wavelength of light.
activated amino acidUnit 1: An amino acid that has been joined to a specific transfer RNA molecule using energy from ATP, ready to be added to a growing polypeptide chain during translation.
activation energyUnit 1: The minimum energy that reacting molecules must gain before a chemical reaction can proceed; enzymes and other catalysts speed reactions by lowering it.
active immune responseUnit 4: An immune response in which the body’s own lymphocytes produce antibodies after exposure to an antigen, giving long-lasting immunity.
active siteUnit 1: The specific three-dimensional region of an enzyme, complementary in shape to its substrate, where the substrate binds by weak chemical bonds to form an enzyme-substrate complex.
active transportUnit 2: The movement of molecules or ions across a membrane against their concentration gradient, using carrier proteins and energy supplied by ATP from respiration.
actual lengthUnit 4: The real size of a structure, calculated from its measured image size divided by the magnification.
adaptive radiationUnit 2: The rapid evolution of several different species from a single common ancestor, each becoming adapted to exploit a different niche or way of life.
adaxialUnit 3: Describing the upper surface of a leaf, the side facing towards the stem.
addition (mutation)Unit 4: A gene mutation in which an extra nucleotide is inserted into the DNA sequence, causing a frameshift.
adenineUnit 1: A purine nitrogenous base found in DNA, RNA and nucleotides such as ATP, which pairs with thymine in DNA and with uracil in RNA.
adenosine triphosphate (ATP)Unit 1: The universal energy currency of the cell: a nucleotide of adenine, ribose and three phosphate groups whose hydrolysis to ADP and inorganic phosphate releases energy for cellular processes.
ADH (antidiuretic hormone)Unit 3: A hormone secreted by the posterior pituitary that increases the permeability of the distal convoluted tubules and collecting ducts to water, producing more concentrated urine.
adhesionUnit 2: The attractive force between water molecules and the walls of the xylem, which helps draw water up the plant.
adipocyteUnit 3: A cell specialised for the storage of fat.
adrenal glandUnit 4: An endocrine gland above each kidney that secretes hormones controlling responses such as the body’s reaction to stress.
aerationCore terms: The introduction or circulation of air, and the oxygen it contains, into a liquid or other medium.
aerenchymaUnit 2: A plant tissue containing large air spaces that allow gases to move through the plant and provide buoyancy, found in many plants growing in waterlogged conditions.
aerobic respirationUnit 3: The breakdown of glucose and other respiratory substrates in the presence of oxygen to release energy, producing carbon dioxide and water and yielding a large amount of ATP.
afferent arterioleUnit 3: The small arteriole that carries blood into the glomerulus of a kidney nephron, supplying blood for ultrafiltration.
affinity (for oxygen)Unit 2: The strength of attraction between haemoglobin and oxygen, determining how readily oxygen is taken up or released at a given partial pressure.
agglutinationUnit 4: The clumping together of cells, such as red blood cells or microbes, when antibodies bind to their surface antigens.
aggregationCore terms: The clumping together of separate particles, cells or molecules into a larger mass.
aggressive displayUnit 4: A pattern of threatening behaviour used by an animal to intimidate a rival, often avoiding the need for actual fighting.
aim (of a study)Unit 4: A clear statement of what an investigation is intended to find out or test.
air sacUnit 2: A thin-walled chamber in the gas-exchange system, such as those in bird and insect respiratory systems, that increases the surface area available for the exchange of gases.
albumenUnit 5: The protein-rich egg white that surrounds and provides water and protein to the developing embryo in a bird’s egg.
algal bloomUnit 3: A rapid growth of algae at the water surface, caused by nutrient enrichment such as leached nitrate, that blocks light and contributes to eutrophication.
alimentary canalUnit 2: The continuous muscular tube running from the mouth to the anus along which food passes and is digested and absorbed.
alkalineUnit 2: Describes a solution with a pH greater than 7, containing a relatively low concentration of hydrogen ions.
alleleCore terms: One of the alternative forms of a gene found at a particular locus on a chromosome.
allele frequencyUnit 4: The proportion of all the alleles of a particular gene in a population that are a given allele.
allergenCore terms: A normally harmless substance that triggers an inappropriate immune response in a sensitised individual.
allergic reactionUnit 5: An inappropriate and exaggerated immune response to a normally harmless substance acting as an antigen.
allopatric speciationUnit 5: The formation of new species when populations of the same species become geographically separated, so that genetic differences accumulate until they can no longer interbreed.
allosteric siteUnit 1: A site on an enzyme, separate from the active site, where a regulatory molecule can bind and change the enzyme’s shape and therefore its activity.
alveoliUnit 2: The tiny air sacs at the ends of the bronchioles in the lungs, providing a large, thin, moist surface area for the exchange of gases between air and blood.
alveolusCore terms: A tiny air sac in the lung across whose thin, moist wall gases are exchanged with the blood.
amino acidUnit 1: The monomer unit of proteins, containing an amino group, a carboxyl group and a variable R group attached to a central carbon atom.
amino acid sequenceUnit 2: The order in which amino acids are joined together in a polypeptide, which is the primary structure of a protein and is determined by the sequence of bases in DNA.
ammoniaUnit 3: A soluble, highly toxic nitrogenous compound produced from the breakdown of amino acids, which many aquatic organisms excrete directly.
ammonificationCore terms: The conversion of nitrogen-containing organic compounds in dead matter into ammonium ions by decomposing microorganisms.
amniotic fluidUnit 4: The fluid surrounding a developing embryo or fetus that cushions and protects it.
amplitudeUnit 4: The maximum displacement of a wave from its rest position, corresponding to its size or intensity.
amylopectinUnit 1: A branched polysaccharide of alpha-glucose that, with amylose, makes up starch and acts as an energy store in plants.
amyloseUnit 1: An unbranched, coiled polysaccharide of alpha-glucose joined by 1,4 glycosidic bonds, forming part of starch.
anaemiaUnit 4: A condition in which the blood carries too little oxygen, usually because of a shortage of red blood cells or haemoglobin.
anaerobicUnit 4: Describes a process or organism that does not require oxygen.
anaerobic respirationUnit 4: The release of energy from glucose in the absence of oxygen, producing far less ATP than aerobic respiration and yielding lactate in animals or ethanol and carbon dioxide in yeast and plants.
analogous structureUnit 2: A structure in different species that performs a similar function but has a different evolutionary origin, indicating convergent evolution.
analogous structuresUnit 2: Structures in different species that perform a similar function but have a different evolutionary origin and underlying anatomy (the result of convergent evolution).
anaphaseCore terms: The stage of cell division during which the chromatids or homologous chromosomes are pulled to opposite poles of the cell.
aneuploidyUnit 4: A condition in which a cell has one or more chromosomes more or fewer than the normal number, often due to non-disjunction.
angiospermUnit 4: A flowering plant that produces seeds enclosed within an ovary that develops into a fruit.
annelidUnit 2: A member of the phylum Annelida; a segmented worm with a body divided into ring-like sections.
AnnelidaUnit 2: The phylum of segmented worms, whose bodies are divided into a series of ring-like segments.
antUnit 4: A small social insect; in ecology, ants are often used as an example of organisms living in organised colonies.
antagonistic musclesUnit 4: A pair of muscles that produce opposite movements at a joint, so that as one contracts the other relaxes.
antenna complexUnit 3: A cluster of light-harvesting pigment molecules in a photosystem that absorbs light energy and channels it to the reaction centre chlorophyll.
anteriorUnit 4: Describes the front or head end of an organism or structure.
antherUnit 4: The part of a stamen in which pollen grains are produced.
antibioticUnit 4: A chemical produced by a microorganism (or synthetically) that kills or inhibits the growth of bacteria, without harming the host’s cells.
antibiotic resistanceUnit 4: The situation in which a microorganism that should be affected by an antibiotic is no longer susceptible to it.
antibiotic resistance geneUnit 4: A gene that enables a bacterium to survive exposure to an antibiotic, and which can be used as a marker in genetic engineering.
antibodyUnit 2: A protein produced by the immune system (by B-lymphocytes/plasma cells) in response to a specific antigen, which binds to that antigen.
anticoagulantUnit 1: A substance that prevents or slows the clotting of blood.
anticodonUnit 1: A sequence of three bases on a tRNA molecule that is complementary to, and pairs with, a specific codon on mRNA during translation.
antigenUnit 2: A molecule, usually a protein on a cell surface, that is recognised as foreign by the immune system and triggers the production of specific antibodies.
antigenic typeUnit 4: A subgroup or strain of a microbial species whose members carry the same or very similar surface antigens; used to trace infections and usually identified using antibodies from serum.
antiparallelUnit 1: Describing the two strands of a DNA molecule, which run in opposite directions, one running 5′ to 3′ and the other 3′ to 5′.
antrumUnit 5: The fluid-filled cavity that develops within a maturing (Graafian) follicle in the ovary.
aortaUnit 2: The main artery that carries oxygenated blood away from the left ventricle of the heart to the body.
apoplast pathwayUnit 2: The route by which water moves through the cell walls and intercellular spaces of plant tissue without crossing cell membranes.
aquaporinUnit 3: A membrane protein forming a channel that allows water molecules to pass through the plasma membrane, for example in the collecting duct under the influence of ADH.
aquaticUnit 3: Describing an organism that lives in water.
ArchaeaUnit 2: One of the three domains of life, comprising prokaryotic single-celled organisms that are biochemically distinct from bacteria and often live in extreme environments.
archipelagoUnit 2: A group or chain of islands clustered together in the sea, often studied because their isolation promotes the evolution of distinct species.
arterioleUnit 2: A small branch of an artery that carries blood from arteries towards the capillaries; its muscular wall can constrict or dilate to control blood flow into a tissue.
arteriosclerosisCore terms: The hardening and loss of elasticity of artery walls.
ArthropodaUnit 2: A phylum of invertebrates with a hard exoskeleton, a segmented body and jointed limbs, including insects, crustaceans and arachnids.
articulationCore terms: A joint or point of contact between two bones that allows movement.
artificial geneUnit 4: A gene synthesised in the laboratory rather than isolated from an organism, for use in genetic engineering.
ascending limbUnit 3: The part of the loop of Henle that carries filtrate up towards the cortex, actively pumping out sodium and chloride ions while being impermeable to water.
aseptic techniqueUnit 3: A set of procedures used to prevent contamination of cultures by unwanted microorganisms, for example by sterilising equipment and media and working near a flame.
asexual reproductionUnit 4: Production of offspring from a single parent without the fusion of gametes, so the offspring are genetically identical to the parent.
atheromaUnit 1: A fatty deposit that builds up in the wall of an artery, narrowing the lumen and contributing to cardiovascular disease.
ATPUnit 1: Adenosine triphosphate, the universal energy currency of cells, which releases usable energy when its terminal phosphate bond is hydrolysed to form ADP and inorganic phosphate.
ATP synthaseUnit 3: The enzyme embedded in a membrane that catalyses the formation of ATP from ADP and inorganic phosphate as protons flow through it down their gradient.
ATP synthesisUnit 3: The formation of ATP from ADP and inorganic phosphate, an energy-requiring reaction that stores energy for use by the cell.
ATP-ADP cycleUnit 1: The continuous interconversion in which ATP is hydrolysed to ADP and phosphate to release energy for cell processes, and ADP is rephosphorylated to ATP using energy from respiration.
atrial systoleUnit 2: The stage of the cardiac cycle in which the atria contract, forcing blood into the ventricles.
atrio-ventricular node (AVN)Unit 2: A region of cardiac tissue at the base of the right atrium that receives the wave of excitation from the SAN and, after a short delay, passes it to the ventricles, triggering ventricular contraction.
atriumUnit 2: One of the two upper chambers of the heart that receives blood returning to the heart and pumps it into the ventricle below.
attenuatedUnit 4: Describing a pathogen that has been weakened so that it can no longer cause disease but still stimulates an immune response, as used in some vaccines.
auditory cortexUnit 4: The region of the cerebral cortex that receives and interprets nerve impulses from the ears, allowing sound to be perceived.
autolysisUnit 2: The self-digestion of a cell brought about when its own hydrolytic enzymes are released and break down its contents.
autonomicUnit 4: Relating to the part of the nervous system that controls involuntary, automatic processes such as heart rate, ventilation, blood pressure and digestion, comprising the sympathetic and parasympathetic divisions.
autonomic nervous systemUnit 4: The part of the nervous system that controls involuntary functions of the body, divided into the sympathetic and parasympathetic divisions.
autoradiographUnit 2: An image produced on photographic film by radiation from a radioactively labelled substance, used to show where that substance is located in a specimen.
autosome / autosomalUnit 4: A chromosome that is not a sex chromosome; autosomal describes genes or inheritance involving such chromosomes.
autotrophicUnit 5: Describing an organism that synthesises its own complex organic molecules from simple inorganic sources, using either light energy (photoautotrophic) or chemical energy (chemoautotrophic).
autotrophic organismUnit 1: An organism that synthesises its own complex organic molecules from simple inorganic sources such as carbon dioxide, using light or chemical energy.
axonUnit 3: The long fibre of a neurone that carries the nerve impulse away from the cell body towards other cells.
axon terminalUnit 3: The fine ending of an axon that forms a synapse and releases neurotransmitter to pass the impulse on.

B

bactericidalUnit 4: Describing an antibiotic or agent that kills bacteria.
bacteriochlorophyllUnit 3: A light-absorbing pigment found in photosynthetic bacteria, similar to the chlorophyll of plants.
bacteriostaticUnit 4: Describing an antibiotic or agent that prevents bacteria from growing and reproducing without killing them.
base compositionUnit 1: The relative proportions of the four nitrogenous bases in a sample of DNA, used to compare DNA from different sources.
base deletionCore terms: A gene mutation in which one or more nucleotide bases are removed from a DNA sequence.
base insertionCore terms: A gene mutation in which one or more nucleotide bases are added into a DNA sequence.
base pairsUnit 4: Two complementary nitrogenous bases joined by hydrogen bonds across the two strands of a DNA molecule, with adenine pairing with thymine and cytosine with guanine.
base substitutionUnit 4: A gene mutation in which one base in the DNA sequence is replaced by a different base, which may alter the codon and the amino acid coded for.
basement membraneUnit 3: A thin layer of extracellular material that underlies and supports an epithelium, acting as a filter in structures such as the glomerulus.
belt transectUnit 5: A sampling method in which species are recorded within a series of quadrats placed along a line across a habitat, used to study how a community changes along an environmental gradient.
beta thalassemiaUnit 4: An inherited blood disorder caused by a mutation affecting the beta-globin chains of haemoglobin, resulting in reduced or abnormal haemoglobin and anaemia.
beta-glucoseUnit 1: An isomer of glucose in which the hydroxyl group on carbon 1 lies above the ring, allowing the formation of straight, unbranched chains such as cellulose.
bicepsUnit 4: A skeletal muscle of the upper arm that contracts to flex the elbow, working antagonistically with the triceps.
biconcaveUnit 2: Describes the shape of a red blood cell, which is curved inwards on both faces, giving a large surface area to volume ratio and a short diffusion distance for oxygen.
bicuspid valveUnit 2: The atrioventricular valve on the left side of the heart, made of two flaps, that prevents the backflow of blood from the left ventricle into the left atrium.
bidirectionalUnit 2: Able to occur or move in two opposite directions, as seen in the transport of dissolved substances through phloem.
bileUnit 2: An alkaline fluid produced by the liver and stored in the gall bladder, released into the small intestine to emulsify fats and to neutralise acidic chyme.
bile saltsUnit 2: Components of bile that emulsify lipids by breaking large fat droplets into many smaller ones, increasing the surface area available for lipase to act on.
bimodal distributionUnit 2: A frequency distribution with two distinct peaks, indicating two separate groups or most common values within the data.
binary fissionUnit 1: The asexual reproduction of prokaryotic cells in which the cell replicates its circular DNA and then divides into two genetically identical daughter cells.
binding siteUnit 4: A specific region on a molecule, such as an enzyme or antibody, with a shape complementary to another molecule, allowing the two to attach.
binomial systemUnit 2: The system of naming species using two Latin names: the genus name followed by the species name.
biochemical testUnit 1: A qualitative laboratory test used to detect the presence of a particular biological molecule, such as a reducing sugar, starch, protein or lipid.
biodiversityUnit 2: The number of different species and the number of individuals of each species present in a given area, and also the genetic variety within those species.
biofuelUnit 3: A fuel derived from recently living biological material or biomass, such as ethanol or biogas, rather than from fossil sources.
biogeochemical flowUnit 3: The movement of a chemical element such as carbon or nitrogen between living organisms and the non-living environment.
biohazardUnit 5: A biological agent, such as a pathogen or infectious material, that poses a risk to the health of humans or other organisms.
biomassUnit 3: The total mass of living material present, usually measured as dry mass per unit area or per trophic level.
biosensorUnit 1: A device that uses an immobilised enzyme (or other biological molecule) to detect and measure the concentration of a specific substance.
biotic factorUnit 5: A living component of an ecosystem, such as predation, competition or disease, that affects the size and distribution of a population.
birth rateUnit 3: The number of new individuals produced by reproduction in a population over a given period of time.
bivalentUnit 1: A pair of homologous chromosomes lying alongside each other during prophase I of meiosis, allowing crossing over to occur.
blastocystUnit 4: An early embryonic stage formed by repeated mitotic division (cleavage) of the zygote, consisting of a hollow ball of cells that implants in the endometrium.
blood clotUnit 1: A semi-solid mass formed when soluble fibrinogen is converted to insoluble fibrin, trapping blood cells to seal a damaged vessel.
blood flowUnit 2: The movement of blood through the vessels of the circulatory system, driven by the pumping of the heart.
blood plasmaUnit 1: The straw-coloured liquid part of the blood that carries cells, dissolved nutrients, hormones, waste products, proteins and gases around the body.
blubberUnit 4: A thick layer of fat beneath the skin of some mammals that provides thermal insulation and an energy store.
body mass index (BMI)Unit 4: A measure of body mass relative to height, calculated by dividing mass in kilograms by the square of height in metres, used to assess whether a person is over- or underweight.
bond (chemical)Unit 1: A force of attraction that holds atoms together in a molecule or compound, such as a covalent, ionic or hydrogen bond.
bone densityUnit 4: The mass of mineralised bone tissue per unit volume, which determines the strength of a bone and decreases in conditions such as osteoporosis.
bone matrixUnit 4: The hard extracellular material of bone, made of collagen fibres impregnated with mineral salts such as calcium phosphate (hydroxyapatite), giving bone its strength.
bony fishUnit 2: A fish whose skeleton is made of bone, possessing a gill cover (operculum) and using a countercurrent system across the gills for efficient gas exchange.
booster (dose)Unit 4: An additional dose of a vaccine given after the initial course to raise antibody levels and the number of memory cells, strengthening long-term immunity.
Bowman’s capsuleUnit 3: The cup-shaped start of a nephron that surrounds the glomerulus and receives the fluid filtered out of the blood under pressure during ultrafiltration.
bradycardiaUnit 2: An abnormally slow resting heart rate, shown on an electrocardiogram as a longer than normal interval between successive heartbeats.
brainUnit 3: The central organ of the nervous system that receives, integrates and coordinates information and controls the activity of the rest of the body.
brittle bone diseaseUnit 4: An inherited condition (osteogenesis imperfecta) caused by a mutation in the gene for collagen, producing an imbalance between the organic and inorganic components of the bone matrix so that bones break easily.
bronchioleUnit 2: One of the fine airways that branch from the bronchi within the lung and carry air to the alveoli.
bronchusCore terms: One of the two main airways that branch from the trachea and carry air into a lung.
brown adipose tissueUnit 3: A specialised fat tissue, rich in mitochondria, that releases heat directly rather than generating ATP, contributing to heat production in some mammals.
buccal cavityUnit 2: The mouth cavity, the first part of the digestive tract, where food is taken in and mechanical and initial chemical digestion begin.
buffer solutionUnit 1: A solution that resists changes in pH when small amounts of acid or alkali are added, used to keep conditions constant during an experiment.
bundle of HisUnit 2: A band of specialised conducting fibres that carries the wave of electrical excitation from the atrioventricular node down through the septum to the ventricles.

C

caecumUnit 2: A blind-ending pouch at the junction of the small and large intestine, enlarged in many herbivores where it houses microorganisms that help digest cellulose.
calcium ion channelUnit 3: A protein pore in a membrane that allows calcium ions to pass through, for example opening at a synapse to admit calcium ions when an impulse arrives.
calcium ionsUnit 3: Positively charged calcium particles whose movement into a cell triggers events such as the release of neurotransmitter from a synaptic knob or muscle contraction.
calibration curveUnit 1: A graph of a measurable quantity, such as absorbance, plotted against known concentrations of a standard, used to find the concentration of an unknown sample.
calyxUnit 4: The outermost whorl of a flower, made up of the sepals, which protects the flower while it is in bud.
cambiumUnit 5: A layer of dividing cells in the stem and root of a plant that produces new xylem and phloem, allowing growth in thickness.
canaliculiUnit 4: Tiny channels in bone tissue that connect the lacunae, allowing nutrients and waste to pass between osteocytes.
canine toothUnit 2: A pointed tooth between the incisors and premolars, well developed in carnivores for gripping, piercing and tearing prey.
capacitationUnit 4: Changes to the membrane of a sperm cell, occurring in the female reproductive tract, that increase its fluidity and allow the acrosome reaction to take place.
capillaryUnit 2: The smallest blood vessel, with a wall one cell thick, where exchange of materials such as oxygen, nutrients and wastes takes place between blood and tissues.
capillary bedUnit 2: A dense network of capillaries supplying a tissue, providing a very large surface area for the exchange of substances between blood and cells.
capsidUnit 1: The protein coat that surrounds and protects the genetic material of a virus.
capsule (bacterial)Unit 1: A protective slime layer of polysaccharide surrounding some bacterial cells, helping them resist desiccation and evade the host immune system.
capture-mark-recaptureUnit 3: A method of estimating the size of an animal population in which a sample is caught, marked and released, and the proportion of marked individuals in a later sample is used to calculate the total.
carbaminohaemoglobinUnit 2: The compound formed when carbon dioxide binds to the haemoglobin in red blood cells, providing one means by which carbon dioxide is carried in the blood.
carbohydrateCore terms: An organic molecule made of carbon, hydrogen and oxygen, including sugars, starches and cellulose, used mainly as an energy source and for structure.
carbon cycleUnit 3: The continual recycling of carbon between the atmosphere and living things through photosynthesis, respiration, decomposition, the formation of fossil fuels and combustion.
carbon fixationUnit 3: The stage of photosynthesis in which carbon dioxide is combined with a five-carbon acceptor (ribulose bisphosphate) by the enzyme RuBisCO to form an organic compound.
carbon footprintUnit 3: The total amount of carbon dioxide attributable to the actions of an individual, product or service over a period of one year.
carbonic acidUnit 2: The weak acid formed when carbon dioxide dissolves and reacts with water in the blood; it dissociates to release hydrogen ions and hydrogencarbonate ions.
carbonic anhydraseUnit 3: An enzyme in red blood cells that speeds up the reaction between carbon dioxide and water to form carbonic acid, aiding the transport of carbon dioxide in the blood.
carboxylaseCore terms: An enzyme that catalyses the addition of a carboxyl group, or carbon dioxide, to a substrate molecule.
carboxysomeUnit 3: A protein-bound compartment in some bacteria that concentrates carbon dioxide and the enzyme RuBisCO to make carbon fixation more efficient.
carcinogenUnit 4: An agent, such as a chemical or form of radiation, that can cause cancer by inducing mutations in DNA.
cardiac cycleUnit 2: The sequence of events in one complete heartbeat, comprising contraction (systole) and relaxation (diastole) of the atria and ventricles.
cardiac muscleUnit 2: The specialised muscle of the heart wall that contracts rhythmically and without fatigue throughout life, generating its own beat (myogenic).
cardiac outputUnit 2: The volume of blood pumped out of the heart by one ventricle in one minute, equal to heart rate multiplied by stroke volume.
cardiovascular diseaseUnit 1: Any disease affecting the heart or blood vessels, such as coronary heart disease, often linked to the build-up of atheroma.
carnassialUnit 2: A specialised cheek tooth in carnivores with a sharp, shearing edge adapted for slicing flesh and cutting through tendon.
carnivoreUnit 2: An animal that feeds on other animals, with teeth and a gut adapted to a diet of meat.
carotenoidUnit 3: An orange or yellow accessory photosynthetic pigment that absorbs light at wavelengths chlorophyll absorbs poorly and passes the energy to the reaction centre.
carpelUnit 4: The female reproductive structure of a flower, consisting of the stigma, style and ovary.
carrier proteinsUnit 1: Membrane proteins that bind specific molecules or ions and change shape to transport them across the membrane, used in facilitated diffusion and active transport.
carrying capacityUnit 3: The maximum population size of a species that a given environment can sustain over time given the available resources.
cartilageUnit 4: A firm but flexible connective tissue found in the airways, at the ends of bones, at the front of the ribs, and in the nose and outer ear.
catalystCore terms: A substance that increases the rate of a chemical reaction by lowering its activation energy and is not used up or permanently changed by the reaction.
categorical dataUnit 5: Data that can be sorted into distinct, named groups or categories rather than being measured on a numerical scale.
cDNAUnit 4: Complementary DNA; a double-stranded DNA molecule synthesised from a messenger RNA template using reverse transcriptase, containing only the coding (exon) sequence.
cellUnit 1: The basic structural and functional unit of all living organisms, bounded by a cell membrane and able to carry out the processes of life.
cell bodyUnit 3: The part of a neurone that contains the nucleus and most of the cytoplasm and organelles, from which the dendrites and axon extend.
cell cycleUnit 1: The ordered sequence of events in a dividing cell, comprising interphase (growth and DNA replication) followed by mitosis and cytokinesis.
cell divisionUnit 4: The process by which a parent cell divides to produce daughter cells, by mitosis for growth and repair or by meiosis to form gametes.
cell lineUnit 1: A population of cells of a single type that can be grown and maintained in culture over many generations.
cell membraneUnit 1: The partially permeable phospholipid bilayer surrounding a cell that controls the movement of substances into and out of it.
cell recognitionUnit 1: The ability of cells to identify one another and distinguish self from non-self, mediated by glycoproteins and glycolipids on the cell-surface membrane.
cell wallUnit 1: A rigid outer layer surrounding the cell membrane of plant, fungal and prokaryotic cells that provides support and prevents the cell from bursting.
cell-mediated responseUnit 4: An immune response involving the direct action of T lymphocytes against pathogens, infected cells and cancerous cells, rather than the release of antibodies.
cellular aerobic respirationUnit 5: The enzyme-controlled breakdown of glucose and other respiratory substrates within cells using oxygen, releasing energy to synthesise ATP.
celluloseUnit 1: A structural polysaccharide of beta-glucose whose straight chains form microfibrils of great tensile strength, making up plant cell walls.
central canalUnit 5: The narrow channel running through the centre of the spinal cord, surrounded by the grey matter, that contains cerebrospinal fluid.
central nervous systemUnit 4: The part of the nervous system consisting of the brain and spinal cord, which coordinates the activity of the body by processing information received from receptors.
centrifugationCore terms: The separation of components of a mixture according to density by spinning the sample at high speed.
centrioleUnit 1: A small cylindrical structure made of microtubules, found in pairs in animal cells, that helps organise the spindle during cell division.
centromereUnit 1: The region of a chromosome where the two sister chromatids are joined and to which spindle fibres attach during cell division.
centrosomeUnit 1: The main microtubule-organising region of an animal cell, containing a pair of centrioles, which forms the spindle during nuclear division.
cerebellumUnit 4: The region of the brain that controls balance and posture and co-ordinates voluntary muscular activity such as writing.
cerebral cortexUnit 4: The outer layer of the cerebrum, responsible for higher functions such as conscious thought, reasoning, language and the interpretation of sensory information.
cerebrumUnit 4: The largest part of the brain, divided into two hemispheres, responsible for higher functions such as thought, memory, sensory processing and voluntary movement.
cervical cancerUnit 1: A malignant tumour of the cervix, strongly associated with infection by certain types of human papillomavirus.
cervixUnit 1: The narrow lower part of the uterus that connects it to the vagina.
channel proteinUnit 1: A protein spanning the cell membrane that forms a water-filled pore, allowing specific ions or polar molecules to cross by facilitated diffusion.
chemical digestionUnit 2: The breakdown of large insoluble food molecules into smaller soluble ones by hydrolysis, catalysed by digestive enzymes.
chemical fertiliserUnit 5: A manufactured substance containing inorganic nutrients such as nitrates and phosphates, added to soil to increase the growth and yield of crops.
chemical pollutionUnit 3: The release of harmful chemical substances into the environment by human activity, damaging organisms or ecosystems.
chemiosmosisUnit 3: The synthesis of ATP driven by the movement of protons down their electrochemical gradient across a membrane through ATP synthase.
chemoautotrophUnit 5: An autotrophic organism that obtains its energy from the oxidation of inorganic chemicals rather than from light, using it to synthesise organic compounds.
chemoautotrophicCore terms: Describing an organism that synthesises its own organic compounds using energy released from chemical reactions rather than light.
chemosyntheticUnit 2: Describes an organism that makes its own organic compounds using energy released from chemical reactions rather than from light.
chemotherapyUnit 4: The treatment of disease, particularly cancer, using chemical drugs that kill or inhibit the division of rapidly dividing cells.
chemotrophicCore terms: Describing an organism that obtains its energy from chemical reactions.
Chi-squared testUnit 4: A statistical test that compares observed results with those expected by chance to determine whether any difference between them is significant.
chiasmaUnit 1: A point at which two homologous chromosomes remain in contact during meiosis, where sections of genetic material may be exchanged by crossing over.
chief cellUnit 2: A cell in the lining of the stomach that secretes pepsinogen, the inactive precursor of the protein-digesting enzyme pepsin.
chitinUnit 2: A tough nitrogen-containing polysaccharide that forms the exoskeleton of arthropods and is also found in fungal cell walls.
chloride shiftUnit 2: The movement of chloride ions into red blood cells in exchange for hydrogencarbonate ions diffusing out, which maintains the electrical neutrality of the cell.
chlorophyllUnit 3: The main green pigment in chloroplasts that absorbs light energy, mainly in the red and blue parts of the spectrum, for use in photosynthesis.
chloroplastUnit 1: An organelle in plant and algal cells, containing chlorophyll on internal membranes, that is the site of photosynthesis.
chlorosisUnit 2: The yellowing of plant leaves caused by a lack of chlorophyll, often resulting from a deficiency of magnesium or other minerals.
choleraUnit 4: A bacterial disease of the intestine, caused by Vibrio cholerae, that produces severe watery diarrhoea and dehydration, usually spread through contaminated water.
cholesterolUnit 1: A lipid found in cell membranes where it regulates fluidity, and a precursor for steroid hormones, bile salts and vitamin D.
chondrocyteUnit 4: A cell embedded in the matrix of cartilage that produces and maintains the cartilage tissue.
ChordataUnit 2: A phylum of animals possessing, at some stage of development, a flexible supporting rod (notochord), a dorsal nerve cord and pharyngeal slits.
chromatidCore terms: One of the two identical copies of a replicated chromosome, joined to its sister at the centromere.
chromatinUnit 1: The complex of DNA and histone proteins that makes up chromosomes, appearing as diffuse threads in a non-dividing nucleus.
chromatogramUnit 3: The pattern of separated substances produced when a mixture is separated by chromatography, used to identify the components present.
chromatographyUnit 3: A technique that separates the components of a mixture according to how strongly they are carried by a moving solvent relative to a stationary medium, used for example to separate chloroplast pigments.
chromosomeUnit 4: A thread-like structure of tightly coiled DNA wound around histone proteins, carrying the genes, found in the nucleus of a eukaryotic cell.
chromosome mutationUnit 4: A change in the number or structure of whole chromosomes, such as the non-disjunction that causes Down’s syndrome.
chymeUnit 2: The acidic, semi-liquid mixture of partly digested food and gastric juice that passes from the stomach into the small intestine.
ciliaUnit 4: Tiny hair-like projections on the surface of some cells that beat rhythmically to move fluid, mucus or the cell itself.
ciliated epitheliumUnit 4: A layer of cells bearing cilia, found lining structures such as the airways and oviduct, whose beating moves substances across the surface.
circular muscleUnit 5: A layer of muscle whose fibres are arranged in rings around a tubular structure; its contraction narrows the lumen, as occurs in gut peristalsis.
circulatory systemUnit 2: The system of the heart and blood vessels that transports blood, and the substances it carries, around the body.
cladogramUnit 2: A branching diagram that shows the inferred evolutionary relationships between organisms based on shared characteristics.
Class (taxonomy)Unit 2: A rank in the classification hierarchy, below phylum and above order, grouping together related orders of organisms.
classical conditioningUnit 5: A form of learned behaviour in which an animal comes to produce a response to a new stimulus after that stimulus has been repeatedly associated with one that naturally triggers the response.
classificationUnit 2: The arrangement of organisms into ordered groups based on their similarities and evolutionary relationships.
cleavage (DNA)Unit 4: The cutting of a DNA molecule, for example by restriction enzymes at specific recognition sequences.
climate changeUnit 3: Long-term changes in global temperature and weather patterns, driven largely by increased atmospheric carbon dioxide, that can alter the distribution of species and cause extinction.
climax communityUnit 3: The stable, self-sustaining community of organisms that represents the final stage of an ecological succession in a given environment.
clinical trialUnit 4: A controlled scientific study used to test the safety and effectiveness of a new drug or medical treatment in humans before it is approved for use.
clonal (expansion)Unit 4: The rapid mitotic division of a selected lymphocyte to produce a large population of genetically identical cells during an immune response.
cloneUnit 4: A group of genetically identical organisms or cells produced from a single parent by asexual reproduction or other non-sexual means.
closed circulatory systemUnit 2: A circulatory system in which blood is contained within vessels at all times and is pumped under pressure around the body.
closed seasonUnit 3: A period during which the catching or killing of a species is prohibited, used as a conservation measure to allow populations to recover or breed.
CnidariaUnit 2: A phylum of simple aquatic animals with radial symmetry, a sac-like body with a single opening, and a nerve net, including jellyfish and sea anemones.
CNS (central nervous system)Unit 3: The brain and spinal cord, which receive and process information from receptors and coordinate the responses of the body.
co-dominanceUnit 4: A form of inheritance in which both alleles of a gene are equally expressed in the phenotype of a heterozygote, so neither is dominant over the other.
coagulationCore terms: The process by which a liquid, such as blood or a protein solution, changes into a semi-solid clot or mass.
coding DNAUnit 4: DNA sequences (exons) that are transcribed and translated into the amino acid sequence of a polypeptide.
coding strandUnit 1: The strand of a DNA double helix that has the same base sequence as the messenger RNA produced, except that thymine replaces uracil, and which is not used as the template during transcription.
codominance / codominantUnit 4: An inheritance pattern in which both alleles of a heterozygous genotype are fully expressed in the phenotype, so neither allele is dominant over the other.
codonUnit 1: A sequence of three bases in mRNA that codes for a specific amino acid (or a start/stop signal) during translation.
coenzymeUnit 3: A small non-protein organic molecule that binds to and is required for the activity of an enzyme, often acting to carry chemical groups or electrons between reactions.
coevolutionUnit 2: The process by which two species that closely interact each act as a selection pressure on the other, so that they evolve together over time.
cofactorCore terms: A non-protein substance that an enzyme requires in order to function.
cohesion-tensionUnit 2: The theory explaining the movement of water up the xylem, in which water lost from the leaves creates tension that pulls up a continuous column of water held together by cohesion between water molecules.
collagenCore terms: A strong, fibrous structural protein found in connective tissues such as tendons, skin and bone.
collecting ductUnit 3: The tubule of the kidney into which several nephrons drain; water is reabsorbed from it by osmosis so that the urine produced becomes more concentrated.
collenchymaUnit 5: A plant tissue made of elongated living cells with unevenly thickened cellulose walls that provides flexible support, especially in young stems and leaves.
coloniseUnit 4: To become established and spread in a new habitat, as when pioneer species occupy a previously bare area during succession.
colonyUnit 2: A group of individual organisms of the same species living closely together; in microbiology, a visible cluster of cells grown from a single parent cell.
columnar epitheliumUnit 5: A lining tissue made of tall, column-shaped cells, often found where absorption or secretion occurs, such as in the lining of the gut.
combustionUnit 3: The burning of organic material or fossil fuels, which releases stored carbon to the atmosphere as carbon dioxide.
commensalismUnit 3: A close relationship between two species in which one benefits while the other is neither helped nor harmed.
common ancestorUnit 2: An ancestral species from which two or more different species have evolved.
common coldUnit 4: A mild, contagious viral infection of the upper respiratory tract.
common nameUnit 2: The everyday, non-scientific name of an organism, which may vary between places and languages, unlike its universal binomial name.
compact boneUnit 4: The dense, hard outer layer of bone that provides strength and support, in contrast to the lighter spongy bone within.
companion cellUnit 2: A cell closely associated with a sieve tube element in phloem that, having a nucleus and many organelles, carries out the metabolic activity needed to load and unload sugars.
competitive inhibitorUnit 1: A molecule with a similar shape to the substrate that binds to the active site of an enzyme, preventing the substrate from binding and so reducing the rate of reaction.
complementary base pairsUnit 1: The specific pairing of nitrogenous bases by hydrogen bonding, in which adenine pairs with thymine, or with uracil in RNA, and cytosine pairs with guanine.
concentrationUnit 1: The amount of a solute dissolved in a given volume of solution, commonly expressed in moles per cubic decimetre.
concentration gradientUnit 2: The difference in the concentration of a substance between two regions, along which the substance tends to move from where it is more concentrated to where it is less concentrated.
conceptionUnit 4: The fertilisation of an egg by a sperm to form a zygote, marking the start of pregnancy.
condensation reactionUnit 1: A reaction in which two molecules are joined together by the formation of a chemical bond, with the removal of a molecule of water.
conduction speedUnit 3: The rate at which a nerve impulse travels along the axon of a neurone.
connective tissueUnit 4: A tissue that supports, binds or separates other tissues and organs, consisting of cells within an extracellular matrix.
contaminationUnit 3: The unwanted introduction of foreign organisms or substances into a culture, sample or environment.
continuous variationUnit 4: Variation in a characteristic that shows a range of values between two extremes, typically controlled by several genes and influenced by the environment, producing a normal distribution.
control (experimental)Unit 3: A part of an experiment kept under conditions identical to the test except for the factor being investigated, used as a comparison to show that factor caused any effect.
control measureUnit 5: An action or precaution taken to reduce or remove the risk presented by a hazard during an investigation.
controlled variableUnit 4: A factor that is kept constant during an experiment so that it does not affect the dependent variable and the test remains fair.
convergent evolutionUnit 2: The independent evolution of similar features in unrelated species, because they have adapted to similar environments or ways of life.
cooperative bindingUnit 2: The property by which the binding of the first oxygen molecule to haemoglobin alters its shape so that subsequent oxygen molecules bind more readily.
coordinationCore terms: The way in which an organism detects changes and produces appropriate, integrated responses through its nervous and hormonal systems.
coppicingUnit 3: A woodland management technique in which trees are cut down to near ground level so that they regrow, providing a sustainable, renewable supply of timber.
copulationCore terms: The act of sexual intercourse by which sperm are transferred from the male to the female reproductive tract.
corona radiataUnit 4: The layer of follicle cells surrounding the secondary oocyte after ovulation, which the sperm must penetrate during fertilisation.
corpus luteumUnit 4: The structure that develops from the remains of a Graafian follicle after ovulation and secretes progesterone to maintain the endometrium.
correlationUnit 4: A statistical relationship between two variables in which a change in one is associated with a change in the other, which does not by itself prove that one causes the other.
corridor (wildlife)Unit 3: A strip of suitable habitat that connects otherwise isolated areas, allowing organisms to move between them and maintaining gene flow between populations.
cortex (root)Unit 2: The layer of packing tissue between the epidermis and the central vascular tissue of a root, through which water passes on its way to the xylem.
cortical reactionUnit 4: The release of contents from cortical granules just beneath the egg membrane after fertilisation, which alters the membrane to prevent the entry of further sperm.
cortisolUnit 4: A steroid hormone released from the adrenal glands in response to stress, which raises blood glucose and whose levels can be affected by experiences in early life.
cotransportUnit 1: A form of facilitated transport in which a carrier protein moves two different substances across a membrane together, often using the gradient of one to move the other against its gradient.
cotyledonUnit 4: A seed leaf of a plant embryo that stores or absorbs food reserves used by the seedling during germination.
counter-current (flow)Unit 2: An arrangement in which two fluids flow in opposite directions (e.g. water and blood across a fish gill), maintaining a diffusion gradient along the whole exchange surface.
countercurrent flowUnit 2: An arrangement in which two fluids flow in opposite directions, as water and blood do across a fish gill, maintaining a diffusion gradient along the whole length so that more oxygen is absorbed.
courtship behaviourUnit 4: A pattern of behaviour by which animals attract a mate and ensure successful reproduction with a member of the same species.
covalent bondUnit 1: A strong chemical bond formed when two atoms share one or more pairs of electrons.
creatine phosphateUnit 1: A high-energy phosphate store in muscle cells that rapidly transfers its phosphate group to ADP to regenerate ATP during short bursts of intense activity.
crenatedUnit 1: Describing an animal cell that has shrunk and developed a wrinkled outline after losing water by osmosis in a solution of lower water potential.
CRH (corticotrophin-releasing hormone)Unit 4: A hormone produced by the hypothalamus that stimulates the pituitary gland to release ACTH, leading to cortisol secretion by the adrenal glands.
CRISPR/Cas9Unit 4: A gene-editing tool that uses a guide RNA and the Cas9 enzyme to cut DNA at a precise sequence, allowing genes to be added, removed or altered.
critical periodUnit 4: A limited window early in an organism’s development during which particular experiences or stimuli are needed for normal behaviour or features to develop correctly.
critical valueUnit 4: In a statistical test, the value from a table that the calculated test statistic is compared with to decide whether to accept or reject the null hypothesis at a given probability level.
critical value of tUnit 5: The value from statistical tables against which a calculated t value is compared to decide whether to accept or reject the null hypothesis at a chosen probability level.
cross (genetic)Unit 4: The deliberate breeding together of two parent organisms of known genotype to study the inheritance of characteristics in their offspring.
cross-bridgeUnit 4: The link formed when a myosin head attaches to an actin filament during muscle contraction, allowing the filaments to be pulled past one another.
cross-linkageUnit 4: A chemical bond joining adjacent molecules or polymer chains together, adding strength or rigidity to a structure such as collagen or cellulose.
cross-linkedUnit 1: Joined by additional chemical bonds between adjacent molecular chains, as in the bonds that stabilise the structure of a protein or polymer.
crossing overUnit 4: The exchange of equivalent sections of genetic material between the chromatids of homologous chromosomes during prophase I of meiosis, producing new allele combinations.
culture (microbial)Unit 1: A population of microorganisms grown deliberately under controlled conditions on or in a suitable nutrient medium.
cumulative species totalUnit 2: The running total of the number of different species recorded as more samples are taken, used to judge when enough sampling has been done.
cumulus cellUnit 5: One of the follicle cells that surround and nourish a developing oocyte and the released egg in the ovary.
curvatureUnit 4: A bending or growth response in a plant shoot or root, often produced by uneven distribution of auxin causing one side to grow faster than the other.
Cushing’s diseaseUnit 4: A disorder caused by an excess of cortisol in the body, leading to symptoms such as weight gain, raised blood glucose and muscle weakness.
cuticle (parasite)Unit 2: The tough, protective outer layer covering a parasite such as a tapeworm, which resists digestion by the host’s enzymes.
cuticular evaporationUnit 2: The loss of water vapour directly through the waxy cuticle covering the surface of a leaf, as distinct from loss through the stomata.
cuttingsUnit 4: Pieces of a parent plant, such as a section of stem, that are removed and used to grow new genetically identical plants by artificial vegetative propagation.
cyanobacteriaUnit 3: A group of photosynthetic bacteria that release oxygen, important as primary producers and in the evolution of the Earth’s atmosphere.
cyanogenesisCore terms: The release of hydrogen cyanide by certain plants when their tissues are damaged, acting as a defence.
cyanogenicUnit 2: Describes an organism, such as some clover plants, that can release hydrogen cyanide as a chemical defence against being eaten.
cyclinUnit 1: A regulatory protein whose changing concentration controls progression through the stages of the cell cycle by activating cyclin-dependent kinases.
cytochrome oxidaseUnit 3: The final electron carrier of the electron transport chain that passes electrons to oxygen, the terminal electron acceptor in aerobic respiration.
cytokinesisUnit 1: The division of the cytoplasm of a cell that follows nuclear division, producing two separate daughter cells.
cytoplasmCore terms: The jelly-like material enclosed by the cell membrane, in which the organelles are suspended and many reactions occur.
cytosineUnit 1: A pyrimidine nitrogenous base found in DNA and RNA that pairs with guanine through three hydrogen bonds.

D

Darwin’s theory of evolutionUnit 4: The proposal that species change over time through natural selection, in which individuals with advantageous inherited variations are more likely to survive and reproduce.
daughter cellUnit 1: A cell produced when a parent cell divides by mitosis or meiosis.
Davson-Danielli modelUnit 1: An early model of membrane structure proposing a phospholipid layer sandwiched between two continuous layers of protein, later replaced by the fluid-mosaic model.
deaminationUnit 3: The removal of the amino group from an amino acid in the liver, forming ammonia which is then converted to urea.
death phaseUnit 3: The stage of growth of a culture during which the death rate exceeds the reproduction rate, so the number of living individuals falls.
death rateUnit 3: The number of individuals in a population that die in a given period of time, often expressed per unit of population.
decarboxylaseUnit 3: An enzyme that catalyses the removal of a carboxyl group from a substrate, releasing carbon dioxide, as occurs in the Krebs cycle.
decarboxylationCore terms: The removal of a carboxyl group from a molecule, releasing it as carbon dioxide; a key step in respiration such as the link reaction and Krebs cycle.
decomposerUnit 3: An organism, such as a bacterium or fungus, that breaks down dead organic matter and releases the nutrients it contains back into the ecosystem.
deep vein thrombosis (DVT)Unit 1: The formation of a blood clot in a deep vein, usually in the leg, which may break free and block vessels elsewhere in the body.
deforestationUnit 5: The large-scale removal of trees and woodland, often to provide land for agriculture or to obtain timber, which reduces biodiversity and habitat.
degenerate codeUnit 1: A feature of the genetic code in which most amino acids are coded for by more than one triplet of bases.
degrees of freedomUnit 4: A value used in a statistical test, calculated from the number of categories or measurements, that determines which critical value to compare the test statistic against.
dehydrationUnit 4: The removal of a molecule of water, for example during a condensation reaction that joins two smaller molecules together.
dehydrogenaseUnit 3: An enzyme that catalyses the removal of hydrogen from a substrate, transferring it to a hydrogen acceptor such as NAD during respiration.
dehydrogenationCore terms: The removal of hydrogen atoms from a molecule, typically by a dehydrogenase enzyme, releasing hydrogen that is carried away by coenzymes such as NAD.
demeUnit 4: A local interbreeding population of a species within a larger population, often somewhat isolated from other such groups.
demethylationCore terms: The removal of a methyl group from a molecule.
demyelinatedUnit 3: Describing a nerve fibre that has lost its myelin sheath, which slows or disrupts the conduction of impulses.
denaturationUnit 1: A permanent change to the tertiary structure and shape of a protein, such as an enzyme; for an enzyme this alters the active site so the substrate can no longer bind and the enzyme stops working. Caused by high temperature or extremes of pH.
denitrificationUnit 5: The conversion of nitrates in the soil into nitrogen gas by denitrifying bacteria, returning nitrogen to the atmosphere.
density-dependent factorUnit 3: A factor whose effect on a population becomes stronger as population density rises, such as competition, predation or disease, helping to regulate population size.
density-independent factorUnit 4: An environmental factor that affects population size to the same extent regardless of how many individuals are present, such as a sudden change in temperature.
dentitionUnit 2: The number, type and arrangement of an animal’s teeth, which are adapted to suit its diet.
deoxygenated bloodUnit 2: Blood that has given up much of its oxygen to the tissues and is being returned to the heart and lungs.
deoxyriboseUnit 1: The five-carbon pentose sugar found in the nucleotides of DNA.
dependent variableUnit 2: The variable that is measured in an investigation and that changes in response to changes in the independent variable.
depolarisationCore terms: The temporary reversal of the charge across a neurone membrane as sodium ions flood in, generating a nerve impulse.
desalinationUnit 3: The removal of salt from seawater to produce fresh water suitable for drinking or irrigation.
descending limbUnit 3: The part of the loop of Henle that carries filtrate down into the medulla; it is permeable to water, so water leaves it by osmosis into the surrounding tissue fluid.
desiccationCore terms: The removal of water from a substance, causing it to dry out.
detectorUnit 3: A structure or cell that senses a change in conditions and provides information to a control system; a receptor.
detritivoreUnit 2: An animal that feeds on dead and decaying organic matter (detritus), helping to break it down and recycle nutrients.
diabetesUnit 4: A condition in which blood glucose concentration is not properly controlled, caused by a lack of insulin or by the body’s cells not responding to it.
diabetes insipidusUnit 3: A condition in which too little antidiuretic hormone is produced or the kidney fails to respond to it, so large volumes of dilute urine are produced.
diabetes mellitusUnit 3: A condition in which blood glucose concentration is not properly regulated, owing to a lack of insulin or a failure of cells to respond to it.
dialysis fluidUnit 3: The solution used in a dialysis machine that has a controlled composition so that wastes and excess substances diffuse out of the blood while needed substances are retained.
diaphragmUnit 2: The sheet of muscle separating the thorax from the abdomen in mammals; its contraction and relaxation change the volume of the thorax to bring about ventilation of the lungs.
diarrhoeaUnit 2: The frequent passing of watery faeces, caused when too little water is reabsorbed in the intestine, often as a result of infection.
diastemaUnit 2: The gap in the lower jaw of herbivores between the front teeth and the molars, through which the cheeks help to move food during mechanical digestion.
diastoleUnit 2: The stage of the cardiac cycle in which the heart muscle relaxes and the chambers fill with blood.
dichotomous keyUnit 5: An identification tool that leads the user to the name of an organism through a series of paired, contrasting choices about its features.
dicotyledonousUnit 2: Describes a flowering plant whose seeds contain two seed leaves (cotyledons).
dietUnit 2: The range of food that an organism habitually eats.
differentiationUnit 4: The process by which an unspecialised cell develops into a particular specialised cell type suited to a specific function.
diffusionCore terms: The net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, until evenly spread.
diffusion distanceUnit 2: The length of the path a substance must travel by diffusion between two points; gas-exchange surfaces are typically thin so this distance is short and exchange is rapid.
diffusion pathwayUnit 2: The route along which a substance moves by diffusion from a region of higher concentration to one of lower concentration, for example across a respiratory surface.
diffusion shellUnit 2: The zone of fluid immediately surrounding a respiratory surface or organism in which the diffusing gas becomes depleted, slowing further exchange unless the fluid is renewed.
dihybrid crossUnit 5: A genetic cross that follows the inheritance of two different characteristics, each controlled by a separate gene, at the same time.
diluteUnit 1: To reduce the concentration of a solution by adding more solvent, usually water.
dilutionUnit 5: The process of reducing the concentration of a solution by adding more solvent, such as water.
diploidUnit 4: Describing a cell or organism containing two complete sets of chromosomes, one inherited from each parent (denoted 2n).
dipoleCore terms: A molecule with a slightly positive region and a slightly negative region because its charge is unevenly distributed, as in a water molecule.
disaccharideUnit 1: A sugar formed when two monosaccharides join by a glycosidic bond, with the loss of a water molecule in a condensation reaction.
discontinuous variationUnit 4: Variation in which individuals fall into a small number of distinct categories with no intermediates, usually controlled by one or a few genes.
disk floretUnit 4: One of the small tubular flowers forming the central part of a composite flower head such as that of a daisy or dandelion.
dislocationUnit 4: An injury in which the bones forming a joint are forced out of their normal position relative to one another.
distal convoluted tubuleUnit 3: The region of the nephron after the loop of Henle where the composition of the filtrate is adjusted, and whose permeability to water is increased by antidiuretic hormone.
distributionUnit 3: The way in which organisms of a species are spread across an area or habitat.
disulfide bridgeUnit 1: A strong covalent bond formed between the sulfur-containing groups of two cysteine amino acids, helping to hold a protein in its tertiary structure.
divergent evolutionUnit 2: The evolution of different features in related species as they adapt to different environments, leading them to become increasingly distinct over time.
diversity indexUnit 2: A numerical measure of the biodiversity of a habitat that takes account of both the number of species present (species richness) and the number of individuals of each species (species evenness).
DNAUnit 1: A double-stranded polynucleotide that carries an organism’s genetic information; its base sequence codes for the amino acid sequence of proteins.
DNA helicaseUnit 1: An enzyme that unwinds the DNA double helix and breaks the hydrogen bonds between the two strands during DNA replication.
DNA hybridisationUnit 2: A technique in which single strands of DNA from two species are mixed so that complementary base sequences pair together; the more closely the strands bind, the more closely related the species are.
DNA methylationUnit 4: The addition of methyl groups to DNA, an epigenetic change that can switch off the expression of a gene without altering its base sequence.
DNA polymeraseUnit 1: An enzyme that joins free DNA nucleotides together to build a new strand against a template during DNA replication.
DNA replicationCore terms: The process by which a DNA molecule is copied, each strand acting as a template so two identical molecules are produced.
Domain (of life)Unit 2: The highest level of taxonomic classification, dividing all living organisms into three large groups (Bacteria, Archaea and Eukarya) based on fundamental cellular and molecular differences.
domain (taxonomy)Unit 3: The highest and broadest level of classification, dividing living things into three groups (Bacteria, Archaea and Eukarya) based on fundamental molecular differences such as ribosomal RNA.
dominance hierarchyUnit 4: A ranking system within an animal social group in which individuals occupy positions of differing status, reducing aggression over food, mates and breeding sites.
dominant (allele)Core terms: An allele that is expressed in the phenotype even when only one copy is present.
dominant (rank)Unit 4: The highest-ranking individual within a social group, having priority of access to resources such as food and mates.
dominant alleleUnit 4: An allele that is expressed in the phenotype even when only one copy is present, masking the effect of a recessive allele of the same gene.
dormantUnit 4: Describing a state of greatly reduced metabolic activity in which growth and development are temporarily suspended, often until conditions become favourable.
dorsal rootUnit 5: The branch of a spinal nerve that carries sensory impulses from the body into the dorsal side of the spinal cord.
dorsal root ganglionUnit 5: A swelling on the dorsal root of a spinal nerve that contains the cell bodies of sensory neurones.
doseUnit 4: A measured quantity of a substance such as a drug administered at one time.
double circulationUnit 2: A circulatory system in which the blood passes through the heart twice for each complete circuit of the body, keeping oxygenated and deoxygenated blood separate.
double fertilisationUnit 4: In flowering plants, the process in which one male gamete fuses with the egg cell to form the zygote while a second fuses with the polar nuclei to form the endosperm.
double helixUnit 1: The shape of a DNA molecule, in which two polynucleotide strands are wound around each other in a spiral, held together by hydrogen bonds between complementary bases.
Duchenne muscular dystrophy (DMD)Unit 4: An inherited, sex-linked recessive condition caused by a faulty gene for dystrophin, leading to progressive weakening and wasting of the muscles.
duodenumUnit 2: The first part of the small intestine, immediately after the stomach, where food is mixed with bile and pancreatic secretions to continue digestion.
dura materUnit 5: The tough, fibrous outer layer of the meninges that protects the brain and spinal cord.
dystrophinUnit 4: A muscle protein that helps connect the contractile fibres of a muscle cell to the cell membrane; its absence causes Duchenne muscular dystrophy.

E

EchinodermataUnit 2: A phylum of marine invertebrates characterised by a spiny skin, radial symmetry in the adult and a water vascular system, for example starfish and sea urchins.
ecosystemUnit 3: A self-sustaining unit made up of all the interacting living organisms (the community) together with the non-living components of their environment.
ecotourismCore terms: Responsible travel to natural areas that conserves the environment and improves the wellbeing of local people.
ectoparasiteUnit 2: A parasite that lives on the outer surface of its host, obtaining nutrition at the host’s expense, for example a head louse.
edge effectsUnit 3: The changes in environmental conditions and species composition that occur at the boundary between two different habitats.
effectorUnit 3: A muscle or gland that carries out a response when stimulated by a nerve impulse from the nervous system.
efferent arterioleUnit 3: The small artery that carries blood away from the glomerulus of a kidney nephron after filtration.
egestionUnit 2: The elimination of undigested waste material from the body via the anus; unlike excretion, this material has not been made by the body.
ejaculationCore terms: The release of semen from the male reproductive tract.
elastic fibresUnit 2: Stretchable protein fibres in tissues such as artery and alveolar walls that allow the tissue to recoil to its original shape after being stretched.
elastic recoilUnit 2: The springing back of a stretched elastic tissue, such as the alveoli, to its original size, which helps force air out of the lungs during expiration.
elastic tissueUnit 2: Connective tissue rich in elastic fibres that can stretch under tension and return to its original shape, allowing structures such as arteries and alveoli to expand and recoil.
elbowUnit 4: A hinge joint between the upper and lower arm that allows movement in one plane.
electrical conduction systemUnit 2: The specialised tissue of the heart, including the sino-atrial node, atrio-ventricular node and Purkinje fibres, that initiates and coordinates the wave of excitation controlling heartbeat.
electrocardiogram (ECG)Unit 2: A recording of the electrical activity of the heart over time, used to assess heart rhythm and to detect abnormalities in the cardiac cycle.
electrochemical gradientUnit 3: A combined concentration and electrical gradient of ions across a membrane that drives the movement of those ions, for example protons during chemiosmosis.
electroencephalography (EEG)Unit 4: A technique that records the electrical activity of the brain using electrodes placed on the scalp.
electromyogram (EMG)Unit 4: A recording of the electrical activity produced by muscle fibres, used to study muscle function.
electron acceptorUnit 3: A molecule that receives electrons from another molecule during a redox reaction, becoming reduced in the process.
electron carrierUnit 3: A molecule embedded in a membrane that passes electrons from one carrier to the next along an electron transport chain.
electron micrographUnit 3: An image of a specimen produced using an electron microscope, allowing very small structures to be seen at high resolution.
electron microscopyUnit 1: A technique that uses beams of electrons rather than light to produce images, giving much higher resolution and magnification than light microscopy.
electron transport chainUnit 3: A series of membrane-bound carriers that pass electrons from one to the next, releasing energy used to pump protons and ultimately synthesise ATP.
ELISAUnit 4: An immunological test that uses an enzyme-linked antibody to detect and measure the presence of a specific antigen or antibody in a sample.
elongationCore terms: The stage of protein synthesis or DNA replication during which the growing chain is extended by the addition of further subunits.
embryoUnit 4: The early stage of development of an organism formed after fertilisation, before it becomes recognisable as a young individual.
embryo sacUnit 4: The large cell within the ovule of a flowering plant that contains the egg cell and polar nuclei and in which the embryo develops after fertilisation.
embryonic stem cellUnit 4: An unspecialised cell from an early embryo that is able to divide and differentiate into any of the cell types of the body.
emigrationUnit 3: The movement of individuals out of a population, which tends to decrease its size.
emphysemaUnit 2: A lung disease in which the walls of the alveoli are damaged and break down, reducing surface area for gas exchange and the elastic recoil of the lungs so that less air is forced out.
emulsion testUnit 5: A food test for lipids in which the sample is mixed with ethanol and then water; a cloudy white emulsion forms if a lipid is present.
end-pointUnit 5: The point in a titration or reaction at which it is judged to be complete, usually shown by a colour change.
endangered (species)Unit 3: A species whose numbers have fallen so low that it is at risk of becoming extinct.
endemicUnit 4: Describing a species or disease that is native to, and found only within, a particular geographical region.
endemic speciesUnit 2: A species that is found naturally in only one geographical area or region and nowhere else in the world.
endergonicUnit 1: Describing a reaction that requires an input of energy to proceed because the products contain more energy than the reactants.
endocrine glandUnit 3: A ductless gland that secretes hormones directly into the blood, which then carries them to their target organs.
endocytosisUnit 1: The bulk uptake of material into a cell by the membrane folding inwards to enclose it within a vesicle, requiring energy from ATP.
endodermUnit 2: The innermost germ layer of an embryo, which gives rise to the lining of the gut and associated structures.
endodermisUnit 2: A layer of cells surrounding the vascular tissue in a plant root that, by means of the Casparian strip, controls the movement of water and mineral ions into the xylem.
endometriumUnit 4: The lining of the uterus, which thickens during the menstrual cycle to prepare for implantation of an embryo and is shed if fertilisation does not occur.
endomitosisCore terms: Replication of the chromosomes within a cell without division of the nucleus or cell, increasing the chromosome number.
endoparasiteUnit 2: A parasite that lives inside the body of its host, for example a tapeworm in the gut, obtaining nutrition at the host’s expense.
endopeptidaseCore terms: An enzyme that breaks peptide bonds within the interior of a protein chain.
endoplasmic reticulumCore terms: A network of membranes within the cytoplasm involved in the synthesis and transport of proteins and lipids.
endospermUnit 4: The nutritive tissue formed during double fertilisation in a flowering plant seed that provides a food store for the developing embryo.
endotheliumUnit 2: The single layer of flattened cells that lines the inner surface of blood vessels and the heart.
enduranceUnit 4: The ability of a muscle or the body to sustain physical activity over a prolonged period without fatigue.
energy currencyUnit 3: A term for ATP, reflecting its role as the immediate source of usable energy that powers cellular processes.
energy flowUnit 3: The one-way passage of energy through an ecosystem from sunlight to producers and then along the trophic levels of food chains.
energy profileUnit 3: A diagram showing how the energy of reactants and products changes during a reaction, including the activation energy required.
enucleateUnit 2: Describing a cell that lacks a nucleus, such as a mature mammalian red blood cell.
environmental resistanceUnit 3: The combined effect of limiting factors (such as shortage of food, space or oxygen, and the build-up of waste or disease) that prevents a population from growing to its full biotic potential.
enzymeUnit 1: A biological catalyst, usually a globular protein, that speeds up a specific reaction by lowering its activation energy without being used up.
enzyme-linked antibodyUnit 4: An antibody joined to an enzyme so that, when it binds to its target, the enzyme catalyses a colour change that reveals the presence of that target.
enzyme-substrate complexUnit 1: The temporary structure formed when a substrate binds to the active site of an enzyme, allowing the reaction to be catalysed.
epidemicUnit 4: A significant increase in the usual number of cases of a disease in an area, often associated with a rapid spread.
epidermisUnit 2: The outer protective layer of cells covering a plant or animal, for example the single cell layer covering a leaf, which in plants is usually covered by a waterproof cuticle.
epididymisUnit 4: A coiled tube next to the testis in which sperm are stored and mature.
epigeneticUnit 4: Describing heritable changes in gene expression that do not involve a change in the DNA base sequence, such as those caused by DNA methylation.
epiphyteUnit 3: A plant that grows on the surface of another plant for support but does not take nutrients from it.
epistasisUnit 4: An interaction between genes in which the allele of one gene affects or masks the expression of a different gene.
epitheliumUnit 1: A tissue made of one or more layers of closely packed cells that lines or covers surfaces of the body.
equator (of cell)Unit 1: The central plane of a dividing cell, midway between the two poles, where chromosomes line up during metaphase.
erythrocyteUnit 2: A red blood cell; a biconcave, enucleate cell containing haemoglobin and specialised for the transport of respiratory gases.
ester bondCore terms: The chemical bond formed when an acid group reacts with an alcohol group, as in the linkage between glycerol and fatty acids.
estuaryUnit 3: The region where a river meets the sea and fresh water mixes with salt water.
EubacteriaUnit 2: One of the three domains, comprising the true bacteria; prokaryotic organisms whose ribosomal RNA distinguishes them from the Archaea.
Eukarya (Domain)Unit 2: The domain containing all eukaryotic organisms, that is animals, plants, fungi and protoctists, whose cells have a nucleus and membrane-bound organelles.
eukaryoteCore terms: An organism whose cells contain a true membrane-bound nucleus and membrane-bound organelles.
eukaryoticUnit 1: Describing a cell or organism whose cells contain a true membrane-bound nucleus and membrane-bound organelles.
eukaryotic cellUnit 3: A cell that has a true nucleus enclosed in a membrane and other membrane-bound organelles.
eutrophicationUnit 3: The enrichment of water with nutrients such as nitrates and phosphates, which causes algal blooms that block light; the resulting plant death and microbial decay deoxygenate the water and kill aquatic organisms.
evaporationUnit 1: The change of a liquid into a vapour, which occurs when molecules with enough energy escape from the surface of the liquid.
excretionUnit 3: The removal from the body of the waste products of metabolism, such as carbon dioxide and urea.
exergonicUnit 1: Describing a reaction that releases energy because the products contain less energy than the reactants.
exocytosisUnit 3: The active process by which a cell secretes substances by fusing a membrane-bound vesicle with the cell-surface membrane to release its contents.
exodermisUnit 5: The outermost layer of cells of the root cortex, lying beneath the epidermis, whose walls help control the passage of water and ions.
exonUnit 4: A section of a gene that codes for protein and is retained in the final messenger RNA after splicing.
exon skipping gene therapyUnit 4: A treatment that uses molecules to make the cell’s machinery skip a faulty section of a gene during processing, allowing a shortened but functional protein to be made.
exopeptidaseCore terms: An enzyme that breaks peptide bonds at the ends of a protein chain, removing amino acids one at a time.
expected ratioUnit 4: The proportion of offspring of each type predicted by genetic theory, used as the basis for comparison with observed results in a statistical test.
expected valueUnit 4: In a statistical test such as chi-squared, the result predicted by the null hypothesis, which is compared with the observed value.
expiration (breathing)Unit 2: The phase of breathing in which air is forced out of the lungs, brought about by relaxation of the diaphragm and intercostal muscles and the elastic recoil of the lungs.
exponential growthUnit 3: A pattern of population growth in which numbers increase ever more rapidly because the rate of increase is proportional to the current size of the population.
exposureUnit 4: The extent to which an organism or surface is subjected to an environmental factor such as wave action, wind or a harmful agent.
extensor (muscle)Unit 4: A muscle that contracts to straighten or extend a joint.
extinctUnit 4: Describing a species that no longer has any living members.
extinctionUnit 3: The complete and permanent loss of a species, when its last surviving members die.
extracellular fluidUnit 1: The fluid that surrounds cells and lies outside the cell membrane, such as tissue fluid and plasma.
extremophileUnit 2: An organism, typically an archaean, that lives in environmental conditions that are extreme for life, such as extremes of temperature, pH, salinity or pressure.

F

facilitated diffusionUnit 1: The passive movement of molecules or ions across a membrane down a concentration gradient, through specific carrier or channel proteins, without the use of energy from the cell.
FADUnit 3: A coenzyme that accepts hydrogen atoms and electrons during respiration, becoming reduced to FADH2 and then donating electrons to the electron transport chain.
faecesUnit 2: The undigested waste material, together with bacteria and dead cells, that is removed from the gut by egestion through the anus.
Family (taxonomy)Unit 2: A taxonomic rank in the classification hierarchy, ranking below order and above genus, that groups together related genera.
fast-twitch fibreUnit 4: A type of muscle fibre that contracts rapidly and powerfully but fatigues quickly, relying mainly on anaerobic respiration.
fatigue / tirednessUnit 2: A state of reduced capacity to function, for example of a muscle that can no longer contract effectively after prolonged or repeated activity.
faulty geneUnit 4: An allele containing a mutation that prevents it from producing a fully functional protein, which may lead to a genetic disorder.
feedback loopUnit 3: A control mechanism in which the output of a process feeds back to influence the process itself, either reducing or amplifying the original change.
feral childUnit 4: A child who has grown up with little or no human contact during early development, used to study the role of experience in behaviour and language.
fermenterUnit 3: A large vessel in which microorganisms are grown under controlled conditions to produce a useful product on a commercial scale.
fertilisationUnit 4: The fusion of a male gamete with a female gamete to form a zygote, restoring the diploid number of chromosomes.
fertilisation membraneUnit 4: A barrier that forms around a fertilised egg shortly after a sperm enters, preventing further sperm from fusing with it.
fertiliserUnit 3: A substance added to soil to supply mineral nutrients, such as nitrogen, and increase plant growth and crop yield.
fertilityUnit 4: The capacity of an organism to produce offspring.
fibrinUnit 1: An insoluble fibrous protein formed from fibrinogen that creates a mesh of fibres to trap blood cells and form a clot.
fibrinogenUnit 1: A soluble plasma protein that is converted into insoluble fibrin during the clotting of blood.
fibrosisCore terms: The formation of excess fibrous connective tissue in an organ, often as a result of damage or disease.
fibrous proteinUnit 1: A protein with long, parallel polypeptide chains that is insoluble and has a structural role, such as collagen or keratin.
field studyUnit 3: An investigation carried out in the natural environment rather than in a laboratory.
field trialUnit 4: A controlled experiment carried out under natural conditions, for example to test the performance or safety of a new crop variety.
fight-or-flight responseUnit 4: The body’s rapid physiological reaction to a perceived threat, coordinated mainly by adrenaline and the sympathetic nervous system, preparing it for vigorous action.
filtrateUnit 3: The fluid that has passed through a filter or filtering surface, such as the liquid forced out of the blood into the Bowman’s capsule.
filtrationCore terms: The separation of suspended solids from a fluid by passing it through a barrier that holds back particles above a certain size.
final electron acceptorUnit 3: The last molecule to receive electrons at the end of an electron transport chain; in aerobic respiration this is oxygen.
final hydrogen acceptorUnit 3: The last molecule to receive hydrogen at the end of a metabolic pathway; in aerobic respiration this is oxygen, forming water.
first degree heart blockUnit 2: A condition in which conduction of the electrical impulse from the atria to the ventricles is delayed, lengthening the interval between atrial and ventricular contraction.
fish farmingUnit 3: The controlled rearing of fish in enclosures to provide a managed source of food.
fixed action patternUnit 4: An innate, stereotyped sequence of behaviour that, once triggered by a specific stimulus, is carried through to completion without further input.
flaccidUnit 2: Describing a plant cell that has lost water so that the cytoplasm and vacuole no longer press the cell membrane against the cell wall, leaving the cell limp.
flagellumUnit 1: A long, whip-like structure projecting from some cells that beats to bring about movement.
flexor (muscle)Unit 4: A muscle that contracts to bend or flex a joint.
floretUnit 4: One of the small individual flowers that together make up a composite flower head.
flotationCore terms: A method of separating materials based on differences in their tendency to float in a liquid.
fluid mosaic modelUnit 1: The model of the cell-surface membrane as a phospholipid bilayer within which proteins are embedded and free to move, giving a fluid, mosaic-like arrangement.
food chainUnit 3: A sequence showing the transfer of energy from one organism to the next as each is eaten by the one above it in the chain.
food webUnit 3: A set of interconnected food chains showing the many feeding relationships within a community.
forced expirationUnit 2: The active expulsion of air from the lungs using muscular effort, beyond the volume removed during normal quiet breathing.
forced inspirationUnit 2: The taking in of air to the lungs using additional muscular effort, beyond the volume drawn in during normal quiet breathing.
forced vital capacityUnit 2: The maximum volume of air that can be forced out of the lungs in one breath following the deepest possible inhalation.
fossilUnit 4: The preserved remains or traces of an organism from the geological past, providing evidence about earlier life and evolution.
fossil fuelsUnit 3: Fuels such as coal, oil and natural gas formed over millions of years from the remains of dead organisms.
founder effectUnit 4: The reduced genetic variation that results when a new population is established from a small number of individuals separated from a larger original population.
fracture (bone)Unit 4: A break or crack in a bone.
frontal lobeUnit 4: The region at the front of each cerebral hemisphere of the brain, associated with planning, decision-making and the control of voluntary movement.
fructolysisUnit 3: The breakdown of fructose to provide energy, for example as a respiratory substrate for sperm cells.
fructoseUnit 1: A monosaccharide containing six carbon atoms (a hexose), found in fruit and combining with glucose to form sucrose.
fruitUnit 4: The structure that develops from the ovary of a flowering plant after fertilisation, containing and protecting the seed or seeds.
FSHUnit 4: A hormone secreted by the anterior pituitary gland that stimulates the maturation of a follicle in the ovary and the production of oestrogen.
functional geneUnit 4: A gene carrying a correct DNA sequence that codes for a working protein, as opposed to a defective allele that causes disease.
fungal sporeUnit 2: A small reproductive unit produced by a fungus that can disperse and germinate to form a new individual.
fungiUnit 2: A kingdom of heterotrophic eukaryotes with cell walls made of chitin, most of which are made of thread-like hyphae and reproduce by spores.

G

galactoseUnit 1: A monosaccharide containing six carbon atoms (a hexose) that combines with glucose to form the disaccharide lactose.
game reserveUnit 3: A protected area of land where wild animals are conserved and may not be hunted.
gameteUnit 4: A specialised haploid sex cell that fuses with another gamete during fertilisation to form a diploid zygote.
ganglionUnit 3: A swelling in the nervous system containing a cluster of nerve cell bodies.
gas exchangeUnit 1: The movement of respiratory gases between an organism and its environment across a surface, with oxygen taken up and carbon dioxide removed.
gastric glandUnit 2: A gland in the lining of the stomach that secretes substances such as hydrochloric acid and the inactive enzyme pepsinogen.
gastric pitUnit 2: A narrow depression in the lining of the stomach into which the gastric glands open and release their secretions.
gel electrophoresisUnit 4: A technique that separates fragments of DNA, RNA or protein by size by applying an electric field across a gel, with smaller fragments moving further.
geneUnit 2: A length of DNA on a chromosome that codes for a specific polypeptide.
gene editingUnit 4: A technique that makes precise additions, deletions or changes to specific sequences within an organism’s DNA.
gene expressionUnit 4: The process by which the information in a gene is transcribed and translated to produce a protein, and which can be switched on or off.
gene flowUnit 4: The transfer of alleles from one population to another through the movement and interbreeding of individuals.
gene mutationUnit 4: A spontaneous, random change in the base sequence of the DNA of a gene, providing a source of genetic variation.
gene poolUnit 4: All of the alleles of all the genes present in the individuals of a population at a given time.
gene therapyUnit 4: Treating a genetic disorder by introducing functional DNA sequences into a patient’s cells to counteract the effect of a defective allele.
general formulaUnit 1: A formula that shows the ratio of elements common to all members of a group of compounds, such as (CH2O)n for carbohydrates.
generation timeUnit 3: The time taken for a population of microorganisms to double in number.
genetic codeUnit 1: The sequence of bases in DNA and mRNA that specifies the order of amino acids in a polypeptide, read in non-overlapping triplets.
genetic disorderUnit 4: An inherited condition caused by a faulty allele or chromosome abnormality in an individual’s DNA.
genetic driftUnit 4: The change in allele frequencies of a population between generations due to chance, having its greatest effect in small populations.
genetic engineeringUnit 3: The deliberate alteration of an organism’s genetic material by adding, removing or changing genes, often to transfer a gene from one species to another.
genetic materialUnit 1: The DNA (or in some viruses RNA) of a cell or organism, which carries the inherited coded instructions.
genetic modificationUnit 4: The deliberate alteration of an organism’s genome by inserting, removing or changing genes, often using genes from another species.
genetic polymorphismUnit 2: The existence of two or more distinct inherited forms (phenotypes) of a species within a population at frequencies too high to be explained by recurrent mutation alone.
genetically identicalUnit 4: Describing cells or organisms that share exactly the same DNA sequence, such as those produced by mitosis or asexual reproduction.
genomeUnit 4: The complete set of genetic material, or all of the DNA, of an organism.
genotypeUnit 4: The alleles an organism carries for a particular characteristic, or its full set of alleles.
genusUnit 2: A taxonomic group containing closely related species that share many similar features but are usually unable to interbreed to produce fertile offspring.
geographical isolationUnit 4: The separation of populations of a species by a physical barrier that prevents them from interbreeding.
germ-line gene therapyUnit 4: The introduction of functional DNA into gametes or an early embryo, so that the change is heritable and passed to future generations.
germinal epitheliumUnit 5: The layer of cells lining the seminiferous tubules of the testis, or covering the ovary, from which the gametes are produced.
germinationUnit 4: The process by which a seed begins to grow into a seedling when supplied with suitable conditions of water, oxygen and warmth.
gestation periodUnit 4: The length of time an embryo develops in the uterus, from fertilisation until birth.
gibberellic acidUnit 4: A specific gibberellin that acts as a plant growth substance, promoting seed germination and stem elongation.
gibberellinUnit 4: A plant growth substance that, among other effects, stimulates seed germination and stem elongation.
gill plate / lamellaUnit 2: One of the thin, plate-like folds projecting from a gill filament that greatly increase the surface area available for gas exchange in fish.
gill rakerUnit 2: A comb-like projection attached to the gill arch that filters solid particles from the water, preventing damage to the delicate gill filaments.
gill slitsUnit 2: The openings or spaces between the gills through which water leaves after passing over the gas-exchange surface.
gillsUnit 1: Respiratory organs of aquatic animals, with a large, thin surface area through which gases are exchanged with the surrounding water.
gizzardUnit 5: A muscular part of the gut in some animals that grinds and breaks up food, often with the aid of swallowed grit.
gland cellUnit 2: A specialised cell that synthesises and secretes a substance such as mucus, an enzyme or a hormone.
global warmingUnit 3: The gradual rise in the average temperature of the Earth’s surface and atmosphere, largely caused by increased concentrations of greenhouse gases from human activity.
globular proteinUnit 1: A protein whose polypeptide chain is folded into a compact, roughly spherical shape, making it soluble and able to carry out metabolic roles such as enzymes.
glomerular filtrateUnit 3: The fluid forced out of the blood in the glomerulus into the Bowman’s capsule during ultrafiltration.
glomerulusUnit 3: A knot of capillaries enclosed within the Bowman’s capsule of a nephron, where blood is filtered under pressure.
glucoseUnit 1: A monosaccharide containing six carbon atoms (a hexose) that is the main respiratory substrate and the monomer of many polysaccharides.
glucose monomerUnit 1: A single glucose molecule, the monosaccharide subunit from which polysaccharides such as starch, glycogen and cellulose are built.
glycerate-3-phosphate (GP)Unit 3: A three-carbon compound formed when carbon dioxide combines with ribulose bisphosphate in the Calvin cycle, and which is then reduced to triose phosphate.
glycerolUnit 1: A three-carbon alcohol that combines with fatty acids to form lipids such as triglycerides and phospholipids.
glycocalyxUnit 1: The layer of glycoproteins and glycolipids on the outer surface of the cell membrane, involved in cell recognition and adhesion.
glycogenUnit 3: A branched polysaccharide of glucose that acts as the main carbohydrate energy store in animal cells.
glycolipidUnit 1: A lipid with a carbohydrate chain attached, found in the cell membrane where it acts in cell recognition.
glycolysisCore terms: The first stage of respiration, occurring in the cytoplasm, in which glucose is split into pyruvate with a small net yield of ATP.
glycoproteinUnit 1: A protein with a carbohydrate chain attached, found in the cell membrane where it acts in cell recognition and as a receptor.
glycosidicUnit 1: Describing the covalent bond that joins two monosaccharide units together, formed by a condensation reaction.
glycosidic bondCore terms: The chemical bond that joins two sugar molecules together, formed by a condensation reaction.
glycosylationCore terms: The addition of one or more sugar groups to a protein or lipid molecule.
goblet cellUnit 2: A cell in an epithelium, such as that lining the airways or gut, that secretes mucus.
GolgiUnit 1: An organelle made of stacked, flattened membrane sacs that modifies, packages and secretes proteins and other molecules.
gonorrhoeaUnit 3: A sexually transmitted infection caused by bacteria.
gradient (of a graph)Unit 3: A measure of the steepness of a line on a graph, found by dividing the change in the vertical value by the change in the horizontal value.
Gram stainUnit 3: A staining technique that classifies bacteria as Gram-positive or Gram-negative according to the structure of their cell wall, based on whether they retain a violet dye after washing.
Gram-negativeUnit 3: Describing bacteria that do not retain the purple Gram stain because of their thin cell wall and outer membrane, appearing red after counterstaining.
Gram-positiveUnit 4: Describing bacteria that retain a purple stain in the Gram test because they have a thick peptidoglycan cell wall.
grazerUnit 2: A herbivore that feeds on low-growing vegetation such as grasses.
grey matterUnit 5: The inner region of the spinal cord, and parts of the brain, made up largely of neurone cell bodies and unmyelinated fibres.
groomingUnit 4: An innate social behaviour in which animals clean themselves or others, helping to maintain hygiene and reinforce social bonds.
growth curveUnit 3: A graph showing how the number of organisms in a population changes over time, typically with lag, exponential and stationary phases.
guanineUnit 1: A nitrogenous base found in DNA and RNA that pairs with cytosine through three hydrogen bonds.
guard cellCore terms: One of a pair of specialised cells flanking a stoma in a leaf or stem epidermis whose changes in turgor open or close the pore, controlling gas exchange and water loss.
guide RNAUnit 4: A short RNA molecule that directs an editing enzyme to a specific complementary sequence in the DNA during gene editing.
gut / intestineUnit 2: The tubular part of the digestive system through which food passes and in which digestion and absorption take place.

H

habitat fragmentationUnit 3: The breaking up of a continuous habitat into smaller, separated patches, reducing the area available to species.
habitatsUnit 2: The places where particular organisms live, providing the conditions and resources they need to survive.
habituationUnit 4: A simple form of learned behaviour in which an animal stops responding to a repeated stimulus that brings neither reward nor punishment, saving energy.
haemocyaninUnit 2: A blue, copper-containing respiratory pigment found in the blood of some invertebrates that transports oxygen, performing a role similar to haemoglobin.
haemodialysisUnit 3: A treatment for kidney failure in which a patient’s blood is passed through a machine that removes waste products and excess substances across a partially permeable membrane.
haemoglobinUnit 2: A red, iron-containing globular protein found in red blood cells that binds reversibly with oxygen to transport it around the body.
haemophiliaUnit 5: An inherited, sex-linked disorder in which the blood is unable to clot normally because a clotting factor is missing or defective.
half-lifeUnit 4: The time taken for half of the radioactive atoms in a sample, or half of the quantity of a substance, to decay or be removed.
haploid numberUnit 1: The number of chromosomes present in a single set, as found in a gamete, conventionally written as n.
hazardUnit 5: Something with the potential to cause harm or injury during an activity or investigation.
heart blockUnit 2: A condition in which the conduction of electrical impulses through the heart is partially or completely interrupted, disrupting the coordination of contraction.
heart rateUnit 4: The number of times the heart beats per unit of time, usually expressed per minute.
heavy isotopeUnit 1: A form of an element with extra neutrons, making its atoms denser; used to label molecules so they can be tracked, as in studies of DNA replication.
helicalUnit 1: Having the shape of a spiral or helix, as in the coiled structure of a DNA strand or an alpha-helix in a protein.
herbivoreUnit 2: An animal that feeds only on plant material.
hermaphroditeUnit 2: An organism that possesses both male and female reproductive organs, as in the tapeworm, allowing self-fertilisation.
heterotrophicUnit 5: Describing an organism that obtains its complex organic molecules by consuming other organisms or organic material, as it cannot synthesise them from inorganic sources.
heterotrophic organismUnit 2: An organism that cannot make its own organic food and must obtain ready-made organic molecules by consuming other organisms.
heterozygousUnit 3: Having two different alleles of a particular gene at the same point on a pair of homologous chromosomes.
hexaploidCore terms: Describing a cell or organism whose nuclei contain six complete sets of chromosomes.
hexose sugarUnit 3: A monosaccharide containing six carbon atoms, such as glucose or fructose.
hilum (kidney)Unit 5: The indented region on the inner edge of the kidney where the renal artery, renal vein and ureter join the organ.
hippocampusUnit 4: A region of the brain involved in the formation of memories and in spatial learning.
histone acetylationUnit 4: The addition of acetyl groups to histone proteins, which loosens the coiling of DNA and tends to increase gene expression.
histone modificationUnit 4: Chemical changes to histone proteins that alter how tightly DNA is coiled and therefore whether genes can be expressed.
histonesUnit 1: Proteins around which DNA is wound and coiled to package it into chromosomes in eukaryotic cells.
holozoicUnit 2: A type of heterotrophic nutrition in which an organism ingests food, then digests it internally before absorbing the products, as seen in most animals.
homeostasisUnit 3: The maintenance of a constant internal environment within narrow limits, keeping conditions in a state of dynamic equilibrium despite changes outside the organism.
homologousUnit 2: Describing structures or chromosomes that share the same fundamental origin; homologous chromosomes carry the same genes in the same order, one from each parent.
homozygous genotypeUnit 4: A genotype in which the two alleles of a gene are identical.
homunculusUnit 4: A distorted map of the body represented on the motor cortex, in which the area devoted to each body part is proportional to the degree of fine motor control it requires.
hormoneUnit 4: A chemical messenger produced by an endocrine gland and carried in the blood to target cells or organs elsewhere in the body, where it brings about a response.
horny padUnit 2: A tough, hardened pad on the upper jaw of a herbivore, such as a ruminant, against which the lower teeth cut and grind plant material.
host (organism)Unit 2: An organism in or on which a parasite lives and from which it obtains its nutrition at the host’s expense.
host cellUnit 1: A cell that is invaded by a virus or other pathogen and used to make new copies of it.
human chorionic gonadotrophin (hCG)Unit 4: A hormone produced by the early embryo that maintains the corpus luteum so it continues to secrete progesterone during early pregnancy.
Human Genome ProjectUnit 4: An international scientific project that determined the complete base sequence of the human genome.
human growth hormone (HGH)Unit 4: A hormone secreted by the anterior pituitary gland that promotes growth of tissues, and which can be produced by genetic engineering.
humoral responseUnit 4: An immune response in which B lymphocytes produce and secrete antigen-specific antibodies into the blood and tissue fluid.
hyaline cartilageUnit 4: A firm but flexible connective tissue that covers the ends of bones at joints and reduces friction.
hybridUnit 4: The offspring produced by a cross between two genetically different parents, often of different varieties or species.
hybrid DNAUnit 2: A double-stranded molecule formed when single DNA strands from two different species pair together; the strength of pairing indicates how closely related the species are.
hydrogen acceptorUnit 3: A molecule that receives hydrogen atoms during a redox reaction, becoming reduced.
hydrogen bondUnit 4: A weak attractive force between a slightly positively charged hydrogen atom in one molecule and a slightly negatively charged atom, such as oxygen or nitrogen, in another.
hydrolysisUnit 1: The breakdown of a larger molecule into smaller ones by the addition of water, used to split bonds during digestion.
hydrophilicUnit 1: Describing a molecule or part of a molecule that is attracted to water and readily dissolves in or associates with it.
hydrophobicUnit 1: Describing a molecule or part of a molecule that is repelled by water and tends not to dissolve in or mix with it.
hydrophyteUnit 2: A plant that is adapted to live in water or in very wet conditions, for example the water lily.
hydrostatic pressureUnit 2: The pressure exerted by a fluid, such as blood in a capillary, that tends to push fluid out through the vessel wall.
hydroxyl groupUnit 1: A functional group consisting of an oxygen atom bonded to a hydrogen atom (-OH), which is polar and makes molecules more soluble.
hyperpolarisationUnit 3: A change that makes the inside of a neurone membrane more negative than the normal resting potential.
hypertonicUnit 1: Describing a solution with a lower water potential (more solutes) than the cell or solution it is compared with, so water moves out of the cell.
hyphaUnit 2: One of the thread-like filaments that make up the body of most fungi.
hypothalamusUnit 4: A region of the brain that links the nervous and endocrine systems and helps regulate functions such as body temperature and water balance.
hypothesisUnit 1: A proposed, testable explanation for an observation, which can be supported or rejected by experiment.
hypotonicUnit 1: Describing a solution with a higher water potential (fewer solutes) than the cell or solution it is compared with, so water moves into the cell.

I

IgGUnit 4: A class of antibody that is the most abundant in blood and provides long-lasting immunity, and which can cross the placenta.
IgMUnit 4: A class of antibody produced first in an immune response, with several binding sites making it effective at clumping pathogens.
ileumUnit 5: The final part of the small intestine, where the digestion of food is completed and most absorption of the products takes place.
imbibition (imbibed)Unit 4: The uptake and absorption of water by a seed, which activates metabolism and triggers germination.
immigrationUnit 3: The movement of individuals into a population from elsewhere, tending to increase its size.
immobilisationCore terms: The fixing or trapping of enzymes onto or within an inert support so that they remain in place and can be reused while their substrate flows past.
immobilised enzymeUnit 1: An enzyme that has been fixed to an inert support or trapped in a matrix, so that it can be reused and its stability is improved.
immune deficiencyUnit 2: A condition in which the immune system is unable to respond effectively to pathogens, leaving the organism more vulnerable to infection.
immune systemUnit 1: The cells, tissues and molecules of the body that work together to defend it against pathogens.
immunisationUnit 4: The process of giving a person protection against a disease, typically by vaccination, so they develop immunity without infection.
immunityUnit 4: The ability of the body to resist infection by a pathogen, acquired actively or passively through the action of antibodies and lymphocytes.
immunodeficiency disorderUnit 4: A condition in which part of the immune system fails to work properly, leaving the individual less able to resist infection.
immunogenicUnit 4: Describing an antigen or vaccine that produces a strong immune response.
implantationUnit 4: The sinking of the developing blastocyst into the lining of the uterus (endometrium).
imprintingUnit 5: A form of learned behaviour in which a young animal forms a strong attachment to the first large moving object it sees, usually a parent, during a sensitive early period.
in vitro fertilisation (IVF)Unit 4: A fertility treatment in which egg cells are fertilised by sperm outside the body before an embryo is placed in the uterus.
inactivationUnit 1: The loss of an enzyme’s catalytic activity, for example when its active site is permanently altered by extreme heat or pH.
incidenceUnit 4: The number of new cases of a disease arising in a population over a given period of time.
incipient plasmolysisUnit 1: The point at which a plant cell’s protoplast is just beginning to pull away from the cell wall, where the pressure potential has fallen to zero.
incisorUnit 2: A chisel-shaped tooth at the front of the jaw used for biting and cutting food.
independent assortmentUnit 1: The random orientation and separation of the maternal and paternal homologous chromosomes at metaphase and anaphase of meiosis I, generating genetic variation in the gametes.
independent variableUnit 2: The variable that is deliberately changed by the investigator in an experiment in order to observe its effect on the dependent variable.
induced fitUnit 1: The change in the shape of an enzyme’s active site as the substrate enters and binds, so that the active site moulds closely around the substrate.
infectionUnit 4: The invasion and multiplication of a pathogen within a host, which may lead to disease.
infectious diseaseUnit 1: A disease caused by a pathogen that can be passed from one organism to another.
infertilityUnit 4: The inability of an individual to reproduce naturally.
inflammationCore terms: A localised protective response to injury or infection, marked by redness, heat, swelling and pain as blood flow and white cells increase at the site.
ingestionUnit 2: The taking of food into the body, for example through the mouth.
inhibitorCore terms: A molecule that reduces or stops the activity of an enzyme by interfering with its active site or function.
initiation codonUnit 1: The first codon of an mRNA molecule to be translated, which signals the start of protein synthesis and codes for the first amino acid.
innate behaviourUnit 4: Inherited behaviour that an animal performs without learning, such as reflexes, kineses and taxes.
inorganic compoundsUnit 3: Chemical compounds that do not contain carbon-hydrogen bonds, such as water and mineral ions.
inorganic ionUnit 1: A charged particle that does not contain carbon-hydrogen bonds and is needed by organisms for many functions, such as sodium, calcium or nitrate ions.
insectivorous plantUnit 5: A plant that traps and digests insects and other small animals to obtain nutrients, typically nitrogen, often as an adaptation to nutrient-poor soils.
insolubleUnit 1: Describing a substance that does not dissolve in a particular solvent.
inspiration (breathing)Unit 2: The phase of breathing in which air is drawn into the lungs as the external intercostal muscles and diaphragm contract, increasing the volume and lowering the pressure inside the thorax.
inspiratory capacityUnit 2: The maximum volume of air that can be breathed in following a normal, resting exhalation.
instinctive behaviourUnit 4: Inherited behaviour shown by an animal without prior learning or experience.
intensive agricultureUnit 3: A method of farming that maximises yield from a given area of land using high inputs such as fertilisers, pesticides and machinery.
intercostal muscleUnit 2: One of the muscles lying between the ribs whose contraction and relaxation moves the ribcage to bring about ventilation of the lungs.
intermembrane spaceUnit 3: The compartment between the inner and outer membranes of a mitochondrion, where protons accumulate during chemiosmosis.
internal diameterUnit 2: The width of the space (lumen) enclosed by a tubular structure such as a blood vessel, measured across the inside of its walls.
internodeUnit 3: The region of a plant stem between two successive nodes where leaves are attached.
interphaseUnit 1: The stage of the cell cycle between divisions, during which the cell grows, carries out its functions and replicates its DNA.
interspecific competitionUnit 3: Competition between members of different species for the same limited resource.
intracellular enzymeUnit 1: An enzyme that catalyses reactions inside the cell that made it.
intrasexual selectionUnit 4: A form of sexual selection in which members of one sex compete with one another for access to mates.
intraspecific competitionUnit 3: Competition between members of the same species for the same limited resource.
intrinsic proteinUnit 1: A protein embedded within and spanning the phospholipid bilayer of the cell membrane, often acting as a channel or carrier.
introduced speciesUnit 3: A species that has been brought into an area where it does not naturally occur, often through human activity.
intronUnit 4: A section of a gene that does not code for protein and is removed from the messenger RNA before it is translated.
invasive speciesUnit 3: An introduced species that spreads rapidly and harms native species or the ecosystem.
inversion (mutation)Unit 4: A chromosome mutation in which a segment of a chromosome is reversed end to end.
ionUnit 1: A charged particle formed when an atom or group of atoms gains or loses one or more electrons.
irritationUnit 5: A hazard category describing a substance that can cause inflammation or discomfort on contact with the skin, eyes or other tissues.
isotonicUnit 1: Describing two solutions that have the same water potential, so there is no net movement of water between them.

J

jawboneUnit 5: A bone of the jaw that bears the teeth and, in the lower jaw, allows the chewing of food.

K

kick samplingUnit 5: A technique for sampling invertebrates in flowing water in which the substrate is disturbed by foot and the dislodged organisms are swept into a net held downstream.
kidney failureUnit 3: A condition in which the kidneys can no longer filter the blood effectively and remove waste products.
kinesisUnit 4: A non-directional change in the rate of random movement of an organism in response to a stimulus, increasing the chance of reaching more favourable conditions.
kinetic energyUnit 1: The energy that a particle or object possesses because of its movement.
KingdomUnit 2: One of the broadest taxonomic groups used to classify living organisms, ranking just below domain and containing related phyla.
kite diagramUnit 5: A chart that displays the abundance or percentage cover of species along a transect, with each species shown as a symmetrical band whose width represents its abundance at each point.
klinokinesisCore terms: A non-directional response in which an organism changes its rate of turning according to the intensity of a stimulus, so that it spends longer in favourable conditions.
knee jointUnit 4: A hinge synovial joint between the femur and tibia that allows flexion and extension of the leg.

L

lactaseUnit 1: An enzyme that hydrolyses the disaccharide lactose into glucose and galactose.
lactealUnit 2: A blind-ended lymph vessel found at the centre of a villus that absorbs the products of fat digestion from the small intestine.
lactic acid / lactateUnit 4: A three-carbon compound produced from pyruvate in animal cells during anaerobic respiration, regenerating NAD so that glycolysis can continue.
lactoseUnit 1: A disaccharide made of glucose and galactose joined by a glycosidic bond, found in milk.
lactose intolerant / lactose intoleranceUnit 1: The inability to digest lactose, caused by a lack of the enzyme lactase, leading to digestive symptoms when milk is consumed.
lag phaseUnit 3: The initial period of slow growth in a culture of micro-organisms, during which the cells are adjusting to their new conditions and synthesising the enzymes needed before they begin to reproduce rapidly.
laminaUnit 4: The broad, flattened blade of a leaf, providing a large surface area for the absorption of light and gas exchange.
lamina (leaf)Unit 2: The flat, broad blade of a leaf, providing a large surface area for the absorption of light and the exchange of gases.
lamina propriaUnit 5: A thin layer of loose connective tissue lying beneath an epithelium, forming part of the mucosa of organs such as the gut and trachea.
language processingUnit 4: The function of specific areas of the cerebrum involved in the comprehension and production of speech and written language.
large intestineUnit 2: The final region of the gut, where most of the remaining water is reabsorbed from the undigested material before it is egested.
larynxUnit 2: The region at the top of the trachea, containing the vocal cords, through which air passes during ventilation.
last universal common ancestor (LUCA)Unit 2: The most recent population of organisms from which all living things on Earth are descended.
latent learningUnit 4: A form of learning in which an animal acquires information about its surroundings without any immediate reward, using it later when it becomes useful.
latent periodUnit 4: The short delay following exposure to an antigen before a detectable immune response develops; on a second exposure this period is much shorter due to memory cells.
leachingUnit 3: The washing of soluble minerals, such as nitrate ions, downwards out of the soil by water draining through it, reducing the nutrients available to plants.
leaf adaptationUnit 2: A structural or physiological feature of a leaf that improves its efficiency at carrying out photosynthesis or gas exchange, or at conserving water.
left ventricleUnit 2: The lower left chamber of the heart, which has a thick muscular wall and pumps oxygenated blood out through the aorta to the rest of the body.
leghaemoglobinUnit 3: An oxygen-binding pigment found in the root nodules of leguminous plants that removes free oxygen, maintaining the low-oxygen conditions that nitrogen-fixing bacteria need to fix nitrogen.
length-tension curveUnit 4: A graph showing how the force a muscle can generate depends on the degree of overlap between actin and myosin filaments at different muscle lengths.
level of significanceUnit 5: The probability threshold, typically 0.05, used in a statistical test to decide whether a result is significant; below it the null hypothesis is rejected.
leverUnit 4: A rigid structure, such as a bone, that turns about a fixed point (the joint) to transmit force and produce movement.
LHUnit 4: Luteinising hormone; a pituitary hormone that triggers ovulation and stimulates the formation of the corpus luteum in the ovary.
life cycleUnit 2: The sequence of developmental stages an organism passes through from its formation until it produces the next generation.
ligamentUnit 4: A band of tough, slightly elastic connective tissue that joins bone to bone and stabilises a joint.
ligaseUnit 4: An enzyme that joins together two sections of DNA by catalysing the formation of phosphodiester bonds between adjacent nucleotides.
light compensation pointUnit 3: The light intensity at which the rate of photosynthesis exactly equals the rate of respiration in a plant, so there is no net exchange of carbon dioxide or oxygen.
light intensityUnit 2: A measure of the amount of light energy falling on a given area, which acts as a limiting factor for the rate of photosynthesis.
light-dependent reactionUnit 3: The first stage of photosynthesis, occurring in the thylakoid membranes, in which light energy is absorbed by chlorophyll and used to produce ATP and reduced NADP and to split water, releasing oxygen.
light-independent reactionUnit 3: The stage of photosynthesis in the stroma of the chloroplast in which carbon dioxide is fixed and reduced to carbohydrate using the ATP and reduced NADP made in the light-dependent reaction; also called the Calvin cycle.
ligninUnit 2: A tough, waterproof material deposited in the walls of xylem and sclerenchyma cells, providing support and strength.
limiting factorUnit 1: A factor whose level controls the rate of a process such as photosynthesis when it is in shortest supply, holding back the rate even when other factors are favourable.
line of best fitUnit 3: A straight or smooth curved line drawn through plotted points on a graph so that it best represents the overall trend, with points scattered evenly on either side.
line of dehiscenceUnit 5: A line of weakness along which a structure such as a seed pod or anther splits open to release its contents.
line transectUnit 3: A sampling method in which a measuring tape is laid across an area and the organisms touching or crossing the line are recorded at set intervals to study the distribution of species along an environmental gradient.
linear DNAUnit 1: DNA arranged as a straight molecule with two ends, characteristic of the chromosomes of eukaryotic cells.
linear scaleUnit 5: A scale on which equal divisions represent equal increments, so that values are spaced evenly along its length.
link reactionUnit 3: The stage of aerobic respiration in the mitochondrial matrix in which pyruvate is decarboxylated and dehydrogenated and combined with coenzyme A to form acetyl coenzyme A, releasing carbon dioxide and reduced NAD.
linkage (genetic)Unit 5: The tendency of genes located close together on the same chromosome to be inherited together rather than assorting independently.
lipaseUnit 2: An enzyme that catalyses the hydrolysis of triglycerides (fats) into fatty acids and glycerol.
lipidCore terms: A non-polar biological molecule, insoluble in water but soluble in organic solvents, including triglycerides and phospholipids, used for energy storage, membranes and insulation.
lipid dropletUnit 1: A store of lipid held within the cytoplasm of a cell.
lipopolysaccharideUnit 4: A molecule of lipid joined to a polysaccharide that forms the outer membrane of Gram-negative bacterial cell walls.
liposomeUnit 4: A tiny spherical vesicle bounded by a phospholipid bilayer, used to enclose and deliver substances such as drugs into cells.
living fossilUnit 2: A living species that closely resembles organisms known only from fossils and which has changed little over a very long period of time.
load (lever)Unit 4: The resistance or weight that a lever acts against when force is applied to produce movement.
lobuleUnit 1: One of the many small functional units that make up the liver.
local anaestheticUnit 3: A drug that blocks the transmission of nerve impulses in a restricted region of the body, preventing pain sensations from reaching the brain without causing loss of consciousness.
local currentUnit 3: The small flow of ions along a neurone membrane between a depolarised region and the adjacent resting region, which causes the next part of the membrane to reach threshold and so propagates the action potential.
lock and key theoryUnit 1: A model of enzyme action in which the substrate has a shape complementary to the rigid active site, so that only that specific substrate fits to form an enzyme-substrate complex.
log phaseUnit 3: The phase of microbial population growth during which numbers increase exponentially because nutrients are plentiful and few factors are limiting reproduction; also called the exponential phase.
long-term memoryUnit 4: The lasting store of information in the brain, formed when short-term memories are consolidated and retained over extended periods.
longitudinal muscleUnit 5: A layer of muscle whose fibres run lengthways along a structure such as the gut wall, shortening it when they contract.
longitudinal sectionUnit 5: A cut made along the long axis of a structure to reveal its internal features in that plane.
loop of DNAUnit 1: A circular molecule of DNA with no free ends, characteristic of prokaryotic cells and of mitochondria and chloroplasts.
loop of HenleUnit 3: The long hairpin-shaped section of the kidney nephron, dipping into the medulla, that sets up a concentration gradient of ions in the surrounding tissue fluid allowing water to be reabsorbed and concentrated urine to be produced.
low-density lipoprotein (LDL)Unit 1: A lipid-protein complex that transports cholesterol in the blood from the liver to the body’s cells; high blood levels are associated with deposition of cholesterol in artery walls.
low-power planUnit 5: A simple outline drawing made under low magnification showing the position and boundaries of the tissues in a specimen, without individual cells.
lowland heathUnit 2: A low-lying habitat characterised by acidic, nutrient-poor soils dominated by dwarf shrubs such as heather.
lumbar (region)Unit 4: The lower region of the vertebral column, between the thoracic vertebrae and the sacrum.
lumenUnit 2: The internal space enclosed within a tubular or hollow structure, such as a blood vessel or the gut.
lymph vesselUnit 2: A thin-walled vessel that drains excess tissue fluid (lymph) from the tissues and returns it to the blood circulation.
lymphocyteUnit 1: A type of white blood cell involved in the specific immune response, including B-lymphocytes which produce antibodies and T-lymphocytes which co-ordinate and carry out cell-mediated responses.
lysineUnit 1: A basic amino acid, one of the twenty amino acids used to build proteins.
lysosomeCore terms: A membrane-bound organelle containing digestive (hydrolytic) enzymes that breaks down worn-out organelles, ingested material or whole cells.
lysozymeUnit 1: An enzyme found in secretions such as tears, saliva and mucus that destroys bacteria by breaking down the peptidoglycan in their cell walls.

M

macrocyteUnit 2: An abnormally large red blood cell.
macrocytic anaemiaUnit 2: A form of anaemia in which the red blood cells produced are abnormally large and reduced in number, lowering the blood’s capacity to carry oxygen.
macrophageUnit 4: A large phagocytic white blood cell that engulfs and digests pathogens and cell debris and presents antigens to lymphocytes.
magnificationUnit 1: The number of times larger an image of an object appears compared with the actual size of the object.
malariaUnit 4: A disease caused by the protoctistan parasite Plasmodium, transmitted by the bite of an infected female Anopheles mosquito.
malateUnit 2: A negatively charged organic ion which, together with potassium ions, accumulates in guard cells to lower their water potential, drawing in water so that the cells become turgid and the stoma opens.
male reproductive systemUnit 4: The group of organs that produce, store and deliver sperm, together with the hormones that control these functions.
maltaseCore terms: An enzyme that hydrolyses the disaccharide maltose into two glucose molecules.
maltoseUnit 1: A disaccharide made of two glucose units joined by a glycosidic bond, produced by the digestion of starch.
mammary glandUnit 1: A gland in female mammals that produces and secretes milk to nourish offspring.
manureUnit 2: Animal waste added to soil as an organic fertiliser to return nutrients such as nitrogen to the ground.
mass flowUnit 2: The bulk movement of a fluid down a pressure gradient, as proposed for the transport of dissolved sugars through the phloem from source to sink.
matrix (mitochondrial)Unit 3: The fluid-filled space enclosed by the inner mitochondrial membrane, containing enzymes for the link reaction and Krebs cycle along with DNA, ribosomes and the substrates for aerobic respiration.
meanUnit 4: The arithmetic average of a set of values, calculated by dividing the sum of the values by the number of values.
measlesUnit 4: An infectious viral disease spread by droplets, against which vaccination provides protection.
mechanical digestionUnit 2: The physical breakdown of food into smaller pieces, increasing its surface area for the action of digestive enzymes, without changing it chemically.
medianUnit 5: The middle value of a set of data when the values are arranged in order of size.
medulla oblongataUnit 4: The region of the hindbrain that controls vital involuntary functions such as heart rate, breathing rate and blood pressure.
megasporeUnit 4: A haploid spore produced in the ovule that develops, through mitotic divisions, into the embryo sac in flowering plants.
meiosisUnit 1: A form of nuclear division producing four genetically different haploid cells from one diploid parent cell, halving the chromosome number for gamete formation.
membraneUnit 1: A thin boundary layer, typically formed from a phospholipid bilayer with associated proteins, that surrounds cells and organelles and controls the passage of substances.
membrane fluidityUnit 1: The ability of the phospholipids and proteins within a cell membrane to move freely within the layer, allowing the membrane to be flexible and self-sealing.
membrane-bound organelleUnit 3: A discrete structure within a eukaryotic cell that is surrounded by one or two membranes, separating its contents and reactions from the rest of the cytoplasm.
memory cellUnit 4: A long-lived lymphocyte produced after exposure to an antigen that remains in circulation and divides rapidly to give a fast secondary response if the same antigen is met again.
memory consolidationUnit 4: The process by which short-term memories are stabilised and transferred into long-term storage in the brain.
meningesUnit 5: The three protective membranes that surround and cover the brain and spinal cord; their inflammation is known as meningitis.
menopauseUnit 4: The natural ending of a woman’s reproductive cycles, when ovulation and menstruation cease.
meristemCore terms: A region of unspecialised, actively dividing plant cells from which new tissues and organs grow.
mesophyteUnit 2: A plant adapted to live in habitats with an adequate, but not excessive, supply of water.
mesosomeUnit 1: An infolding of the cell-surface membrane in some prokaryotic cells, thought to be associated with respiration or with the attachment of DNA during division.
metabolic rateUnit 3: The rate at which an organism uses energy in its chemical reactions, usually measured as the rate of oxygen consumption or heat production per unit time.
metabolic reactionsUnit 2: The chemical reactions that take place within an organism to maintain life, including those that build up and break down molecules.
metabolic waterUnit 4: Water produced inside an organism as a by-product of metabolic reactions, particularly aerobic respiration.
metabolismCore terms: The sum of all the chemical reactions taking place within an organism, including those that build up molecules (anabolism) and those that break them down (catabolism).
metaboliteUnit 1: A substance that takes part in or is produced by the chemical reactions of metabolism within a cell.
metaphaseUnit 1: The stage of mitosis in which the chromosomes line up along the equator of the cell, attached to the spindle fibres by their centromeres.
micro-organismUnit 3: An organism too small to be seen without a microscope, such as a bacterium, virus, fungus or protoctist.
microfibrilsUnit 1: Bundles of cellulose molecules held together by hydrogen bonds, which group together to form the fibres that give plant cell walls their strength.
micrographUnit 1: An image of a specimen produced using a microscope, for example a light micrograph or an electron micrograph.
micrometreUnit 5: A unit of length equal to one thousandth of a millimetre, used to measure microscopic structures.
micropinocytosisUnit 1: The uptake of small droplets of liquid into a cell by the inward folding of tiny portions of the cell-surface membrane to form vesicles.
micropyleUnit 4: A small pore in the integuments of an ovule through which the pollen tube grows to reach the embryo sac for fertilisation.
microtubuleUnit 1: A hollow cylindrical structure made of the protein tubulin that forms part of the cytoskeleton and makes up the spindle fibres, centrioles and the core of cilia and flagella.
microvilliUnit 2: Microscopic, finger-like folds of the cell-surface membrane of epithelial cells that greatly increase the surface area for absorption.
middle piece (sperm)Unit 4: The region of a sperm cell between the head and the tail that is packed with mitochondria, providing the energy for movement.
midribUnit 2: The central, main vein of a leaf, containing vascular tissue and providing support to the lamina.
migrationCore terms: The movement of individuals or populations of organisms from one region or habitat to another, often seasonal.
mineral ionCore terms: An inorganic charged particle, such as nitrate, magnesium or iron, required by organisms for healthy growth and metabolic function.
minute volumeUnit 2: The volume of air breathed in or out of the lungs per minute, calculated as tidal volume multiplied by breathing rate.
mitochondriaUnit 1: Membrane-bound organelles in eukaryotic cells that are the site of aerobic respiration, producing most of the cell’s ATP.
mitochondrial DNAUnit 4: The small circular DNA molecule found within mitochondria, inherited only from the mother.
mitochondrionCore terms: A double-membraned organelle, with an inner membrane folded into cristae, that is the main site of aerobic respiration and ATP production.
mitosisCore terms: A type of nuclear division that produces two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.
mitotic cell divisionUnit 4: A type of cell division producing two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.
mitotic indexUnit 1: The proportion of cells in a sample that are undergoing mitosis, calculated as the number of cells in mitosis divided by the total number of cells.
MN blood groupUnit 4: A human blood group system, determined by codominant alleles, used as an example in studying allele frequencies in populations.
mode (statistics)Unit 4: The value that occurs most frequently in a set of data.
molarUnit 2: A broad, flat-surfaced tooth at the back of the jaw, adapted for grinding and crushing food.
molecular clockUnit 2: A technique that uses the rate at which mutations accumulate in DNA or proteins to estimate the time since two species diverged from a common ancestor.
molecular formulaUnit 1: A chemical formula that shows the actual number of atoms of each element present in one molecule of a compound, for example C6H12O6 for glucose.
monocultureUnit 3: The agricultural practice of growing a single crop species over a large area of land.
monosaccharideUnit 1: A single sugar unit, the simplest type of carbohydrate, which acts as the building block of larger carbohydrates, e.g. glucose.
morphologyUnit 2: The study of the form and structure of organisms.
motilityUnit 4: The ability of an organism or cell to move by itself.
motor cortexUnit 4: The region of the cerebrum that controls voluntary movement, with different parts controlling different parts of the body.
motor neuroneUnit 3: A nerve cell that carries impulses from the central nervous system to an effector such as a muscle or gland.
mRNAUnit 1: Messenger RNA; a single-stranded molecule that carries a copy of the genetic code from the DNA in the nucleus to the ribosomes, where it is translated into a polypeptide.
mucosaUnit 2: The innermost lining layer of the gut wall, which secretes mucus and enzymes and, in the small intestine, absorbs the products of digestion.
mucous glandUnit 5: A gland that secretes mucus to lubricate and protect the surface lining an organ.
mucusUnit 2: A slimy, lubricating secretion that protects and moistens epithelial surfaces such as the gut lining and airways.
multiple allelesUnit 2: The existence of three or more different alleles of the same gene within a population, although any one diploid individual can carry only two of them.
mumpsUnit 4: An infectious viral disease that causes swelling of the salivary glands, against which vaccination provides protection.
mureinUnit 1: The polymer of sugars and amino acids that forms the cell wall of bacteria, also known as peptidoglycan.
muscle contractionUnit 4: The shortening of a muscle produced when myosin filaments pull actin filaments past them, using energy from ATP.
muscle fatigueUnit 4: A reduction in the ability of a muscle to contract following prolonged or intense activity, associated with depletion of energy reserves and the build-up of lactate.
muscular dystrophyUnit 4: An inherited disorder causing progressive weakening and wasting of muscle tissue.
muscularis mucosaUnit 5: A thin layer of smooth muscle within the mucosa that lies beneath the lamina propria in the wall of the gut.
musculoskeletal systemUnit 4: The system of bones, joints and muscles that supports the body and brings about movement.
mutagenCore terms: An agent, such as certain chemicals or ionising radiation, that increases the rate of mutation in the DNA of an organism.
mutantUnit 4: An organism or cell carrying a mutation that gives it a phenotype differing from the normal form.
mutationUnit 2: A change in the base sequence or amount of DNA in a cell, which may alter the protein produced and can be passed to daughter cells.
mutualismUnit 3: A close relationship between two species in which both partners benefit.
myceliumUnit 2: The mass of thread-like hyphae that makes up the body of a fungus.
myelinUnit 3: The fatty insulating sheath formed by Schwann cells around some nerve axons that speeds up impulse transmission by allowing the action potential to jump between gaps in the sheath.
myelinationUnit 3: The process by which a myelin sheath is laid down around a nerve axon, increasing the speed at which impulses are conducted.
myofibrilUnit 4: One of the many thread-like contractile units running the length of a muscle fibre, made of repeating sarcomeres of actin and myosin filaments.
myofilamentUnit 4: One of the protein filaments (actin or myosin) arranged within a muscle fibre that slide past one another to bring about contraction.
myogenicUnit 2: Describing muscle, such as cardiac muscle, that is able to contract and relax rhythmically of its own accord without stimulation by a nerve.
myoglobinUnit 4: An oxygen-storing protein found in muscle that has a higher affinity for oxygen than haemoglobin, releasing it when oxygen levels in the muscle fall.
myometriumUnit 4: The thick muscular wall of the uterus that contracts during childbirth.
myosinUnit 4: The protein that forms the thick filaments of a myofibril and forms cross-bridges with actin to bring about muscle contraction.
myosin binding siteUnit 4: The site on an actin filament to which a myosin head attaches to form a cross-bridge during muscle contraction.
myosin filamentUnit 4: A thick filament in a myofibril made of myosin, whose projecting heads pull on actin filaments during contraction.

N

NADUnit 3: A coenzyme that acts as a hydrogen and electron carrier in respiration, becoming reduced when it accepts hydrogen during glycolysis, the link reaction and the Krebs cycle.
NADPUnit 3: A coenzyme that acts as a hydrogen and electron carrier in photosynthesis, becoming reduced in the light-dependent reaction and supplying reducing power for the light-independent reaction.
native (species)Unit 2: A species that occurs naturally in a particular area, having arrived and established there without human introduction.
native speciesUnit 3: A species that occurs naturally in a given area, having arrived and established there without human introduction.
natural selectionUnit 4: The process by which individuals with phenotypes best suited to their environment are more likely to survive and reproduce, passing on the alleles for those advantageous characteristics to the next generation.
nectarUnit 2: A sugary fluid secreted by flowers that attracts pollinating animals.
negative feedbackCore terms: A control mechanism in which a change in a condition away from its set point triggers responses that return the condition back towards the set point.
nephronUnit 3: The functional unit of the kidney, a tubule consisting of a renal capsule, proximal and distal convoluted tubules, loop of Henle and collecting duct, in which the blood is filtered and urine is formed.
nerve cellUnit 3: A specialised cell that generates and conducts electrical impulses; a neurone.
nerve fibreUnit 3: The long thread-like extension of a neurone, such as an axon, along which a nerve impulse is conducted.
nerve impulseUnit 3: A wave of depolarisation that travels along the membrane of a neurone as an action potential, transmitting information from one part of the body to another.
nerve netUnit 3: A simple nervous system, found in Cnidaria, consisting of an interconnected web of nerve cells that conduct impulses in all directions rather than along defined pathways.
nervous systemUnit 4: The network of neurones that detects stimuli and coordinates rapid responses by transmitting electrical impulses.
netUnit 5: A piece of apparatus used to capture mobile animals when sampling a habitat, for example a sweep net for insects or a pond net for aquatic invertebrates.
net primary productivityUnit 3: The rate at which producers store chemical energy as biomass after subtracting the energy lost in their own respiration, expressed per unit area per unit time.
neural pathwayUnit 4: A connected series of neurones along which nerve impulses travel between parts of the nervous system.
neuroneCore terms: A specialised cell that conducts electrical impulses, consisting of a cell body with dendrites and a long axon, forming the functional unit of the nervous system.
neuroplasticityUnit 4: The ability of the brain to change and adapt by forming new connections between neurones, for example in response to new information, learning or injury.
neurosecretory cellUnit 3: A specialised nerve cell that synthesises and releases a hormone into the blood in response to nervous stimulation, such as the cells producing antidiuretic hormone.
neurotoxinUnit 1: A poisonous substance that disrupts the functioning of nerve cells, for example by blocking the transmission of impulses.
neurotransmitterUnit 3: A chemical released from the end of a neurone at a synapse that diffuses across the gap and binds to receptors on the next cell to relay the impulse.
nicheUnit 3: The role of an organism within its ecosystem, including its position in food webs and its interactions with the biotic and abiotic features of its environment.
nitrateUnit 1: A nitrogen-containing ion (NO3-) absorbed from the soil by plants and used to make amino acids, proteins and nucleic acids.
nitrificationUnit 3: The conversion of ammonium ions to nitrites and then to nitrates by nitrifying bacteria in the soil under aerobic conditions.
nitrifying bacteriaUnit 3: Soil bacteria that carry out nitrification, oxidising ammonium ions to nitrite and then nitrite to nitrate, making nitrogen available to plants.
nitrogenUnit 2: An element essential for living organisms as a component of amino acids, proteins and nucleic acids, which cannot be used directly from the air by most organisms.
nitrogen cycleUnit 3: The natural recycling of nitrogen between the atmosphere, soil and living organisms through the processes of nitrogen fixation, ammonification, nitrification and denitrification, largely driven by bacteria.
nitrogen-fixing bacteriaUnit 3: Bacteria that convert atmospheric nitrogen gas into nitrogen-containing compounds such as ammonium ions, which can then be used by plants to make amino acids and proteins.
nitrogenaseUnit 3: The enzyme used by nitrogen-fixing bacteria to convert atmospheric nitrogen gas into ammonia; it is inactivated by oxygen, so requires anaerobic conditions.
nitrogenous baseUnit 1: A nitrogen-containing molecule that forms part of a nucleotide; in DNA these are adenine, guanine, cytosine and thymine, and in RNA thymine is replaced by uracil.
node of RanvierUnit 3: A short gap in the myelin sheath of a nerve axon where the membrane is exposed, allowing the action potential to jump from node to node in saltatory conduction.
non-coding DNAUnit 4: Regions of DNA that do not code for proteins, including the introns lying between the exons of a gene.
non-competitive inhibitorUnit 1: A molecule that reduces enzyme activity by binding to the enzyme at a site other than the active site, changing the shape of the active site so that the substrate can no longer bind.
non-cyclic photophosphorylationUnit 3: The light-dependent pathway in which electrons pass from photosystem II through photosystem I and are not returned, generating ATP, reduced NADP and oxygen from the photolysis of water.
non-disjunctionUnit 4: The failure of homologous chromosomes or sister chromatids to separate correctly during meiosis, resulting in gametes with an abnormal number of chromosomes.
non-endospermic seedUnit 4: A seed in which the food store is held in the cotyledons rather than in endosperm, the endosperm having been absorbed during development.
non-functional proteinUnit 4: A protein that cannot carry out its normal role, often because a mutation has altered its amino acid sequence and therefore its shape.
non-polar moleculeUnit 1: A molecule in which the electrical charge is evenly distributed, so it has no positive or negative regions and does not dissolve readily in water.
non-reducing sugar testUnit 5: A food test in which a sample is first boiled with acid to hydrolyse the sugar, neutralised, then heated with Benedict’s reagent; a colour change from blue to an orange-red precipitate indicates a non-reducing sugar.
noradrenalineUnit 4: A neurotransmitter released by most sympathetic neurones that produces excitatory effects such as increasing heart rate and ventilation rate.
nuclear membraneUnit 1: The double membrane that surrounds the nucleus of a eukaryotic cell, separating the genetic material from the cytoplasm; also called the nuclear envelope.
nuclear poreUnit 1: A small channel through the nuclear envelope that allows the exchange of materials, such as mRNA and ribosomal subunits, between the nucleus and the cytoplasm.
nucleic acidUnit 1: A large molecule made of nucleotide units, such as DNA and RNA, that stores and transfers genetic information.
nucleolusUnit 1: A dense, rounded structure within the nucleus that manufactures ribosomal RNA and assembles ribosomes.
nucleotideCore terms: The monomer of a nucleic acid, consisting of a pentose sugar, a phosphate group and a nitrogenous base.
nucleusUnit 1: The membrane-bound organelle in a eukaryotic cell that contains the chromosomes and controls the cell’s activities.
null hypothesisUnit 4: A statement, used in statistical testing, that there is no significant difference or association between the variables being studied, any apparent difference being due to chance.
nutrientUnit 3: A substance that an organism takes in and uses to provide energy, materials for growth and repair, or to support metabolic processes.
nutritionUnit 4: The process by which organisms obtain and use the materials and energy needed for growth, repair and metabolism.
nymph (insect)Unit 2: An immature stage in the life cycle of certain insects that resembles a smaller, wingless version of the adult and develops without a pupal stage.

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obesityUnit 4: The condition of having an excessive accumulation of body fat that may harm health.
obligate anaerobeUnit 3: An organism that can only survive in the absence of oxygen, for which oxygen is toxic.
observationUnit 4: The act of noting and recording a fact or event, often forming the starting point for a scientific investigation.
occipital lobeUnit 4: The rear region of the cerebrum that processes visual information.
ocean acidificationUnit 3: The fall in the pH of seawater caused by the absorption of increasing amounts of atmospheric carbon dioxide, which dissolves to form carbonic acid.
oedemaUnit 3: The accumulation of excess tissue fluid in the spaces between cells, causing swelling, for example when too little fluid is returned to the capillaries or lymphatic system.
oesophagusUnit 2: The muscular tube that carries food from the mouth to the stomach by waves of peristalsis.
oestrogenUnit 4: A female sex hormone, secreted mainly by the ovary, that stimulates repair and growth of the uterus lining and brings about female secondary sexual characteristics.
oestrous cycleUnit 4: The recurring cycle of reproductive and hormonal changes in female mammals that prepares the body for ovulation and possible pregnancy.
oestrusUnit 4: The period in the oestrous cycle when a female is fertile and receptive to mating.
offspringUnit 4: The new individuals produced by reproduction of a parent or parents.
omnivoreUnit 2: An animal that feeds on both plant and animal material.
oncogeneUnit 4: A mutated form of a normal gene (a proto-oncogene) that promotes uncontrolled cell division and can lead to the formation of a cancer.
one gene one polypeptide hypothesisUnit 1: The idea that each gene carries the coded information for the production of a single polypeptide.
oogenesisUnit 5: The process by which female gametes are produced in the ovary, leading to the formation of secondary oocytes.
operant conditioningUnit 5: A form of learning in which the likelihood of a behaviour changes according to whether it is followed by reward or punishment.
opercular cavityUnit 2: The space behind the gills of a bony fish, covered by the operculum, through which water is drawn over the gills during ventilation.
operculumUnit 2: The bony flap covering and protecting the gills of a bony fish, whose movements help to maintain a flow of water over the gills.
optical densityUnit 3: A measure of how much light is absorbed or blocked as it passes through a suspension, used with a colorimeter to estimate the number of cells or amount of micro-organisms present.
optimum pHUnit 1: The pH at which an enzyme catalyses a reaction at its fastest rate.
oral vaccineUnit 4: A vaccine that is taken by mouth rather than by injection, stimulating an immune response and the formation of memory cells against a specific pathogen.
organelleCore terms: A specialised structure within a cell that carries out a particular function, such as a mitochondrion, ribosome or nucleus.
organellesUnit 1: Specialised structures within a cell, each carrying out a particular function.
organic moleculesUnit 2: Carbon-containing molecules made by living organisms, such as carbohydrates, lipids, proteins and nucleic acids.
orthokinesisCore terms: A non-directional response in which an organism changes its speed of movement according to the intensity of a stimulus, settling where conditions are favourable.
osmoreceptorUnit 3: A receptor cell, found in the hypothalamus, that detects changes in the water potential of the blood and helps to regulate the body’s water balance.
osmoregulationUnit 3: The control of the water content and solute concentration of the body fluids, such as the blood and tissue fluid.
osmosisUnit 1: The net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
osmotic lysisUnit 1: The bursting of a cell that occurs when it takes in so much water by osmosis that the cell-surface membrane can no longer withstand the internal pressure.
osmotic pressureUnit 2: The pressure that must be applied to a solution to prevent the net movement of water into it across a partially permeable membrane.
osteoarthritisUnit 4: A degenerative joint condition in which the cartilage at the ends of bones wears away, causing pain, stiffness and reduced movement.
osteoblastUnit 4: A bone-forming cell embedded in the bone matrix that lays down the inorganic mineral component, building new bone tissue.
osteoclastUnit 4: A cell that breaks down bone tissue, reabsorbing the matrix as part of the continuous remodelling of bone.
osteogenesis imperfecta (OI)Unit 4: An inherited condition, often called brittle bone disease, in which faulty collagen leads to weak bones that fracture easily.
osteomalaciaUnit 4: The softening of bones in adults caused by a deficiency of vitamin D, leading to inadequate calcium deposition and resulting in pain and weakness.
osteopaeniaUnit 4: A reduction in bone mineral density below normal levels, but less severe than osteoporosis, that increases the risk of fractures.
osteoporosisUnit 4: A condition in which bones lose mineral density and become porous and fragile, increasing the likelihood of fractures.
outbreakUnit 4: The occurrence of cases of a disease in a community or region at a rate clearly above the level normally expected.
outbreedingUnit 3: Breeding between unrelated or distantly related individuals of a species, which tends to maintain genetic variation within a population.
ovaryUnit 4: In animals, the female reproductive organ that produces ova and secretes sex hormones; in flowering plants, the structure containing ovules that develops into the fruit.
overfishingUnit 3: The harvesting of fish at a rate faster than the population can replace itself, leading to a decline in stocks.
oviductUnit 4: The tube that carries an ovum from the ovary towards the uterus and is usually the site of fertilisation.
ovulationUnit 4: The release of a secondary oocyte from a mature Graafian follicle in the ovary into the oviduct.
ovuleUnit 4: The structure within the ovary of a flowering plant that contains the egg cell and develops into a seed after fertilisation.
ovumUnit 4: The mature female gamete, formed when the secondary oocyte completes meiosis II following fertilisation.
oxidationUnit 3: A chemical reaction involving the loss of electrons or hydrogen, or the gain of oxygen, by a molecule, atom or ion.
oxidative phosphorylationUnit 3: The synthesis of ATP that takes place on the inner mitochondrial membrane, driven by energy released as electrons pass along the electron transport chain to oxygen, the final electron acceptor.
oxidisedUnit 5: Describing a substance that has lost electrons or hydrogen, or gained oxygen, during a chemical reaction.
oxygen dissociation curveUnit 2: A graph showing how the percentage saturation of haemoglobin with oxygen varies with the partial pressure of oxygen.
oxygen saturationUnit 2: The percentage of the available oxygen-binding sites on haemoglobin that are occupied by oxygen at a given partial pressure of oxygen.
oxygenated bloodUnit 2: Blood that has passed through the lungs and is carrying a high concentration of oxygen, mostly as oxyhaemoglobin.
oxyhaemoglobinUnit 2: The bright red compound formed when haemoglobin combines reversibly with oxygen.
oxyntic cellUnit 2: A cell in the lining of the stomach that secretes hydrochloric acid.
oxytocinUnit 4: A hormone that stimulates contraction of the uterine muscle during birth and the release of milk during lactation.

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pacemakerUnit 2: The sinoatrial node, a region of specialised tissue in the wall of the right atrium that initiates and sets the rhythm of the heartbeat.
palisade mesophyllUnit 2: A layer of closely packed, column-shaped cells beneath the upper epidermis of a leaf that contains many chloroplasts and is the main site of photosynthesis.
pancreasUnit 2: An organ that secretes digestive enzymes into the small intestine and produces the hormones insulin and glucagon.
pandemicUnit 4: The spread of an infectious disease across a very large area, such as several countries or continents, affecting a large number of people.
parallel flowUnit 2: A gas-exchange arrangement in fish gills in which blood and water flow in the same direction, so equilibrium is reached and only partial oxygen uptake is achieved.
parasiteUnit 2: An organism that lives in or on a host of another species, obtaining nutrients from it and causing the host harm.
parasitismCore terms: A close relationship between two species in which one, the parasite, gains nutrients at the expense of the other, the host, which is harmed.
parasympathetic divisionUnit 4: The part of the autonomic nervous system that generally has inhibitory effects, slowing processes such as heart rate, with neurones that mostly release acetylcholine.
parenchymaUnit 5: An unspecialised plant packing tissue of thin-walled living cells that provides support and stores substances.
partial pressure of oxygenUnit 2: A measure of the concentration of oxygen in a mixture of gases or dissolved in a liquid, equal to the pressure oxygen alone would exert.
passiveUnit 1: Describing a process such as diffusion that occurs down a concentration gradient and does not require energy from the cell (ATP).
passive immunityUnit 4: Immunity gained by receiving antibodies made by another individual, giving short-lived protection because no memory cells are produced.
patellaUnit 4: The kneecap; a small bone at the front of the knee joint that protects it and improves the leverage of the muscles that straighten the leg.
pathogenUnit 4: A microorganism or agent that causes disease in its host.
pedicleUnit 4: In bone structure, the short stalk of bone that connects the body of a vertebra to its posterior arch.
pedigree chartUnit 4: A diagram showing the inheritance of a characteristic through several generations of a family, used to trace the pattern of transmission of an allele.
pelvisUnit 4: The basin-shaped ring of bones at the base of the trunk that supports the body and connects the spine to the legs.
penicillinUnit 4: An antibiotic produced by a fungus that kills bacteria by preventing the formation of their cell walls.
pentoseUnit 1: A monosaccharide sugar containing five carbon atoms, such as ribose or deoxyribose.
pepsinUnit 2: An endopeptidase, active in the acidic conditions of the stomach, that hydrolyses internal peptide bonds of proteins.
pepsinogenUnit 2: The inactive form in which pepsin is secreted by the cells of the stomach, converted to active pepsin by hydrochloric acid.
peptidasesUnit 2: Enzymes that hydrolyse the peptide bonds in proteins and peptides, releasing shorter peptides or individual amino acids.
peptide bondUnit 1: The covalent bond formed between the amino group of one amino acid and the carboxyl group of another during a condensation reaction, linking them in a polypeptide.
peptidoglycanUnit 1: The polymer of sugar chains cross-linked by short peptides that forms the rigid cell wall of bacteria.
percentage change in massUnit 5: The change in mass of a specimen expressed as a percentage of its original mass, used to measure net water movement in osmosis experiments.
percentage coverUnit 5: The proportion of the ground within a sample area, such as a quadrat, that is occupied by a particular species, expressed as a percentage.
percentage errorUnit 2: A measure of the uncertainty in a measurement, expressed as the smallest measurable division as a percentage of the value measured.
perichondriumUnit 5: The layer of dense connective tissue that surrounds cartilage.
pericycleUnit 5: A layer of cells just inside the endodermis of a root from which lateral roots arise.
peripheral nervous systemUnit 4: All the nerves outside the brain and spinal cord that carry impulses between the central nervous system and the rest of the body.
peristalsisUnit 2: Waves of coordinated contraction and relaxation of the circular and longitudinal muscles in the gut wall that move food along the alimentary canal.
permeableUnit 1: Describing a material or membrane that allows substances to pass through it.
pesticideUnit 3: A chemical used to kill organisms regarded as pests, such as insects, weeds or fungi, that damage crops or livestock.
petalUnit 4: A part of a flower, often coloured in insect-pollinated species, that surrounds the reproductive structures and may attract pollinators.
phagocyteCore terms: A white blood cell that engulfs and digests pathogens and debris by phagocytosis.
phagocytosisUnit 1: The process by which a cell engulfs solid material, such as a pathogen, by surrounding it with its cell-surface membrane to form a vesicle.
phenotypeUnit 4: The observable characteristics of an organism, produced by the interaction of its genotype with the environment.
phenylalanineUnit 1: An aromatic amino acid, one of the twenty amino acids used to build proteins.
pheromoneUnit 4: A chemical released by an organism that influences the behaviour or physiology of other members of the same species.
phloemUnit 2: The vascular tissue in plants that transports sugars and other organic solutes from sources to sinks, made up of sieve tube elements and companion cells.
phosphataseCore terms: An enzyme that catalyses the removal of a phosphate group from a molecule.
phosphate ionUnit 1: A negatively charged ion (PO4 3-) that forms part of the backbone of nucleotides and is involved in the formation of ATP and phospholipids.
phosphodiester bondUnit 1: The covalent bond that links the phosphate group of one nucleotide to the sugar of the next, forming the sugar-phosphate backbone of a nucleic acid.
phospholipidUnit 1: A lipid molecule in which one fatty acid of a triglyceride is replaced by a phosphate group, giving it a water-attracting head and water-repelling tails; the main component of cell membranes.
phospholipid bilayerUnit 1: A double layer of phospholipid molecules arranged with their water-attracting heads facing outwards and their water-repelling tails facing inwards, forming the basic structure of cell membranes.
photo-phosphorylationCore terms: The synthesis of ATP using light energy during the light-dependent stage of photosynthesis, occurring by cyclic or non-cyclic pathways.
photoautotrophUnit 5: An organism that uses light energy to synthesise its own organic molecules from inorganic raw materials such as carbon dioxide and water.
photoautotrophicCore terms: Describing an organism that uses light energy to synthesise its own organic compounds from inorganic raw materials.
photolysisCore terms: The splitting of water molecules using light energy during the light-dependent stage of photosynthesis, producing hydrogen ions, electrons and oxygen.
photosynthesisUnit 2: The process by which green plants and other autotrophs use light energy to synthesise organic molecules such as glucose from carbon dioxide and water.
photosynthetic organismUnit 3: An organism that makes its own organic compounds from carbon dioxide and water using light energy trapped by photosynthetic pigments.
photosystem I / IIUnit 3: Light-harvesting complexes of pigments and proteins in the thylakoid membrane; Photosystem II provides electrons from photolysis of water, and Photosystem I reduces NADP.
phototrophicCore terms: Describing an organism that obtains its energy from light.
phylogeneticUnit 2: Relating to the evolutionary relationships and lines of descent between different groups of organisms.
phylogenetic treesUnit 2: Branching diagrams that represent hypotheses about the evolutionary relationships and common ancestry between groups of organisms.
PhylumUnit 2: A taxonomic group ranking below kingdom and above class, containing organisms sharing a common body plan.
physical barrierUnit 4: A structural feature, such as skin or mucous membranes, that helps prevent pathogens from entering the body.
physiological processUnit 3: A process concerned with the normal functioning and activity of a living organism or its parts.
physiological temperatureUnit 1: The normal range of body temperature at which an organism’s metabolic processes operate, for example around 37 C in humans.
pia materUnit 5: The innermost, delicate membrane of the meninges, closely covering the surface of the brain and spinal cord.
piercing mouthpartsUnit 2: Specialised insect mouthparts adapted to penetrate tissue and withdraw fluids such as plant sap or blood.
pigment-protein complexUnit 3: A grouping of photosynthetic pigments held with proteins in the thylakoid membrane that absorbs light and channels its energy to a reaction centre.
pinocytosisCore terms: A form of endocytosis in which a cell takes in droplets of extracellular fluid by enclosing them in vesicles formed from the cell membrane.
pioneer speciesUnit 3: The first organisms to colonise a bare or newly formed habitat, beginning a succession by gradually changing the conditions to make them suitable for other species.
pith / medulla (stem)Unit 5: The central tissue of a stem, made of parenchyma, that stores substances and provides support.
placentaUnit 4: The organ formed during pregnancy that allows exchange of gases, nutrients and waste between the mother and foetus while keeping their blood supplies separate, and that secretes hormones.
planetary boundariesUnit 3: A set of nine Earth system thresholds that define a safe operating space for humanity, beyond which there is a risk of abrupt and irreversible environmental change.
plant tissueUnit 4: A group of similar plant cells working together to perform a particular function.
plasma membraneUnit 3: The selectively permeable membrane of phospholipids and proteins that surrounds a cell and controls the movement of substances into and out of it.
plasma proteinUnit 3: A protein dissolved in blood plasma, such as albumin, fibrinogen or globulins, with roles including transport, clotting and maintaining water potential.
plasmidUnit 1: A small, circular loop of DNA found in bacteria that is separate from the main chromosome and can replicate independently.
plasmodesmataUnit 1: Fine cytoplasmic strands that pass through gaps in adjacent plant cell walls, connecting the cytoplasm of neighbouring cells.
plasmolysedUnit 1: Describing a plant cell that has lost so much water by osmosis that its cell-surface membrane and cytoplasm have pulled away from the cell wall.
pleural membraneUnit 2: A thin membrane lining the chest cavity and covering the lungs, enclosing the fluid-filled pleural cavity.
plumuleUnit 4: The part of a plant embryo that develops into the shoot of the seedling.
pluripotentUnit 4: Describing a stem cell that can differentiate into many, but not all, of the cell types of the body.
pneumoniaUnit 3: An infection of the lungs in which the air sacs become inflamed and filled with fluid, reducing gas exchange.
point mutationUnit 4: A gene mutation that changes a single base in the DNA, for example by substitution, which may alter the protein produced.
polar nucleiUnit 4: The two nuclei in the centre of the embryo sac of a flowering plant that fuse with a male nucleus to form the triploid primary endosperm nucleus.
polarisationCore terms: The maintenance of a potential difference across a membrane, as in the resting neurone where the inside is negative relative to the outside.
polarityUnit 1: The uneven distribution of electrical charge within a molecule, producing slightly positive and slightly negative regions, as seen in water.
pollenUnit 4: The grains produced in the anthers of a flowering plant, each containing the male gametes.
pollen grainUnit 5: The structure produced in the anther of a flowering plant that contains the male gamete.
pollen sacUnit 5: One of the chambers within an anther in which pollen grains are produced.
pollen tubeUnit 4: The tube that grows from a germinating pollen grain down through the style to deliver the male gametes to the embryo sac.
pollinationUnit 4: The transfer of pollen from the anther to the stigma of a flower, allowing fertilisation to take place.
pollinatorUnit 4: An animal, such as an insect, that transfers pollen from the anther of one flower to the stigma of another.
polymerUnit 1: A large molecule made up of many similar smaller units (monomers) joined together by repeated condensation reactions.
Polymerase Chain Reaction (PCR)Unit 4: A technique that rapidly makes many copies of a section of DNA by repeated cycles of heating and cooling using primers and a heat-stable DNA polymerase.
polymerisationUnit 1: The joining together of many small repeating molecules (monomers) to form a large molecule (polymer), usually with the removal of water.
polymorphic lociUnit 2: Positions on the chromosomes that exist in two or more allelic forms within a population, used as a measure of genetic diversity.
polypeptideCore terms: A chain of many amino acids joined by peptide bonds.
polypeptide chainUnit 1: A chain of many amino acids joined together by peptide bonds, which folds to form a protein.
polyploidyUnit 4: A condition in which an organism has more than two complete sets of chromosomes.
polysaccharideCore terms: A large carbohydrate polymer formed from many monosaccharide units joined by glycosidic bonds, such as starch, glycogen or cellulose.
polyspermyUnit 4: The fertilisation of an egg cell by more than one sperm, which is normally prevented by the formation of the fertilisation membrane.
populationUnit 3: All the individuals of a single species living together in the same area at the same time, able to interbreed.
population densityUnit 4: The number of individuals of a species per unit area or volume of a habitat.
posteriorUnit 4: Relating to or situated towards the back or rear of the body or a structure.
posterior pituitaryUnit 3: The rear lobe of the pituitary gland, which stores and releases hormones such as antidiuretic hormone produced by the hypothalamus.
postsynaptic membraneUnit 3: The cell-surface membrane of the neurone receiving a signal at a synapse, bearing receptors that bind neurotransmitter.
potential differenceUnit 3: The difference in electrical charge between two points, such as across a cell membrane, measured in volts.
potometerUnit 2: An apparatus used to measure the rate of water uptake by a cut plant shoot as an estimate of the rate of transpiration.
practical analysis taskUnit 5: An assessment in which candidates analyse and evaluate given experimental data and methods rather than carrying out the practical themselves.
practical examinationUnit 5: A formal assessment in which candidates carry out practical work and are marked on their experimental skills.
predationUnit 4: An interaction in which one organism, the predator, kills and eats another, the prey.
predatorUnit 2: An organism that hunts, kills and feeds on other living organisms (its prey).
pregnancyUnit 4: The condition in which a developing embryo or foetus is carried in the uterus from implantation until birth.
premolarUnit 2: A type of cheek tooth positioned between the canines and molars, used for grinding and crushing food.
prenatal diagnosisUnit 4: The testing of an embryo or foetus before birth to detect genetic conditions or developmental abnormalities.
pressure potentialUnit 1: The pressure exerted by the cell contents pushing outwards against the cell wall, which raises the water potential of a plant cell.
presynaptic membraneUnit 3: The cell-surface membrane at the end of the neurone before a synapse, from which neurotransmitter is released into the synaptic cleft.
preyUnit 5: An animal that is hunted and eaten by a predator.
primary consumerUnit 3: An organism that feeds on producers; a herbivore occupying the second trophic level of a food chain.
primary endosperm nucleusUnit 4: The triploid nucleus formed in double fertilisation when a male gamete fuses with the two polar nuclei, which divides to form the endosperm.
primary follicleUnit 4: An early stage in the development of an ovarian follicle, consisting of an oocyte surrounded by a layer of follicle cells.
primary immune responseUnit 4: The relatively slow response on first exposure to an antigen, during which a low level of antibody is produced and memory cells are formed.
primary oocyteUnit 5: A diploid cell in the ovary that is formed from an oogonium and undergoes the first meiotic division during oogenesis.
primary structureUnit 1: The specific sequence of amino acids in a polypeptide chain, determined by the order of bases in a gene.
primerUnit 4: A short single strand of nucleotides that binds to a complementary region of a DNA template, providing a starting point for DNA polymerase, for example in PCR.
probabilityUnit 4: A measure of how likely an event is to occur, expressed as a value between 0 and 1 or as a fraction or percentage.
probe (gene/DNA)Unit 4: A short, single-stranded length of DNA with a complementary base sequence to a target gene, labelled (e.g. radioactively or fluorescently) so that the target can be located.
producer (trophic)Unit 3: An autotrophic organism, usually a photosynthesising plant or alga, that forms the first trophic level by making organic compounds from inorganic raw materials.
progesteroneUnit 4: A hormone that maintains the lining of the uterus during the menstrual cycle and pregnancy, and inhibits the release of FSH and LH.
proglottid / segmentUnit 5: One of the repeated body sections of a tapeworm, each containing a complete set of reproductive organs.
prokaryoteCore terms: An organism whose cells lack a true membrane-bound nucleus and membrane-bound organelles, e.g. a bacterium.
prokaryoticUnit 1: Describing cells, such as bacteria, that lack a true nucleus and membrane-bound organelles, with their DNA free in the cytoplasm.
prolactinUnit 4: A hormone secreted by the anterior pituitary gland that stimulates and maintains milk production in the mammary glands after birth.
prolineUnit 1: An amino acid, one of the twenty amino acids used to build proteins, with a distinctive ring structure.
promoter regionUnit 4: A region of DNA near the start of a gene to which RNA polymerase and transcription factors bind to begin transcription.
proof-reading (DNA)Unit 1: The process during DNA replication in which DNA polymerase checks each newly added nucleotide and corrects mismatched bases, reducing the rate of mutation.
prophaseCore terms: The first stage of mitosis, in which chromosomes condense and become visible, the spindle begins to form and the nuclear envelope breaks down.
proprioceptionUnit 4: The sense by which an organism detects the position and movement of its own body, using receptors in muscles, tendons and joints.
proprioceptorUnit 4: A receptor that detects the position, movement and tension of muscles and joints, providing information about the body’s posture.
prostate glandUnit 4: A gland in the male reproductive system that secretes a fluid forming part of the semen.
proteasesUnit 2: Enzymes that catalyse the breakdown of proteins into peptides and amino acids by hydrolysis of peptide bonds.
proteinCore terms: A large molecule made of one or more polypeptide chains of amino acids folded into a specific three-dimensional shape that determines its function.
protein synthesisUnit 1: The process by which cells build polypeptides, involving the transcription of a gene into mRNA and its translation at the ribosomes.
proto-oncogeneUnit 4: A normal gene that controls cell division and which, if mutated into an oncogene, can lead to uncontrolled cell division.
protonUnit 3: A positively charged subatomic particle; in biology, a hydrogen ion whose movement across membranes drives ATP synthesis in chemiosmosis.
proton gradientUnit 3: A difference in hydrogen ion concentration across a membrane that stores potential energy used to drive ATP synthesis during chemiosmosis.
proton pumpUnit 3: A membrane protein that uses energy from excited electrons to move hydrogen ions across a membrane, building up a proton gradient.
proximal convoluted tubuleUnit 3: The coiled first section of the nephron after the Bowman’s capsule, where most selective reabsorption of useful substances back into the blood occurs.
pulmonary circulationUnit 2: The part of the mammalian double circulatory system that carries blood between the heart and the lungs.
pupaUnit 5: The non-feeding stage in the life cycle of an insect undergoing complete metamorphosis, during which the larva is reorganised into the adult form.
purineUnit 1: A nitrogenous base with a double-ring structure; in DNA and RNA the purines are adenine and guanine.
putrefactionUnit 3: The breakdown of dead organic matter, especially proteins, by microorganisms, releasing simpler substances and often foul-smelling products.
pyrimidineUnit 1: A nitrogenous base with a single-ring structure; in DNA and RNA the pyrimidines are cytosine, thymine and uracil.
pyruvateUnit 3: The three-carbon compound produced by glycolysis, which may be converted to acetyl coenzyme A for the Krebs cycle or used in anaerobic respiration.

Q

quadricepsUnit 4: The group of four muscles at the front of the thigh that act together to straighten the leg at the knee.
quaternary structureUnit 1: The structure formed when two or more polypeptide chains, sometimes with a non-protein group, combine to make a single functional protein, as in haemoglobin.
quotaUnit 3: A set limit on the amount of a resource, such as fish, that may be taken in a given period to allow populations to recover and remain sustainable.

R

radicleUnit 4: The part of a plant embryo that develops into the root of the seedling.
random matingUnit 4: Mating in which any individual has an equal chance of breeding with any other, one of the conditions required for the Hardy-Weinberg principle to hold.
range barsUnit 1: Lines drawn on a graph to show the spread of a set of repeat measurements, extending from the lowest to the highest value recorded.
rateUnit 1: A measure of how much something changes in a given period of time, such as the amount of product formed per second in a reaction.
rate of photosynthesisUnit 3: The amount of product formed or raw material used by photosynthesis per unit time, often measured as the volume of oxygen released or carbon dioxide taken up.
ratioUnit 4: A comparison of two or more quantities showing their relative sizes, such as the proportions of different phenotypes in offspring.
raw resultsUnit 5: The original measurements or observations recorded directly during an investigation, before any processing or calculation.
ray floretUnit 4: One of the strap-shaped outer flowers of a composite flower head, often functioning to attract pollinators.
reactantUnit 3: A substance that is used up and changed into products during a chemical reaction.
reactionUnit 3: A process in which substances are chemically changed into different substances.
reaction centreUnit 3: The site in a photosystem to which light energy absorbed by accessory pigments is funnelled, where an electron is excited and passed to an electron acceptor.
reading (data)Unit 5: A single value obtained from a measuring instrument during an investigation.
receptacle (flower)Unit 4: The enlarged tip of a flower stalk to which the other parts of the flower are attached.
receptorCore terms: A cell or molecule that detects a specific stimulus or signal and initiates a response.
recessive (allele)Core terms: An allele whose effect on the phenotype is masked in the heterozygote and is only expressed when present on both homologous chromosomes.
recessive alleleUnit 4: An allele whose effect on the phenotype is shown only when two copies are present, being masked by a dominant allele in a heterozygote.
recombinant plasmidUnit 4: A plasmid into which a piece of foreign DNA has been inserted, used as a vector to introduce a gene into a host cell.
red blood cellsUnit 1: Specialised cells that transport oxygen in the blood; in mammals they are biconcave discs lacking a nucleus and packed with haemoglobin.
reduced (chemically)Unit 5: Describing a substance that has gained electrons or hydrogen, or lost oxygen, during a chemical reaction.
reduced NADUnit 3: The form of the coenzyme NAD that has gained hydrogen and electrons during respiration, carrying them to the electron transport chain to generate ATP.
reduced NADPUnit 3: The form of the coenzyme NADP that has gained hydrogen and electrons in the light-dependent stage, supplying reducing power for the light-independent reactions.
reducing agentUnit 5: A substance that donates electrons to another substance, causing it to be reduced while the agent itself is oxidised.
reducing sugarUnit 1: A sugar that can donate electrons to another molecule and therefore gives a positive (brick-red precipitate) result with Benedict’s solution, such as glucose and maltose.
reducing sugar testUnit 5: A food test in which a sample is heated with Benedict’s reagent; a colour change from blue to an orange-red precipitate indicates the presence of a reducing sugar.
reductionUnit 3: A chemical change in which a substance gains electrons or hydrogen, or loses oxygen.
reflexCore terms: A rapid, automatic and involuntary response to a stimulus that does not require conscious control by the brain.
reflex actionUnit 4: A rapid, automatic and involuntary response to a stimulus that does not require conscious thought and often protects the body.
reflex arcUnit 3: The nerve pathway of a reflex, in which a receptor, sensory neurone, relay neurone, motor neurone and effector bring about a rapid, automatic, protective response.
regenerationCore terms: The regrowth or replacement by an organism of cells, tissues or body parts that have been lost or damaged.
reintroductionUnit 3: The release of a species back into an area of its former range from which it had been lost, to re-establish a wild population.
relay neuroneUnit 3: A neurone within the central nervous system that connects sensory neurones to motor neurones, for example in a reflex arc.
reliabilityUnit 1: The degree to which a measurement or result can be trusted, judged by how consistent repeated readings are.
renal capsuleUnit 5: The tough, fibrous membrane that encloses and protects the kidney.
renal pelvisUnit 5: The funnel-shaped central cavity of the kidney that collects urine and channels it into the ureter.
renal pyramidUnit 5: One of the cone-shaped regions of the kidney medulla that contain the collecting ducts and parts of the nephrons.
repeatabilityUnit 1: The closeness of agreement between results when the same person repeats the same measurement using the same method and equipment.
repeatableUnit 1: Describing results that are similar each time the same investigator repeats the measurement under the same conditions.
replica platingUnit 4: A technique in which the pattern of bacterial colonies on one agar plate is transferred to other plates, used to identify colonies with particular characteristics.
replication forkUnit 1: The Y-shaped region of a DNA molecule where the two strands have been separated so that each can act as a template during DNA replication.
repolarisationCore terms: The restoration of the resting potential across a neurone membrane after an action potential, as potassium ions move out of the cell.
representative (sample)Unit 5: A sample that reflects the characteristics of the whole population from which it is taken.
reproducibleUnit 5: Describing results that can be obtained again when the investigation is repeated by a different person or with different equipment.
reservoirUnit 2: A store or pool in which a substance, such as carbon or nitrogen, is held within a nutrient cycle.
residual volumeUnit 2: The volume of air remaining in the lungs after a maximum forced exhalation.
resistantUnit 4: Able to survive and reproduce despite exposure to a factor such as an antibiotic, pesticide or pathogen that would normally be harmful.
resolutionUnit 1: The smallest distance by which two points can be separated and still be distinguished as separate, determining the level of detail a microscope can show.
respirationUnit 1: The series of enzyme-controlled reactions in cells that release energy from organic molecules such as glucose to produce ATP.
respiratory inhibitorUnit 3: A substance that blocks one or more stages of respiration, for example by stopping the transfer of electrons along the electron transport chain.
respiratory quotient (RQ)Unit 3: The ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a given time, indicating the type of substrate being respired.
respiratory substrateUnit 3: An organic molecule, such as a carbohydrate, lipid or protein, that is oxidised in respiration to release energy.
respiratory tractUnit 1: The passages through which air travels to and from the lungs, including the trachea, bronchi and bronchioles.
resting membrane potentialUnit 3: The potential difference of about -70 mV maintained across the membrane of a neurone that is not transmitting an impulse, kept by sodium-potassium pumps and the membrane’s permeability.
resting potentialUnit 4: The potential difference of about -70 mV maintained across the membrane of a neurone when it is not conducting an impulse, established mainly by the sodium-potassium pump.
restriction endonucleaseUnit 4: An enzyme that cuts DNA at a specific base sequence, producing fragments often with complementary single-stranded ends used in genetic engineering.
restriction enzymeUnit 4: An enzyme that cuts a DNA molecule at a specific recognition sequence of bases.
reverse transcriptaseUnit 1: An enzyme that catalyses the synthesis of a complementary strand of DNA using an RNA molecule as the template.
rewardUnit 4: In animal behaviour, a positive outcome of an action that increases the likelihood the behaviour is repeated.
rheumatoid arthritisUnit 4: An autoimmune condition in which the immune system attacks the joints, causing inflammation, pain, swelling and stiffness.
riboseUnit 1: A five-carbon (pentose) sugar that forms part of the nucleotides of RNA and of molecules such as ATP.
ribosomal RNA (rRNA)Unit 1: A type of RNA that combines with proteins to form ribosomes, the structures on which polypeptides are assembled.
ribosomeUnit 1: A small structure made of rRNA and protein, found free in the cytoplasm or on the rough endoplasmic reticulum, that is the site of protein synthesis.
ribosome (70S/80S)Unit 1: The two sizes of ribosome: the smaller 70S type found in prokaryotes, mitochondria and chloroplasts, and the larger 80S type found in the cytoplasm of eukaryotic cells.
ricketsUnit 4: A bone disorder in children caused by defective calcification of bone resulting from a deficiency of vitamin D or a lack of calcium in the diet.
riskUnit 5: The chance that a hazard will cause harm during an investigation.
risk assessmentUnit 5: An evaluation of the hazards in a procedure and of the measures needed to reduce the chance of harm.
RNA polymeraseUnit 1: The enzyme that catalyses transcription by joining RNA nucleotides together to form a molecule of mRNA using a DNA strand as a template.
rod (bacterial shape)Unit 3: A bacterium with an elongated, rod-like cell shape, also known as a bacillus.
root hair cellUnit 2: An epidermal cell of a root with a long, thin extension that greatly increases the surface area for the absorption of water and mineral ions.
root noduleUnit 3: A swelling on the root of a leguminous plant that houses nitrogen-fixing bacteria living in a mutualistic relationship with the plant.
root pressureUnit 2: The pressure generated in the roots by the active transport of mineral ions into the xylem, which lowers the water potential and draws water in, helping to push it up the stem.
rough endoplasmic reticulumUnit 1: A system of membrane-bound channels studded with ribosomes, which transports the proteins made by those ribosomes through the cell.
rubellaUnit 4: A viral infectious disease (German measles) that can cause serious harm to a developing fetus if a woman is infected during pregnancy.
RUBISCOUnit 3: The enzyme that catalyses the fixation of carbon dioxide by combining it with ribulose bisphosphate in the light-independent stage of photosynthesis.
RuBP / ribulose bisphosphateUnit 3: The five-carbon acceptor molecule that combines with carbon dioxide in the Calvin cycle and is regenerated from triose phosphate.
rumenUnit 2: The largest of the chambers in the stomach of a ruminant, where mutualistic microorganisms digest cellulose.
ruminantUnit 2: A cud-chewing herbivore (e.g. a cow) with a stomach divided into chambers, the largest being the rumen, containing mutualistic microbes that digest cellulose.

S

safe operating spaceUnit 3: The region within the planetary boundaries in which human activity can continue without triggering dangerous and irreversible environmental change.
salinityUnit 3: The concentration of dissolved salts in water or soil.
salivaUnit 2: A watery secretion in the mouth that lubricates food and, in some animals, contains amylase to begin the digestion of starch.
saltatory conductionUnit 3: The rapid propagation of a nerve impulse along a myelinated neurone, in which the action potential jumps from one node of Ranvier to the next, greatly increasing the speed of conduction.
sample sizeUnit 4: The number of individuals or measurements taken from a population; a larger sample gives more reliable results and reduces the effect of anomalies.
sampling techniqueUnit 3: A method, such as using quadrats or transects, for estimating the abundance and distribution of organisms in a habitat without counting every individual.
saprobiontUnit 2: An organism, such as a fungus or bacterium, that feeds on dead or decaying organic matter by secreting enzymes onto it and absorbing the digested products.
saprotrophUnit 3: An organism, such as a fungus or bacterium, that feeds on dead organic matter by secreting enzymes onto it and absorbing the products of external digestion.
saprotrophicUnit 2: Describing an organism that obtains its nutrition from dead or decaying organic matter by external digestion and absorption.
saprotrophic nutritionUnit 2: A mode of heterotrophic nutrition in which enzymes are secreted onto dead organic material, which is digested externally and the soluble products are then absorbed, e.g. in fungi.
sarcomereUnit 4: The functional contractile unit of a striated muscle fibre, lying between two Z lines and containing overlapping actin and myosin filaments.
sarcoplasmic reticulumUnit 4: The specialised endoplasmic reticulum of a muscle fibre that stores and releases calcium ions to trigger contraction.
saturated fatty acidUnit 1: A fatty acid whose hydrocarbon chain contains only single carbon-to-carbon bonds, so it holds the maximum possible number of hydrogen atoms.
scale barUnit 1: A line of known length marked on a micrograph or drawing that shows the actual size represented, allowing real dimensions to be calculated.
scientific nameUnit 2: The two-part Latin name given to a species under the binomial system, consisting of its genus and species names.
sclerenchymaUnit 5: A plant tissue of cells with thick, lignified walls that are dead at maturity, providing strength and support.
scoliosisUnit 4: A condition in which the spine is curved sideways from its normal vertical alignment.
scrotumUnit 4: The external sac that holds the testes outside the abdomen, keeping them at a slightly lower temperature suitable for sperm production.
secondary consumerUnit 3: An organism that feeds on primary consumers; a carnivore occupying the third trophic level of a food chain.
secondary follicleUnit 5: A developing ovarian follicle in which a fluid-filled cavity is beginning to form around the oocyte.
secondary immune responseUnit 4: The faster and stronger immune response produced on second exposure to an antigen, due to memory cells formed during the first exposure.
secondary oocyteUnit 4: The haploid cell produced when a primary oocyte completes the first meiotic division, released at ovulation and able to be fertilised.
secondary productivityUnit 3: The rate at which consumers convert the energy in their food into new body tissue (biomass), usually expressed per unit area per unit time.
secondary structureUnit 1: The regular folding of a polypeptide chain into shapes such as the alpha helix or beta-pleated sheet, held in place by hydrogen bonds.
secondary successionUnit 3: The sequence of community change that occurs on land that has previously been colonised, where soil, seeds and nutrients are already present.
sedimentUnit 3: Solid particles that settle out of water and accumulate as a layer on the bottom.
seedUnit 4: A structure formed from a fertilised ovule, containing an embryo and a food store, surrounded by a protective coat.
seed coatUnit 1: The protective outer covering of a seed, formed from the wall of the ovule, that protects the embryo inside.
seed dispersalUnit 4: The movement of seeds away from the parent plant, reducing competition and allowing colonisation of new areas.
selectionUnit 3: The process by which environmental factors allow individuals with advantageous characteristics to survive and reproduce more successfully than others.
selection (for/against)Unit 4: The process by which environmental factors increase or decrease the survival and reproductive success of individuals with particular alleles.
selection pressureUnit 4: An environmental factor that influences which phenotypes survive and reproduce, altering the frequency of alleles in a population over time.
selective advantageUnit 2: A characteristic that increases an organism’s chances of surviving and reproducing, so its alleles become more frequent in the population.
selective reabsorptionUnit 3: The process in the kidney by which useful substances such as glucose, amino acids and some water and ions are taken back into the blood from the filtrate.
selectively permeableUnit 3: Describing a membrane that allows some substances to pass through more readily than others.
self-fertilisationUnit 2: Fertilisation in which the male and female gametes that fuse are produced by the same individual.
self-pollinationCore terms: The transfer of pollen from the anther to the stigma of the same plant, or of another flower on the same plant.
semenUnit 4: The fluid released at ejaculation, containing spermatozoa together with secretions from the accessory glands.
semi-conservative replicationUnit 1: The mode of DNA replication in which the two strands of the parent molecule separate and each acts as a template for a new complementary strand, so that each daughter molecule contains one original strand.
semilunar valveUnit 2: A half-moon-shaped valve at the base of the aorta and pulmonary artery that prevents the backflow of blood into the ventricles.
seminiferous tubuleUnit 5: One of the coiled tubules within the testis in which spermatozoa are produced.
sensory cortexUnit 4: The region of the cerebral cortex that receives and processes sensory information from the body.
sensory neuroneUnit 3: A neurone that carries impulses from a receptor towards the central nervous system.
sequencingUnit 4: Determining the order of bases in a sample of DNA.
seral stageUnit 3: A distinct stage in a succession, dominated by a particular community, during the change from bare ground towards a climax community.
serial dilutionUnit 3: A stepwise series of dilutions in which each dilution is made from the previous one, producing a range of progressively lower concentrations.
serotypeUnit 4: A distinct variety within a species of microorganism, distinguished by its surface antigens.
set pointUnit 3: The normal level of a condition around which the body is regulated, departures from which are corrected by negative feedback.
sex linkageUnit 5: The pattern of inheritance of a gene carried on a sex chromosome (usually the X chromosome), so that the characteristic is inherited differently in males and females.
sex-linkedUnit 4: Describes a gene, or the characteristic it controls, that is carried on a sex chromosome, usually the X chromosome.
sexual dimorphismUnit 2: The existence of marked differences in appearance, such as size or colouration, between the males and females of a species.
sexual reproductionUnit 4: Reproduction involving the fusion of two haploid gametes to form a diploid zygote, producing genetically varied offspring.
sexually transmitted disease (STD)Unit 3: An infectious disease passed from one person to another mainly through sexual contact.
shade leafUnit 2: A leaf adapted to low light intensity, typically thinner with more chlorophyll, found on the shaded parts of a plant.
short-term memoryUnit 4: The store that holds a small amount of information for a brief period before it is either lost or transferred to long-term memory.
sickle cell diseaseUnit 4: An inherited disorder, caused by a mutated haemoglobin allele, in which red blood cells become sickle-shaped and carry oxygen less effectively.
sickle cell traitUnit 4: The condition of being heterozygous for the sickle cell allele, giving partial protection against malaria with few harmful effects.
sieve plateUnit 2: A perforated end wall between adjacent sieve tube elements in phloem, through which the contents flow during translocation.
sieve tube elementUnit 2: An elongated, living phloem cell joined end to end with others and perforated by sieve plates, through which organic solutes are transported.
sieve tubesUnit 2: Columns of elongated phloem cells, joined end to end by sieve plates, that conduct dissolved organic solutes through the plant.
sign stimulusUnit 4: A specific feature of a stimulus that triggers a particular innate behavioural response in an animal.
significant differenceUnit 4: A difference between two sets of data that is unlikely to be due to chance, as shown by a statistical test.
significant figuresUnit 3: The digits in a number that carry meaning about its precision, counted from the first non-zero digit.
simple diffusionUnit 1: The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, without the use of metabolic energy or a transport protein.
simple epitheliumUnit 1: An epithelium made of a single layer of cells, providing a thin surface that is well suited to rapid exchange of substances such as gases or absorbed products.
Simpson’s Diversity IndexUnit 5: A measure of biodiversity that takes account of both the number of species present and the number of individuals in each species.
single circulationUnit 2: A circulatory system in which the blood passes through the heart only once for each complete circuit of the body, as in fish.
single circulatory systemUnit 2: A circulatory system, as in fish, in which the blood passes through the heart only once during each complete circuit of the body.
single-strandedUnit 1: Describes a nucleic acid molecule, such as mRNA, that consists of a single polynucleotide chain rather than two strands held together.
single-stranded DNAUnit 4: DNA consisting of one polynucleotide strand only, rather than the usual double helix.
sink (source-sink)Unit 2: A region of a plant where organic solutes are delivered to be used or stored, such as a growing root or developing fruit.
sino-atrial node (SAN)Unit 2: A region of specialised tissue in the wall of the right atrium that initiates each heartbeat by generating a wave of electrical excitation; the heart’s pacemaker.
sister chromatidsUnit 1: The two identical copies of a chromosome produced by DNA replication, joined together at the centromere until they are separated during cell division.
site of special scientific interest (SSSI)Unit 5: An area protected by law because of its particular wildlife, geological or physical features.
skeletal muscleUnit 2: Striated muscle attached to the skeleton that brings about voluntary movement of the body.
skeletonUnit 4: The supporting framework of an organism that provides shape, allows movement and protects internal organs.
sliding filament theoryUnit 4: The explanation of muscle contraction in which the actin filaments are pulled past the myosin filaments by the formation of cross-bridges, shortening the sarcomere without the filaments themselves shortening.
slow-twitch fibreUnit 4: A muscle fibre that contracts slowly and is resistant to fatigue, relying on aerobic respiration and adapted for endurance activity.
small intestineUnit 2: The region of the gut between the stomach and large intestine where most digestion is completed and the products of digestion are absorbed.
smooth muscleUnit 2: Involuntary, non-striated muscle found in the walls of internal organs such as the gut and blood vessels.
social behaviourUnit 4: Behaviour involving interaction between members of the same species, often organised within a group.
social insectUnit 4: An insect that lives in an organised colony with cooperation and division of labour between individuals.
sodium ion channelUnit 3: A protein channel in the membrane of a neurone through which sodium ions can pass, often voltage-gated, allowing depolarisation during an action potential.
sodium ionsUnit 1: Positively charged sodium particles (Na+) that are actively transported across membranes and are important in processes such as co-transport and the establishment of membrane potentials.
soil erosionUnit 3: The wearing away and removal of the surface layer of soil by wind or water, often worsened by the loss of vegetation cover.
solubleUnit 1: Describes a substance that is able to dissolve in a given solvent, such as water.
soluteUnit 1: A substance that is dissolved in a solvent to form a solution.
solute potentialUnit 1: A measure of the reduction in water potential of a solution caused by the presence of dissolved solute; it has a negative value, becoming more negative as solute concentration increases.
solutionUnit 1: A mixture formed when a solute is fully dissolved in a solvent.
solvent (polar solvent)Unit 1: A liquid in which solutes dissolve; water is a polar solvent whose charged regions surround and separate ions and polar molecules, allowing them to dissolve and be transported.
somatic gene therapyUnit 4: The introduction of a functioning allele into the body cells of an individual to treat a genetic disorder; the change is not passed to offspring.
somatic nervous systemUnit 4: The part of the peripheral nervous system that carries impulses to skeletal muscles and controls voluntary movement.
sourceUnit 2: A region of a plant where organic solutes are made or released, such as a photosynthesising leaf, from which they are translocated.
spatial awarenessUnit 4: An animal’s perception of the position of objects and itself within its surroundings.
spatial memoryUnit 4: The store of information about the position of places and objects in the environment.
specialised cellsUnit 1: Cells that have become differentiated, developing a particular structure suited to carrying out a specific function.
speciationUnit 5: The formation of one or more new species from an existing one, occurring when populations become reproductively isolated and their gene pools diverge.
speciesUnit 1: A group of similar organisms that can interbreed to produce fertile offspring.
specific heat capacityUnit 1: The amount of energy required to raise the temperature of 1 g (or 1 kg) of a substance by 1 degree Celsius.
specimenUnit 5: A single sample of an organism, tissue or material that is collected, prepared or examined in a study.
spermUnit 4: The male gamete, a small motile haploid cell that fertilises the female gamete.
spermatidUnit 5: A haploid cell formed during spermatogenesis that develops into a mature spermatozoon without further division.
spermatogenesisUnit 4: The formation and development of spermatozoa from germ cells in the testes.
spermatogoniaUnit 4: The diploid stem cells in the testes that divide to give rise to spermatozoa.
spinal cordUnit 3: The column of nervous tissue running within the backbone that connects the brain to the rest of the body and coordinates many reflexes.
spindleCore terms: A structure of protein microtubules formed during cell division that attaches to chromosomes and moves them to opposite poles of the cell.
spindle fibresUnit 1: Threads made of microtubules that form during cell division and attach to the centromeres of chromosomes, pulling them towards the poles of the cell.
spinous processUnit 4: The backward-pointing projection of a vertebra to which muscles and ligaments attach.
spiracleUnit 2: A small external opening on the body of an insect through which air enters and leaves the tracheal system.
spongy boneUnit 4: The lighter, porous bone tissue, made of a network of bony struts, found mainly at the ends of long bones and inside flat bones.
spongy mesophyllUnit 2: A layer of loosely packed cells with large air spaces in the lower part of a leaf, allowing gas exchange and some photosynthesis.
spread (of disease)Unit 4: The transmission of a disease from one host to another through a population.
squamous epitheliumUnit 1: An epithelium made of flattened, thin cells, providing a short diffusion pathway suited to surfaces where rapid exchange occurs, such as the walls of the alveoli.
squash techniqueUnit 5: A method of preparing a microscope slide in which a softened piece of tissue is pressed flat to spread the cells into a single layer for viewing.
stage (of a pathway)Unit 3: A distinct step in a biochemical pathway, such as glycolysis or the Krebs cycle in respiration.
stalked particleUnit 3: A structure on the inner mitochondrial membrane containing ATP synthetase, where ATP is made as protons flow through during chemiosmosis.
standard deviationUnit 4: A measure of the spread of a set of data about the mean; a larger value indicates greater variation in the data.
standard errorUnit 5: A measure of how much a sample mean is likely to differ from the true population mean, used to indicate the reliability of the mean.
standard formUnit 3: A way of writing very large or very small numbers as a value between 1 and 10 multiplied by a power of ten.
starchUnit 2: A polysaccharide made of many glucose units that acts as the main energy storage carbohydrate in plants.
stationary phaseUnit 3: The phase of population growth in which the rate of reproduction equals the rate of death, so the population size stays roughly constant at the carrying capacity.
statistical testUnit 5: A mathematical procedure used to decide whether a result is likely to be due to chance or shows a genuine effect.
stem cellUnit 4: An unspecialised cell that can divide repeatedly and develop into one or more specialised cell types.
sterileUnit 3: Free from all living microorganisms and their spores.
sterile (genetics)Unit 4: Unable to produce viable offspring.
sticky endsUnit 4: The short, unpaired single-stranded sequences of bases left at the cut ends of a DNA molecule after it has been cut by a restriction enzyme, allowing complementary fragments to join.
stigmaUnit 4: The receptive surface at the top of the carpel of a flower on which pollen grains land.
stimulusUnit 3: A detectable change in the internal or external environment that produces a response in an organism.
stoma / stomataUnit 2: A pore in the lower epidermis of a leaf, bounded by two guard cells, through which gases are exchanged and water vapour is lost.
stomatal densityUnit 2: The number of stomata per unit area of a leaf surface.
stomatal indexUnit 2: A measure of stomatal frequency expressed as the proportion of epidermal cells that are stomata in a given area.
stop codonUnit 1: A triplet of bases on mRNA that does not code for an amino acid but signals the end of translation, halting the assembly of the polypeptide chain.
stratified epitheliumUnit 1: An epithelium made of several layers of cells, providing protection at surfaces subject to wear, such as the lining of the oesophagus.
striated muscleUnit 4: Skeletal muscle that appears banded under the microscope because of the regular arrangement of actin and myosin filaments.
strip fellingUnit 3: A method of managing woodland in which trees are removed in narrow strips, allowing surrounding areas to seed and regenerate the cleared ground.
stroke (medical)Unit 4: Damage to part of the brain caused by an interruption to its blood supply, either by a blockage or a bleed.
stroke volumeUnit 2: The volume of blood pumped out of a ventricle of the heart during a single contraction.
stromaUnit 3: The fluid-filled matrix surrounding the thylakoids inside a chloroplast, where the light-independent reactions of photosynthesis take place.
stroma (ovary)Unit 5: The supporting connective tissue of the ovary in which the follicles are embedded.
structural formulaUnit 1: A representation of a molecule that shows how its atoms are arranged and bonded together.
structural isomersUnit 1: Molecules that have the same molecular formula but a different arrangement of their atoms.
Student’s t-testUnit 5: A statistical test used to determine whether the means of two sets of data differ significantly from one another.
style (flower)Unit 4: The stalk of the carpel that connects the stigma to the ovary in a flower.
suberinUnit 2: A waxy, waterproof substance deposited in certain plant cell walls, such as those of cork and the endodermis, that makes them impermeable to water.
subjectiveUnit 5: Describing an observation or judgement that is influenced by personal opinion rather than measured objectively.
submissiveUnit 4: Describes behaviour in which an animal yields to a more dominant individual, reducing conflict within a group.
substrateUnit 1: The specific reactant molecule on which an enzyme acts, binding to the enzyme’s active site to form an enzyme-substrate complex.
successful collisionUnit 1: A collision between reacting particles, or between an enzyme and its substrate, that occurs with the correct orientation and sufficient energy for a reaction to take place.
succinic acidUnit 3: A four-carbon organic acid that is one of the intermediate compounds formed during the Krebs cycle in aerobic respiration.
sucraseUnit 1: The enzyme that catalyses the hydrolysis of sucrose into the monosaccharides glucose and fructose.
sucroseUnit 1: A disaccharide formed by condensation of glucose and fructose; it is the main form in which sugar is transported in plants.
sugar-phosphate backboneUnit 1: The repeating chain of alternating deoxyribose (or ribose) sugars and phosphate groups linked by phosphodiester bonds that forms the structural support of a nucleic acid strand.
sulfur bacteriaUnit 3: Bacteria that obtain energy by oxidising sulfur or sulfur-containing compounds, contributing to recycling of sulfur in ecosystems.
sun leafUnit 2: A leaf adapted to high light intensity, typically thicker with a well-developed palisade layer, found on the exposed parts of a plant.
sunken stomataUnit 2: Stomata located in pits below the leaf surface, an adaptation that traps moist air and reduces water loss by transpiration.
surface area : volume ratioUnit 2: The ratio of an organism’s or structure’s surface area to its volume, which decreases as size increases and influences the need for specialised exchange surfaces.
surfactantUnit 2: A substance secreted by cells lining the alveoli that lowers surface tension, preventing the alveoli from collapsing and making lung inflation easier.
surveyUnit 3: A method of sampling an area to estimate the abundance or distribution of organisms present in a habitat.
susceptibleUnit 4: Liable to be harmed by, or to contract, a particular disease or other factor; lacking resistance.
suspensionUnit 3: A mixture in which solid particles are dispersed throughout a liquid but are not dissolved, and which may settle out over time.
sustainableUnit 3: Describes use of a resource at a rate that allows it to be maintained or replaced, so that it remains available for the future.
symbiosisUnit 3: A close and long-term association between two organisms of different species living together.
sympathetic divisionUnit 4: The part of the autonomic nervous system that prepares the body for activity, producing responses such as an increased heart rate.
sympatric speciationUnit 5: The formation of new species from populations that live in the same area, becoming reproductively isolated without any geographical barrier.
symplast pathwayUnit 2: The route by which water moves through a plant via the cytoplasm of cells, passing from cell to cell through the plasmodesmata.
synapseUnit 4: The junction between two neurones, across which a nerve impulse is transmitted by means of a chemical neurotransmitter.
synapsisUnit 1: The pairing of homologous chromosomes during prophase I of meiosis, forming bivalents and allowing crossing over to occur.
synaptic cleftUnit 3: The narrow gap between the membranes of two neurones at a synapse, across which neurotransmitter diffuses.
synaptic knobUnit 3: The swollen ending of a presynaptic neurone that contains vesicles of neurotransmitter and releases them into the synaptic cleft.
synaptic pruningUnit 4: The process during brain development in which unused or weakly stimulated synapses are eliminated, leaving behind more efficient networks of neural connections.
synaptic transmissionUnit 3: The process by which an impulse is passed across a synapse: an action potential triggers calcium entry, vesicles release neurotransmitter into the cleft, and it binds to receptors on the postsynaptic membrane.
synaptic vesicleUnit 3: A small membrane-bound sac in the synaptic knob that stores neurotransmitter and releases it by exocytosis when an impulse arrives.
synergidUnit 4: One of two cells found alongside the egg cell in the embryo sac of a flowering plant, involved in guiding the pollen tube.
synovial jointUnit 4: A freely movable joint in which the bones are separated by a fluid-filled cavity lined with cartilage and enclosed by a capsule, allowing low-friction movement.
syphilisUnit 3: A sexually transmitted infection caused by a bacterium.
systemic circulationUnit 2: The part of the double circulation that carries oxygenated blood from the heart to the body tissues and returns deoxygenated blood to the heart.

T

t valueUnit 4: The statistic calculated in a Student’s t-test, used to judge whether the difference between two means is statistically significant.
T-tubulesUnit 4: Infoldings of the muscle fibre membrane that conduct the wave of depolarisation deep into the fibre, triggering release of calcium ions from the sarcoplasmic reticulum.
tangentUnit 3: A straight line that just touches a curve at a single point, used to estimate the rate of change at that point on a graph.
tapetumUnit 4: A nutritive layer of cells lining the pollen sac of an anther that supplies materials to the developing pollen grains.
target cellUnit 3: A cell that has the specific receptors needed to respond to a particular hormone or signalling molecule.
target sequenceUnit 4: The specific length of DNA that is to be copied, located, or amplified, for example by a complementary primer in PCR.
taxonUnit 3: Any named group used in a system of biological classification, such as a kingdom, phylum, genus or species.
taxonomyUnit 3: The branch of biology concerned with the identification, naming and classification of living organisms.
tearsUnit 1: The watery secretion that bathes the surface of the eye and contains the enzyme lysozyme, which destroys bacteria and so acts as a defence against infection.
telophaseUnit 1: The final stage of mitosis in which the separated chromosomes reach the poles of the cell, the spindle breaks down and new nuclear envelopes form around each set of chromosomes.
templateUnit 1: A strand of DNA whose base sequence is used as a pattern for the synthesis of a complementary strand of DNA or RNA.
temporal lobeUnit 4: A region of the cerebral cortex associated with language, learning, memory, and the processing of auditory information.
tendonUnit 4: A band of tough, inelastic connective tissue that attaches a muscle to a bone, transmitting the force of contraction to move the skeleton.
terminal peptide bondUnit 2: The peptide bond at the free end of a peptide chain (the amino or carboxyl end), which is hydrolysed by exopeptidases during protein digestion.
terrestrialUnit 2: Describing an organism that lives on land rather than in water.
territoryUnit 4: An area defended by an animal against others of the same species, securing resources such as food, mates, or breeding sites.
tertiary structureUnit 1: The specific three-dimensional shape of a protein produced by folding of the polypeptide chain, held in place by bonds between R groups; in an enzyme this folding creates a specifically shaped active site.
testaUnit 4: The tough protective outer coat of a seed, formed from the integuments of the ovule.
testesUnit 4: The male reproductive organs that produce sperm by meiosis and secrete the hormone testosterone.
testisUnit 5: The male reproductive organ that produces sperm and secretes testosterone.
testosteroneUnit 4: The male sex hormone, produced mainly in the testes, that stimulates sperm production and the development of male secondary sexual characteristics.
tetradUnit 4: A group of four haploid cells produced from one cell by meiosis, or a paired pair of homologous chromosomes (bivalent) seen during meiosis.
tetraploidUnit 4: Having four complete sets of chromosomes in the nucleus rather than the normal two.
thecaUnit 5: The layer of cells surrounding a developing ovarian follicle that contributes to the production of hormones.
thermally stableUnit 4: Able to keep its structure and function over a range of temperatures without denaturing or breaking down.
third order leverUnit 4: A lever in which the effort is applied between the load and the fulcrum, as in the action of the human forearm.
thoracicUnit 4: Relating to the chest region; thoracic vertebrae bear facets that articulate with the ribs.
thoraxUnit 2: The region of an animal’s body between the head and the abdomen; in insects it bears the legs and wings and carries some of the spiracles.
three-parent babyUnit 4: A child produced by a technique in which faulty mitochondria in an egg are replaced with healthy mitochondria from a donor, so the offspring inherits nuclear DNA from two parents and mitochondrial DNA from a third.
threonineUnit 1: One of the amino acids used in protein synthesis.
threshold potentialUnit 3: The level of depolarisation that a membrane must reach for an action potential to be generated; stimuli below this value produce no impulse.
thrombinUnit 1: The enzyme that catalyses the conversion of soluble fibrinogen into insoluble fibrin during blood clotting.
thylakoidUnit 3: A flattened membrane-bound sac inside a chloroplast that contains the photosynthetic pigments and is the site of the light-dependent reactions.
thylakoid space / lumenUnit 3: The fluid-filled interior of a thylakoid, where protons accumulate to form the gradient used in chemiosmosis.
thymineUnit 1: A pyrimidine base found in DNA that pairs with adenine through two hydrogen bonds.
tibiaUnit 4: The larger of the two long bones of the lower leg, articulating with the femur at the knee.
tissue fluidUnit 3: The fluid that surrounds and bathes the cells of the body, formed when plasma is forced out of the capillaries; it supplies cells with oxygen and nutrients and removes their waste.
tissue rejectionUnit 4: The immune response in which a recipient’s immune system recognises a transplanted tissue or organ as foreign and attacks it.
tissue repairUnit 2: The replacement or regeneration of damaged cells and tissues to restore normal structure and function.
tissuesUnit 2: Groups of similar cells working together to carry out a particular function.
tonoplastUnit 5: The membrane surrounding the large central vacuole of a plant cell, controlling movement of substances between the vacuole and cytoplasm.
total countUnit 3: An estimate of the total number of cells in a sample, including both living and dead cells.
totipotentUnit 4: Describing a cell that can divide and differentiate into any cell type, including the cells of the placenta, and so give rise to a complete organism.
toxinUnit 3: A poisonous substance produced by a living organism, such as a pathogenic bacterium, that damages the host’s cells or tissues.
tracheaCore terms: The windpipe; the airway, strengthened by rings of cartilage, that carries air between the larynx and the bronchi.
tracheal systemUnit 2: The network of air-filled tubes through which insects exchange gases, carrying air from the spiracles to the tissues via tracheae and tracheoles.
tracheidUnit 2: An elongated, tapering, lignified cell that conducts water in the xylem of ferns, conifers and flowering plants.
tracheoleUnit 2: The finest, blind-ending branches of an insect’s tracheal system that reach the cells and are the main site of gas exchange.
transaminationUnit 3: The transfer of an amino group from an amino acid to another molecule, allowing the liver to produce non-essential amino acids.
transcriptionUnit 4: The process in which a complementary strand of mRNA is synthesised from a DNA template in the nucleus, catalysed by RNA polymerase.
transcytosisUnit 1: The transport of material across a cell by enclosing it in vesicles, taking it in by endocytosis on one side and releasing it by exocytosis on the other.
transectUnit 5: A line or belt laid across a habitat along which the distribution of organisms is recorded to study how it changes with the environment.
transfer RNA (tRNA)Unit 1: A small single-stranded RNA molecule that carries a specific amino acid to the ribosome and has an anticodon that pairs with a complementary mRNA codon during translation.
transgenicUnit 4: Describing an organism that contains one or more genes that have been artificially introduced from a different species.
transition temperatureUnit 1: The temperature at which the membrane lipids change between a more fluid and a more rigid (gel) state, affecting membrane permeability.
translationUnit 1: The stage of protein synthesis in which the base sequence of mRNA is read by ribosomes and used to assemble amino acids into a polypeptide in the correct order.
translocationCore terms: The movement of soluble organic products of photosynthesis, such as sucrose, through the phloem from sources to sinks.
transpirationUnit 2: The evaporation and loss of water vapour from the leaves and other aerial parts of a plant, mainly through the stomata, which draws water up through the xylem in the transpiration stream.
transverse processUnit 4: A bony projection extending sideways from each side of a vertebra that provides a site for muscle and rib attachment.
transverse sectionUnit 2: A cut made at right angles to the long axis of a structure, used to show its internal tissues in cross-section under the microscope.
traumaUnit 4: Physical injury or damage to body tissues caused by an external force.
trendUnit 5: A general pattern or direction shown by a set of data over a range of values.
tricepsUnit 4: The muscle at the back of the upper arm that extends the forearm, acting antagonistically to the biceps.
tricuspid valveUnit 2: The atrioventricular valve between the right atrium and right ventricle, with three flaps, that prevents backflow of blood into the atrium.
triglycerideUnit 1: A lipid formed by the condensation of one glycerol molecule with three fatty acids, joined by ester bonds.
triose phosphate (TP)Unit 3: A three-carbon phosphorylated sugar produced as an intermediate in glycolysis and in the Calvin cycle of photosynthesis.
triplet codeUnit 1: The genetic code in which a sequence of three bases codes for one amino acid, giving 64 possible combinations, more than enough to specify the 20 amino acids.
triploidUnit 4: Having three complete sets of chromosomes in the nucleus, as in the endosperm tissue of a flowering plant seed.
trisomyUnit 4: A condition in which a cell has three copies of a particular chromosome instead of the usual two.
trophic levelUnit 3: A feeding level in a food chain – the position of an organism according to how many times energy has been transferred from the original source along the chain (for example producer, primary consumer, secondary consumer).
trophoblastUnit 4: The outer layer of cells of a blastocyst, which attaches to and implants in the uterine lining and contributes to the formation of the placenta.
tropomyosinUnit 4: An accessory protein wound around the actin filament of striated muscle that, at rest, blocks the myosin-binding sites.
troponinUnit 4: An accessory protein attached at intervals along the actin filament that, on binding calcium ions, changes shape and moves tropomyosin to expose the myosin-binding sites.
true stomachUnit 2: The fourth chamber of a ruminant’s stomach (the abomasum), where enzymic digestion occurs as in a non-ruminant stomach.
trypsinUnit 2: An endopeptidase secreted by the pancreas as inactive trypsinogen, which hydrolyses internal peptide bonds within proteins in the small intestine.
tube nucleusUnit 4: The nucleus of a germinating pollen grain that controls the growth of the pollen tube down through the style.
tuberculosis (TB)Unit 4: An infectious bacterial disease of the lungs caused by Mycobacterium, spread by airborne droplets.
tubulinUnit 1: The globular protein from which microtubules and spindle fibres are assembled.
tumourUnit 1: A mass of cells formed by uncontrolled cell division.
tumour suppressor geneUnit 4: A gene that normally restrains cell division; its loss or inactivation can allow uncontrolled division and the formation of a tumour.
tunica (vessel layer)Unit 5: One of the concentric layers that make up the wall of a blood vessel.
tunica externaUnit 2: The outer layer of an artery or vein, made largely of collagen and elastic fibres, which gives the vessel strength and support.
tunica intimaUnit 2: The innermost lining of a blood vessel, consisting of a smooth layer of endothelium that reduces resistance to blood flow.
tunica mediaUnit 2: The middle layer of an artery or vein, composed of smooth muscle and elastic tissue, which controls the diameter of the vessel.
turgidUnit 1: Describing a plant cell that holds as much water as possible, its protoplast pushing firmly against the cell wall so that further entry of water is prevented.
turgor pressureUnit 2: The outward pressure exerted by the contents of a plant cell against its cell wall when water enters by osmosis, keeping the cell firm.
Turner’s syndromeUnit 4: A condition in females caused by the inheritance of only one X chromosome (XO), resulting from non-disjunction during meiosis.
two-tailed testUnit 4: A statistical test used when a difference between two sets of data could lie in either direction, with no prediction of which way it will go.

U

ulcerUnit 3: An open sore that forms where the surface lining of a tissue, such as the lining of the gut, breaks down.
ultrafiltrationUnit 3: The filtration of blood under high pressure in the glomerulus, forcing small molecules and ions out into the Bowman’s capsule while retaining blood cells and large proteins.
ultravioletUnit 3: Electromagnetic radiation of shorter wavelength than visible light, which carries enough energy to damage DNA.
umbilical arteryUnit 4: A blood vessel that carries deoxygenated, waste-laden blood from the foetus to the placenta.
umbilical cordUnit 4: The structure connecting the foetus to the placenta, carrying blood between them to supply nutrients and remove waste.
umbilical veinUnit 4: A blood vessel that carries oxygenated, nutrient-rich blood from the placenta to the foetus.
uncontrolled cell divisionUnit 1: Cell division that is no longer regulated, producing a mass of cells that may form a tumour.
undifferentiatedUnit 4: Describing a cell that has not yet become specialised for a particular function.
unicellularUnit 1: Describes an organism that consists of a single cell.
universal energy currencyUnit 1: A description of ATP as the molecule used by all cells to transfer energy from energy-releasing reactions to energy-requiring processes.
unlinked genesUnit 4: Genes located on different chromosomes, or far apart on the same chromosome, so that they are inherited independently.
unmyelinatedUnit 3: Describes a nerve fibre that lacks a myelin sheath, so that the impulse travels more slowly along it.
unsaturated fatty acidUnit 1: A fatty acid whose hydrocarbon chain contains one or more carbon-to-carbon double bonds, so it is not fully saturated with hydrogen.
uracilUnit 1: A pyrimidine base found in RNA in place of thymine, pairing with adenine.
ureaUnit 2: A soluble, relatively non-toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids and excreted by the kidneys.
ureterUnit 5: The tube that carries urine from the kidney to the bladder.
urethraUnit 4: The tube carrying urine from the bladder out of the body, which in males also conveys semen.
urinary systemUnit 5: The group of organs that produce, store and remove urine, including the kidneys, ureters, bladder and urethra.
urineUnit 1: The fluid produced by the excretory system, containing water and dissolved waste products such as urea, that is removed from the body.
uterus / wombUnit 4: The muscular organ in which a fertilised egg implants and the embryo and foetus develop during pregnancy.

V

vaccinationUnit 4: The deliberate introduction of antigens, usually a weakened or harmless form of a pathogen, to stimulate an immune response and lasting immunity without causing disease.
vaccineUnit 4: A preparation containing antigens, derived from a pathogen, that is introduced into the body to stimulate an immune response and produce immunity without causing the disease.
vacuoleUnit 1: A fluid-filled sac enclosed by a membrane within the cytoplasm; the large permanent vacuole of a plant cell contains cell sap and helps maintain turgor.
validUnit 3: Describes results or a conclusion that genuinely measure what was intended because the investigation was suitably designed and controlled.
validityUnit 4: The extent to which an investigation actually measures what it sets out to measure, with only the independent variable affecting the result.
valineUnit 4: One of the amino acids; a substitution of valine for glutamic acid in haemoglobin causes sickle-cell anaemia.
variableUnit 1: A factor in an investigation that can change or be changed; the independent variable is altered, the dependent variable is measured, and control variables are kept constant.
varianceUnit 4: A measure of the spread of a set of data about its mean, equal to the square of the standard deviation.
variationUnit 4: The differences in characteristics that exist between individuals of the same species, arising from genetic and environmental causes.
variegated leavesUnit 4: Leaves with patches that lack chlorophyll, so that only the green regions can photosynthesise.
vas deferensUnit 4: The duct that carries sperm from the testis towards the urethra.
vascular bundleUnit 3: A strand of transport tissue in a plant containing xylem and phloem, often together with supporting tissue.
vascular tissueUnit 2: The tissue in plants that conducts water and dissolved substances, made up of xylem and phloem.
vasoconstrictionUnit 2: The narrowing of a blood vessel’s lumen caused by contraction of the smooth muscle in its wall, which reduces blood flow through it.
vectorUnit 4: In genetic engineering, a vehicle such as a plasmid or virus used to carry a gene into a recipient cell; in disease, an organism that transmits a pathogen from one host to another.
veinUnit 5: A blood vessel that carries blood towards the heart, usually under low pressure and containing valves.
velocity of blood flowUnit 2: The speed at which blood moves through a vessel, which falls as the total cross-sectional area of the vessels increases.
ventilation mechanismUnit 2: The process by which an organism moves air or water over its respiratory surface to maintain a steep concentration gradient for gas exchange.
ventral rootUnit 5: The part of a spinal nerve that carries motor nerve fibres away from the spinal cord.
ventricleUnit 2: One of the two lower, thick-walled chambers of the heart that pump blood out into the arteries.
ventricular systoleUnit 2: The phase of the cardiac cycle in which the ventricles contract, raising the pressure of the blood and forcing it out into the arteries.
venuleUnit 2: A small vessel that collects blood from capillaries and carries it to a vein.
vertebraUnit 4: One of the bones forming the backbone, enclosing and protecting the spinal cord.
vertebral archUnit 4: The bony arch on the dorsal side of a vertebra that, with the body of the vertebra, encloses the spinal cord.
vertebral foramenUnit 4: The central opening of a vertebra through which the spinal cord passes.
vertebrateUnit 4: An animal possessing a backbone made up of vertebrae.
vesicleUnit 3: A small membrane-bound sac within a cell used to store or transport substances.
viable countUnit 3: An estimate of the number of living cells in a sample that are capable of growing and dividing.
villiUnit 2: Finger-like projections of the lining of the small intestine that greatly increase the surface area for absorption of digested food.
villusCore terms: One of many finger-like projections of the inner lining of the small intestine that increase the surface area for the absorption of digested food.
viral envelopeUnit 1: An outer membrane derived from the host cell membrane that surrounds some viruses and contains viral proteins.
virulentUnit 2: Describing a pathogen that is highly capable of causing serious disease.
virusUnit 1: A non-cellular infectious particle consisting of nucleic acid enclosed in a protein coat that can only replicate inside a living host cell.
vitamin CUnit 5: A water-soluble vitamin required for the formation of collagen and as an antioxidant, found chiefly in fresh fruit and vegetables.
Volkmann’s canalUnit 4: A small channel running across compact bone that carries blood vessels from the periosteum to connect with the central Haversian canals.
voltage-gated (channel)Unit 3: An ion channel in a cell membrane that opens or closes in response to a change in the membrane’s potential difference.

W

water potentialUnit 1: A measure of the tendency of water molecules to move; water moves from a region of higher (less negative) water potential to one of lower (more negative) water potential. Pure water has a water potential of zero, and adding solute lowers it to a negative value.
water uptakeUnit 2: The absorption of water by an organism, for example by the roots of a plant from the soil.
water vapourUnit 2: Water in its gaseous state, for example the water lost from a leaf during transpiration.
wave of excitationUnit 2: The spread of electrical activity that triggers contraction, as when an impulse passes from the sinoatrial node across the heart muscle.
wavelengthUnit 3: The distance between two corresponding points on consecutive waves, for example of light or other electromagnetic radiation.
waxy cuticleUnit 2: A waterproof layer of wax covering the epidermis of a leaf or stem that reduces water loss by evaporation.
wet mountUnit 5: A temporary microscope slide preparation in which the specimen is suspended in a drop of liquid under a cover slip.
white adipose tissueUnit 3: A type of fat-storing connective tissue in which cells hold a single large droplet of lipid, acting as an energy store.
white matterUnit 5: Nervous tissue consisting mainly of myelinated nerve fibres, giving it a pale appearance.
WHO (World Health Organisation)Unit 4: The agency of the United Nations responsible for coordinating international public health and disease control.
wind-pollinatedUnit 4: Describing a flower adapted to transfer pollen from one plant to another by means of the wind.
woundUnit 4: An injury in which the skin or other body tissue is broken or damaged.

X

X chromosomeUnit 4: One of the two sex chromosomes; females have two X chromosomes and males have one X and one Y.
X-linkedUnit 4: Describing a gene or characteristic carried on the X chromosome, so that its pattern of inheritance differs between the sexes.
X-SCIDUnit 4: A severe inherited disorder of the immune system, caused by a faulty gene on the X chromosome, in which functioning lymphocytes fail to develop.
xeromorphicUnit 2: Describing structural features of a plant that reduce water loss and adapt it to survive in dry conditions.
xerophyteUnit 2: A plant adapted to live in dry conditions, having features that reduce water loss, as shown by marram grass.
xylemUnit 2: The water-conducting tissue of a plant, made of dead, lignified cells joined end to end to form hollow tubes that carry water and dissolved minerals upwards from the roots.
xylem vesselUnit 5: A continuous hollow tube of dead, lignified cells that conducts water and dissolved minerals up through a plant.

Y

yieldUnit 3: The amount of useful product obtained from a process or crop, often expressed per unit area or per unit input.

Z

Z lineUnit 4: The boundary at each end of a sarcomere to which the actin filaments are attached.
Z schemeUnit 3: The zig-zag arrangement of electron carriers in the light-dependent stage of photosynthesis through which excited electrons pass during non-cyclic photophosphorylation, raising and lowering in energy as they move between photosystem II and photosystem I.
zona pellucidaUnit 4: The glycoprotein layer surrounding the secondary oocyte, which the sperm must penetrate during fertilisation.
zone of inhibitionUnit 4: The clear region around an antimicrobial agent on an agar plate where the growth of microorganisms has been prevented.
zygoteUnit 4: The diploid cell formed by the fusion of a male and a female gamete at fertilisation.

Definitions are written for WJEC & Eduqas AS & A-level Biology.

Accuracy & sources. These definitions are compiled from WJEC / Eduqas mark schemes, the official teaching guidance and specification, and written in original wording pitched to A-level. While care has been taken, no liability is accepted for any errors or omissions — always confirm a definition against your specification and an authoritative textbook before relying on it in an exam.