Biological Molecules – A-Level Biology Revision Notes
Comprehensive revision notes covering carbohydrates, lipids, proteins, nucleic acids and water. Written by a Chartered Biologist and former WJEC/Eduqas & Edexcel examiner with 25+ years of teaching experience. Covers all UK exam boards with examiner insight into where students lose marks.
Last updated: February 2026
Why Biological Molecules Matters
Biological molecules is the foundation topic for A-Level Biology. Every exam board covers it in Year 12, and it underpins everything you will study afterwards – from cell membranes (phospholipids) to respiration (glucose) to genetics (nucleic acids). If you get this topic solid, the rest of the course makes far more sense.
Having taught this topic for over 25 years and examined it for both WJEC/Eduqas and Edexcel, I can tell you that the most common reason students lose marks here is not a lack of knowledge. It is imprecise language. Examiners are looking for very specific terminology, and near-enough answers often score zero. I have seen thousands of students write “same shape” instead of “complementary shape”, or describe lipids as “polymers” when they are categorically not. These are the kinds of errors that turn an A into a C.
On this page, I will take you through every aspect of biological molecules that your exam board requires. More importantly, I will tell you exactly where students lose marks and what the examiners actually want to see. This is the insight you will not get from a textbook.
Water – The Molecule That Makes Biology Possible
Water is the most abundant molecule in living organisms, making up between 65% and 95% of cell mass depending on cell type. Every chemical reaction in your body takes place in an aqueous environment. Before we look at the larger biological molecules, you need to understand why water is so uniquely suited to supporting life – and it all comes down to hydrogen bonding.
Structure and Polarity
A water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms. The oxygen atom is more electronegative than hydrogen, which means it pulls the shared electrons in each covalent bond closer to itself. This creates an uneven charge distribution: the oxygen carries a slight negative charge (δ−) and each hydrogen carries a slight positive charge (δ+). This makes water a polar molecule.
Because of this polarity, the δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring water molecule. This electrostatic attraction is called a hydrogen bond. Individually, hydrogen bonds are weak – roughly a twentieth of the strength of a covalent bond. But because water molecules form vast networks of hydrogen bonds simultaneously, the collective effect is enormous.
Properties of Water and Their Biological Significance
High specific heat capacity. Water requires a large amount of energy to raise its temperature. This is because much of the heat energy is used to break hydrogen bonds rather than increase molecular kinetic energy. For organisms, this means body temperature remains relatively stable even when the environment fluctuates. It is why aquatic habitats do not experience extreme temperature swings.
High latent heat of evaporation. A lot of energy is needed to evaporate water because hydrogen bonds must be broken to convert liquid water to vapour. This makes sweating and transpiration highly effective cooling mechanisms. When water evaporates from your skin, it takes substantial thermal energy with it.
Cohesion and surface tension. Hydrogen bonds between water molecules create strong cohesive forces, which means water molecules “stick” to each other. This is essential for water transport in the xylem of plants – the cohesion-tension theory relies on an unbroken column of water molecules being pulled upwards. At surfaces, cohesion creates surface tension, which is strong enough to support small organisms like pond skaters.
Solvent properties. Because water is polar, it can dissolve other polar molecules and ionic compounds. The δ+ and δ− regions interact with the charges on solute particles, separating and surrounding them. This is why water is described as a “universal solvent” – though that is a slight exaggeration, since non-polar molecules such as lipids do not dissolve in it.
Density and ice. Most liquids become denser as they cool. Water does too, but only down to 4°C. Below this temperature, hydrogen bonds hold water molecules in a rigid, open lattice structure – this is ice, and it is less dense than liquid water. That is why ice floats. The ecological significance is huge: floating ice insulates the water beneath it, allowing aquatic organisms to survive in winter.
Carbohydrates – From Simple Sugars to Storage Molecules
Carbohydrates are composed of carbon, hydrogen and oxygen, always in the ratio Cn(H2O)n – which is where the name “carbo-hydrate” comes from. They are classified by size: monosaccharides (single sugars), disaccharides (two sugars joined) and polysaccharides (long chains of sugars).
Monosaccharides
Monosaccharides are the monomers of carbohydrates. They are classified by the number of carbon atoms they contain: trioses (3C, such as glyceraldehyde), pentoses (5C, such as ribose and deoxyribose) and hexoses (6C, such as glucose, fructose and galactose). For A-Level, hexoses are the most important.
Glucose is the key molecule. It exists in two forms: α-glucose and β-glucose. The structural difference is at carbon 1 (C1) only. In α-glucose, the hydroxyl group (–OH) on C1 points below the plane of the ring. In β-glucose, it points above the ring. This seemingly tiny difference has massive consequences – it determines whether the resulting polysaccharide will be starch (from α-glucose) or cellulose (from β-glucose), giving completely different structures and functions.
Condensation and Hydrolysis
When two monosaccharides join together, a molecule of water is released. This is a condensation reaction, and the bond formed between the two sugars is called a glycosidic bond. To break this bond, water must be added back – this is hydrolysis (hydro = water, lysis = splitting).
You need to know the three disaccharides and which monosaccharides form them:
- Maltose = glucose + glucose (joined by a 1,4-glycosidic bond)
- Sucrose = glucose + fructose
- Lactose = glucose + galactose
Polysaccharides – Starch, Glycogen and Cellulose
Starch is the storage carbohydrate in plants. It is actually a mixture of two molecules: amylose and amylopectin. Amylose is an unbranched chain of α-glucose molecules joined by 1,4-glycosidic bonds, which coils into a helix. Amylopectin is branched, with 1,6-glycosidic bonds creating branch points roughly every 20–25 glucose units.
Starch is well suited to storage because: it is insoluble (so it does not affect water potential or diffuse out of cells), it is compact (the helical shape of amylose and the branching of amylopectin allow tight packing), and the branching of amylopectin provides many terminal glucose units that can be rapidly hydrolysed when energy is needed.
Glycogen is the storage carbohydrate in animals. It is structurally similar to amylopectin but much more highly branched. This is a critical distinction. Glycogen is not a mixture of two components – it is a single, heavily branched polymer of α-glucose. The extensive branching means it has many free ends that can be hydrolysed simultaneously, allowing very rapid glucose release. This suits the higher metabolic rate of animals compared to plants.
Cellulose is the structural carbohydrate in plant cell walls. It is made from β-glucose monomers. Because of the orientation of the –OH group in β-glucose, every other monomer must rotate 180° to form a glycosidic bond. This creates long, straight, unbranched chains. Adjacent cellulose chains form hydrogen bonds between their –OH groups, bundling together into microfibrils. These microfibrils are immensely strong and provide rigid structural support for the plant cell wall.
The key structural difference between starch and cellulose comes entirely from using α-glucose versus β-glucose. Starch coils because α-glucose allows a curved chain. Cellulose is straight because β-glucose forces the chain to alternate orientation, preventing coiling. Most organisms cannot digest cellulose because they lack the enzyme cellulase to break the β-1,4-glycosidic bonds. Ruminants like cattle can digest it only because of cellulase-producing bacteria in their gut.
Lipids – Triglycerides and Phospholipids
Lipids are a diverse group of biological molecules that share one key property: they are insoluble in water but soluble in organic solvents like ethanol. The two lipids you must know in detail are triglycerides and phospholipids.
Triglycerides
A triglyceride consists of one molecule of glycerol bonded to three fatty acid chains. Each fatty acid joins to glycerol by a condensation reaction, forming an ester bond and releasing a molecule of water. Three ester bonds form in total, so three water molecules are released when one triglyceride is assembled.
Fatty acids have a carboxyl group (–COOH) at one end and a long hydrocarbon tail. If the hydrocarbon tail contains only single bonds between carbon atoms, the fatty acid is saturated. If it contains one or more carbon-to-carbon double bonds (C=C), it is unsaturated (one double bond = monounsaturated; more than one = polyunsaturated).
Triglycerides are excellent energy storage molecules because: the long hydrocarbon tails contain many C–H bonds (which release energy when oxidised in respiration), they are insoluble in water (so they do not affect osmosis or water potential), they are compact (more energy per gram than carbohydrates or proteins), and they provide thermal insulation and buoyancy.
Phospholipids
Phospholipids have a similar structure to triglycerides, but one of the three fatty acid chains is replaced by a phosphate group. This gives the molecule two distinct regions: a hydrophilic (water-loving) head containing the phosphate and glycerol, and two hydrophobic (water-hating) fatty acid tails.
This amphipathic nature means that in water, phospholipids spontaneously arrange themselves into a bilayer – two rows of phospholipids with their hydrophobic tails facing inwards (away from water) and their hydrophilic heads facing outwards (towards the aqueous environment). This phospholipid bilayer is the structural basis of all cell membranes.
Proteins – Structure, Function and the Four Levels
Proteins are the most functionally diverse molecules in living organisms. They act as enzymes, antibodies, hormones, transport molecules, structural components and receptors. All proteins are built from the same set of roughly 20 amino acids, and it is the specific sequence and arrangement of these amino acids that determines what each protein does.
Amino Acid Structure
Every amino acid has the same general structure: a central carbon atom bonded to an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain). The R group is what makes each amino acid different – it can be as simple as a single hydrogen (glycine) or as complex as a ring structure (tryptophan). Some R groups are polar, some are non-polar, some contain sulphur, and some are positively or negatively charged. These chemical properties determine how the protein folds.
Peptide Bonds and Polypeptides
Amino acids join together by condensation reactions. The amino group of one amino acid reacts with the carboxyl group of another, releasing water and forming a peptide bond. Two amino acids joined together form a dipeptide. A chain of many amino acids joined by peptide bonds is a polypeptide.
The Four Levels of Protein Structure
Primary structure is the specific sequence of amino acids in the polypeptide chain, held together by peptide bonds. This sequence is determined by the gene that codes for the protein. Primary structure is absolutely critical because it determines all higher levels of structure – change even one amino acid, and the entire 3D shape can be affected (as in sickle cell anaemia, where a single amino acid substitution in haemoglobin changes the molecule’s behaviour dramatically).
Secondary structure arises when the polypeptide chain coils or folds into regular structures stabilised by hydrogen bonds between the C=O group of one amino acid and the N–H group of another further along the chain. The two forms are the α-helix (a right-handed coil) and the β-pleated sheet (where parallel sections of the chain lie side by side).
Tertiary structure is the overall 3D shape of a single polypeptide chain. It is maintained by interactions between the R groups of different amino acids. These include: hydrogen bonds (between polar R groups), ionic bonds (between positively and negatively charged R groups), disulfide bridges (strong covalent bonds between the sulphur atoms of two cysteine residues), and hydrophobic interactions (non-polar R groups cluster together in the interior of the protein, away from water). Tertiary structure gives the protein its specific shape, which is essential for its function.
Quaternary structure exists when a functional protein consists of two or more polypeptide subunits held together by the same types of bonds as tertiary structure. Haemoglobin is the classic example: it has four polypeptide subunits (two α chains and two β chains), each with its own haem prosthetic group containing iron. Quaternary structure also applies to proteins that contain non-polypeptide components (prosthetic groups).
Globular vs Fibrous Proteins
Globular proteins fold into compact, roughly spherical shapes. Their hydrophilic R groups face outward, making them generally soluble in water. Examples include enzymes, antibodies, haemoglobin and insulin. Their specific 3D shape is essential to their function.
Fibrous proteins are long, insoluble, and form structural components. Examples include collagen (tendons, bone matrix, skin), keratin (hair, nails) and elastin (blood vessel walls, skin). They consist of repeating units and are held together by many cross-links, giving them great tensile strength.
Nucleic Acids – DNA and RNA
Nucleic acids are the information-carrying molecules of life. DNA (deoxyribonucleic acid) stores the genetic instructions for building proteins, and RNA (ribonucleic acid) plays essential roles in reading and executing those instructions.
Nucleotide Structure
Both DNA and RNA are polymers of nucleotides. Each nucleotide consists of three components: a phosphate group, a pentose sugar, and a nitrogenous base. In DNA, the sugar is deoxyribose. In RNA, the sugar is ribose (which has one more oxygen atom than deoxyribose – specifically, an –OH group on carbon 2 rather than just –H).
The bases in DNA are adenine (A), thymine (T), guanine (G) and cytosine (C). In RNA, thymine is replaced by uracil (U). Adenine and guanine are purines (double-ring structures), while thymine, cytosine and uracil are pyrimidines (single-ring structures).
DNA Structure
Nucleotides join together by condensation reactions forming phosphodiester bonds between the phosphate group of one nucleotide and the sugar of the next. This creates a sugar-phosphate backbone. DNA consists of two such polynucleotide strands running in opposite directions (antiparallel), held together by hydrogen bonds between complementary base pairs: A pairs with T (two hydrogen bonds), and G pairs with C (three hydrogen bonds). This is complementary base pairing.
The two strands twist around each other to form the double helix. Because a purine always pairs with a pyrimidine, the width of the helix is constant. The complementary base pairing means that if you know the sequence of one strand, you can determine the sequence of the other.
DNA Replication
DNA replicates by a semi-conservative mechanism, as demonstrated by Meselson and Stahl (required for Edexcel A). The key steps are:
Unwinding
DNA helicase breaks the hydrogen bonds between complementary base pairs, separating the two strands and “unzipping” the double helix.
Base Pairing
Free DNA nucleotides align along each exposed template strand by complementary base pairing – A with T, G with C.
Joining
DNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides, building the new complementary strand. Edexcel B also requires knowledge of DNA ligase, which joins Okazaki fragments on the lagging strand.
Each new DNA molecule contains one original strand and one newly synthesised strand – hence semi-conservative replication.
Biochemical Tests – How to Identify Biological Molecules
Every exam board expects you to know the qualitative food tests. These come up regularly as short-answer questions and in practical-based questions. Here are the tests you need, with the precise detail examiners require.
| Molecule | Test Reagent | Method | Positive Result |
|---|---|---|---|
| Reducing sugars (glucose, maltose, lactose) | Benedict’s reagent | Add Benedict’s to sample, heat in water bath at 70–90°C for 5 minutes | Blue → green → yellow → orange → brick-red precipitate |
| Non-reducing sugars (sucrose) | HCl then Benedict’s | 1) Boil sample with dilute HCl to hydrolyse. 2) Neutralise with NaHCO3. 3) Add Benedict’s and heat as above | Colour change as above (after negative initial Benedict’s test) |
| Starch | Iodine in KI solution | Add drops of iodine solution to sample at room temperature | Brown/orange → blue-black |
| Proteins | Biuret reagent (NaOH + CuSO4) | Add NaOH to make alkaline, then add dilute CuSO4 drop by drop. Mix gently | Blue → purple/lilac |
| Lipids | Ethanol (emulsion test) | Dissolve sample in ethanol, then pour ethanol into water | Milky white emulsion |
Exam Board Comparison – What Your Board Requires
Biological molecules is covered by every exam board, but the scope and depth varies significantly. This matters because you should not waste time studying content that your board does not examine. Here are the key differences.
| Content | AQA | OCR A | Edexcel A | Edexcel B | WJEC/Eduqas |
|---|---|---|---|---|---|
| Enzymes within this topic? | Yes | Separate (2.1.4) | Topic 2 | Yes | Separate (CC4) |
| Enzyme model required | Induced fit only | Both lock-and-key & induced fit | Both | Induced fit | Both (lysozyme named) |
| Cholesterol | No | Yes | Yes (HDL/LDL) | No | No |
| Named fibrous proteins | No | Yes (collagen, keratin, elastin) | No | No | No |
| Cofactors & coenzymes | No | Yes | No | No | No |
| DNA ligase | No | No | No | Yes | No |
| Chitin | No | No | No | No | Yes |
| Protein synthesis in this topic? | No (Topic 4) | Yes (Module 2) | Yes (Topic 2) | Yes | No (CC5) |
| Inorganic ions detail | H⁺, Fe²⁺, Na⁺, PO₄³⁻ | Extensive (10+ ions) | Limited | Plant-focused | Limited |
If you are on OCR A, you have the most content-heavy version of this topic. AQA and Edexcel B embed enzymes within biological molecules, which means more content in one topic but fewer separate enzyme questions. WJEC and Eduqas are the only boards requiring chitin as a structural polysaccharide. Edexcel A is unusual because it distributes biological molecules across three different topics rather than covering it all in one place.
Common Mistakes – Where Students Lose Marks
Having marked thousands of exam scripts, I can tell you exactly which mistakes come up over and over again. Learn to avoid these and you will immediately outperform most other candidates.
| # | The Mistake | Why It Costs Marks |
|---|---|---|
| 1 | Writing “same shape” instead of “complementary shape” | When describing enzyme-substrate fit, the substrate does not have the same shape as the active site – it has a complementary (matching) shape. This is the single most common error across all exam boards. |
| 2 | Calling lipids “polymers” | Lipids are not made from repeating monomer units. Triglycerides consist of glycerol + three fatty acids – these are different molecules, not repeating subunits. Polymers are polysaccharides, polypeptides and polynucleotides. |
| 3 | Saying glycogen contains “amylose and amylopectin” | Glycogen is a single, highly branched molecule. Only starch is a mixture of amylose and amylopectin. This confusion between plant and animal storage molecules appears in examiner reports every year. |
| 4 | Saying “no double bonds” for saturated fatty acids | The carboxyl group (COOH) contains a C=O double bond. The correct phrase is “no carbon-to-carbon double bonds.” |
| 5 | Mentioning ionic bonds or disulfide bridges for secondary structure | Secondary structure is maintained by hydrogen bonds ONLY (between backbone C=O and N–H groups). R group interactions maintain tertiary structure. |
| 6 | Saying enzymes are “killed” by high temperature | Enzymes are not alive. They are denatured – their 3D shape changes permanently because the bonds maintaining tertiary structure are broken. |
| 7 | Saying hydrogen bonds are “hydrolysed” in DNA replication | Hydrogen bonds are broken, not hydrolysed. Hydrolysis involves water breaking covalent bonds. Hydrogen bonds are weak intermolecular attractions that are simply disrupted by helicase. |
| 8 | Spelling “thymine” as “thyamine” | Thyamine does not exist. Thiamine is vitamin B1 – a completely different molecule. OCR and AQA mark schemes explicitly reject this misspelling. |
Struggling With Biological Molecules?
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Google Frequently Asked Questions About Biological Molecules
The only structural difference is at carbon 1 (C1). In α-glucose, the hydroxyl group (–OH) on C1 points below the plane of the ring. In β-glucose, it points above the ring. This seemingly small difference determines whether the resulting polysaccharide is starch (α-glucose, which coils) or cellulose (β-glucose, which forms straight chains). You must be able to draw both ring structures showing this difference at C1.
Polymers are large molecules made from many repeating identical or similar monomer subunits joined by the same type of bond – think of polysaccharides (many glucose monomers) or polypeptides (many amino acids). A triglyceride is made from one glycerol and three fatty acids. These are not repeating units of the same type. Additionally, three fatty acids is not “many” repeating units. This is why lipids are described as macromolecules but not polymers.
Both are storage polysaccharides made from α-glucose. Starch is the plant storage molecule and is a mixture of two components: amylose (unbranched, helical) and amylopectin (branched). Glycogen is the animal storage molecule and is a single, highly branched molecule – more heavily branched than amylopectin. Glycogen does not contain amylose. The extensive branching of glycogen provides many terminal glucose units for rapid hydrolysis, suiting the higher metabolic demands of animals.
Primary: peptide bonds between amino acids. Secondary: hydrogen bonds between backbone C=O and N–H groups only. Tertiary: hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups. Quaternary: the same types of bonds as tertiary, holding two or more polypeptide subunits together. A very common exam mistake is mentioning ionic bonds or disulfide bridges for secondary structure – these only appear in tertiary and quaternary.
This is a three-step test. First, boil the sample with dilute hydrochloric acid to hydrolyse the glycosidic bond (acid hydrolysis). Second, neutralise the solution by adding sodium hydrogencarbonate (NaHCO3) – this step is essential because Benedict’s reagent is alkaline and the acid would neutralise it. Third, add Benedict’s reagent and heat in a water bath. A colour change from blue through green, yellow, orange to brick-red indicates that reducing sugars are now present (released by hydrolysis of the non-reducing sugar).
No – there are significant differences. AQA and Edexcel B include enzymes within biological molecules; OCR A and WJEC/Eduqas cover enzymes separately. OCR A requires cholesterol, named fibrous proteins (collagen, keratin, elastin) and cofactors/coenzymes, which other boards do not. WJEC and Eduqas are the only boards requiring chitin. Edexcel B uniquely requires DNA ligase. Check your specific exam board section on this page to see exactly what you need to study.
When water cools below 4°C, hydrogen bonds hold the molecules in a rigid, open lattice structure. This means the molecules are spaced further apart than in liquid water, so ice is less dense and floats. Ecologically, this is vital: floating ice forms an insulating layer on the surface of lakes and rivers, preventing the water beneath from freezing solid and allowing aquatic organisms to survive through winter.
Denaturation is the permanent change in a protein’s 3D shape caused by breaking the bonds that maintain tertiary and quaternary structure (hydrogen bonds, ionic bonds, disulfide bridges). It can be caused by high temperatures or extreme pH. The active site shape changes permanently, so the substrate can no longer bind. Inhibition is different – competitive inhibitors reversibly block the active site, and non-competitive inhibitors bind elsewhere and change the active site shape, but neither necessarily involves permanent structural damage.
Disclaimer: The information provided on this page is intended for educational guidance only. While every effort has been made to ensure accuracy, Biology Education and its author accept no responsibility for individual exam outcomes. Students are advised to consult their own teachers, tutors, and official exam board resources as part of their revision.