Cell Membrane Structure & Transport — A-Level Biology Revision Notes
Comprehensive revision notes covering the fluid mosaic model, phospholipid bilayer structure, membrane proteins, simple diffusion, facilitated diffusion, active transport, the Na⁺/K⁺ pump, osmosis in animal and plant cells, water potential calculations, and bulk transport. Written by Tyrone — Chartered Biologist and former WJEC/Eduqas & Edexcel examiner with 25+ years of teaching experience.
Last updated: February 2026
The Fluid Mosaic Model
The structure of the cell membrane was first described by Singer and Nicholson in 1972 as the fluid mosaic model. The membrane is approximately 7–8 nm thick and is the boundary that separates the intracellular contents from the extracellular environment. It is described as “fluid” because the phospholipids are constantly moving within the bilayer, and “mosaic” because the various proteins embedded in it create a pattern resembling a mosaic.
The Phospholipid Bilayer
The foundation of the cell membrane is the phospholipid bilayer. Phospholipids are amphiphilic molecules — they have a hydrophilic (water-loving) phosphate head and two hydrophobic (water-repelling) fatty acid tails. Because cells are surrounded by water both inside (intracellular) and outside (extracellular), the phospholipids spontaneously arrange as a double layer: the hydrophilic heads face outward towards the water on both sides, while the hydrophobic tails face inward, creating a hydrophobic core that excludes water.
This arrangement is critically important for membrane function. The hydrophobic core acts as a barrier that prevents water-soluble (polar, ionic, hydrophilic) substances from passing freely through the membrane. Only lipid-soluble (non-polar, hydrophobic) substances can dissolve in and pass through this region by simple diffusion.
Components of the Cell Membrane
Membrane Proteins
Proteins in the membrane are classified by their position. Extrinsic (peripheral) proteins sit on the surface of the membrane — they have hydrophilic amino acids around their entire circumference, allowing them to interact with the polar phospholipid heads and the surrounding water. Intrinsic (integral) proteins are embedded within the phospholipid bilayer — they have both hydrophilic regions (at the tips, interacting with the polar heads) and hydrophobic regions (in the middle, interacting with the fatty acid tails). Transmembrane proteins are intrinsic proteins that span the entire width of the membrane. These include channel proteins (with a polar pore through which ions and polar molecules can pass) and carrier proteins (with a binding site that changes shape to transport specific molecules).
Other Key Components
| Component | Structure | Function |
|---|---|---|
| Channel proteins | Transmembrane protein with polar pore | Allow facilitated diffusion of ions and polar molecules; substance does not bind |
| Carrier proteins | Transmembrane protein with binding site | Transport specific molecules by facilitated diffusion or active transport; substance binds to complementary site |
| Receptor proteins | Intrinsic or extrinsic protein with specific binding site | Bind hormones and chemical messengers; trigger cell responses |
| Glycoproteins | Protein with branching carbohydrate chains (glycocalyx) | Cell-to-cell recognition; act as antigens for cell identity |
| Glycolipids | Lipid with branching carbohydrate chains | Cell-to-cell recognition and signalling |
| Cholesterol | Amphiphilic molecule that inserts between phospholipids | Regulates membrane fluidity; increases flexibility and stability |
Factors Affecting Membrane Permeability
Temperature
As temperature increases, the phospholipids and proteins gain more kinetic energy. The phospholipids begin to move more rapidly and, at sufficiently high temperatures, move further apart — this makes the membrane slightly more permeable. At even higher temperatures, the membrane proteins are denatured (their tertiary structure is destroyed), causing them to fall out of the bilayer and leaving large holes. The membrane then becomes fully permeable.
The Beetroot Practical
The effect of temperature on membrane permeability can be investigated using beetroot discs. Beetroot cells contain a red pigment (betalain) in their vacuoles. At normal temperatures, the intact cell membrane prevents the pigment from escaping. As temperature increases and the membrane becomes more permeable, pigment leaks into the surrounding solution. The amount of leakage is measured using a colorimeter — the greater the pigment leakage, the lower the percentage transmission of light through the solution.
Alcohol (Ethanol)
Ethanol increases membrane permeability because the phospholipids dissolve in ethanol. At high enough concentrations, ethanol can completely destroy the phospholipid bilayer, making the membrane fully permeable.
Simple Diffusion
Simple Diffusion
The passive net movement of a substance from a region of high concentration to a region of lower concentration, down a concentration gradient. No energy (ATP) is required.
Simple diffusion occurs directly across the phospholipid bilayer — no proteins are involved. Only lipid-soluble (non-polar, hydrophobic) substances can cross by this route because they can dissolve in the hydrophobic core formed by the fatty acid tails. Key examples include oxygen (O₂) and carbon dioxide (CO₂). Polar, ionic, or hydrophilic substances (such as glucose, Na⁺, K⁺) cannot pass through the bilayer by simple diffusion.
Factors Affecting the Rate of Simple Diffusion
Concentration gradient: The steeper the gradient (the greater the difference in concentration), the faster the rate. The relationship is directly proportional — doubling the gradient doubles the rate. If the gradient reaches zero (equal concentrations on both sides), dynamic equilibrium is reached and there is no net movement. In living organisms, concentration gradients are maintained by metabolic processes (e.g. respiration removes O₂, keeping the gradient steep).
Temperature: Higher temperature gives molecules more kinetic energy, so they move faster and diffuse more rapidly.
Size of molecule: Smaller molecules pass between the phospholipids more easily than larger ones, so they diffuse faster.
Lipid solubility: The more lipid-soluble a substance, the more readily it dissolves in the hydrophobic core and the faster it diffuses.
Surface area: A larger membrane surface area provides more space for diffusion to occur, increasing the overall rate.
Facilitated Diffusion
Facilitated Diffusion
The passive net movement of a substance from a region of high concentration to a region of lower concentration, down a concentration gradient, with the aid of a membrane protein (channel or carrier protein). No energy (ATP) is required.
Via Channel Proteins
Channel proteins have a polar pore (hydrophilic channel) running through their centre. Polar, ionic, and hydrophilic substances can freely diffuse through this pore without binding to the protein. The rate of facilitated diffusion through channel proteins is directly proportional to the concentration gradient — the same relationship as simple diffusion.
Via Carrier Proteins
Carrier proteins have a specific binding site with a complementary shape to the substance being transported. The substance binds to the carrier protein, causing a conformational change (change in shape) that moves the substance across the membrane. The carrier then returns to its original shape, ready to transport another molecule.
The Saturation Effect
At low concentrations, the rate of facilitated diffusion via carrier proteins increases proportionally with the concentration gradient. However, there is a limited number of carrier proteins in any membrane. When the concentration of the substance exceeds the number of available carrier proteins, all binding sites are occupied — the carriers are saturated. At this point, the rate reaches a plateau and remains constant regardless of further increases in concentration. This does not mean transport has stopped — it continues at a constant maximum rate.
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Active Transport
The movement of a substance from a region of low concentration to a region of high concentration, against the concentration gradient. This requires energy from ATP, which is produced by aerobic respiration.
Active transport is fundamentally different from diffusion and facilitated diffusion because it moves substances against their natural concentration gradient — from low to high concentration. This requires energy because the substance is being moved in the opposite direction to its natural tendency to diffuse.
The Mechanism
Active transport can be stopped by a lack of oxygen (aerobic respiration cannot produce ATP) or by respiratory inhibitors such as cyanide (which block ATP synthesis). When a respiratory inhibitor is applied, active transport stops immediately and the rate falls to zero.
Like facilitated diffusion with carrier proteins, active transport shows the saturation effect because there is a limited number of carrier proteins in the membrane.
The Sodium-Potassium Pump (Na⁺/K⁺ Pump)
The Na⁺/K⁺ pump is a specific carrier protein that actively transports two different ions in opposite directions (making it an antiport protein). For each cycle, it pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, both against their respective concentration gradients. One molecule of ATP is hydrolysed per cycle.
The Na⁺/K⁺ pump is essential in many cell types, including kidney cells (reabsorption of useful substances), neurones (maintaining the resting potential for nerve impulse transmission), and intestinal epithelial cells (absorbing nutrients).
Bulk Transport — Endocytosis and Exocytosis
Large substances (proteins, polysaccharides, bacteria) cannot cross the membrane through transport proteins. Instead, they are moved by bulk transport, which involves the membrane itself changing shape. All forms of bulk transport require ATP.
Phagocytosis: The cell extends its membrane around a large particle (e.g. a bacterium), engulfing it into a food vacuole (phagosome). The food vacuole then fuses with a lysosome, which digests the contents with hydrolytic enzymes.
Receptor-mediated endocytosis: Specific molecules bind to receptors on the membrane surface. The membrane then infolds around the bound molecules, bringing them into the cell in a vesicle. This is a targeted, specific process.
Pinocytosis (“cell drinking”): The cell continuously takes in small droplets of extracellular fluid by forming tiny vesicles from membrane infolds. This is a non-specific process — no receptors are involved.
Exocytosis: Vesicles inside the cell move to the cell surface membrane and fuse with it, releasing their contents outside the cell. This is how cells secrete proteins, hormones, and neurotransmitters.
Osmosis — Definitions and Water Potential
Osmosis
The passive net diffusion of water molecules from a region of high water potential to a region of lower water potential, through a semi-permeable membrane, down a water potential gradient.
Understanding Water Potential (Ψ)
Water potential (Ψ) is a measure of the tendency of water molecules to move. It is measured in pressure units (kPa or MPa). Pure water has the highest water potential, which is arbitrarily set at 0 kPa. Adding solutes to water lowers the water potential, making it negative. The more solutes dissolved, the more negative the water potential becomes.
Water always moves from a higher (less negative) water potential to a lower (more negative) water potential. The difference in water potential between two regions is called the water potential gradient — this is the driving force for osmosis.
Why Solutes Lower Water Potential
Water is a polar molecule. When solutes are added, they form bonds with water molecules, effectively “trapping” them. These water molecules bonded to solutes are too large to pass through the semi-permeable membrane. This reduces the number of free water molecules available to move, lowering the water potential.
Key Terminology
| Term | Meaning | Water Movement |
|---|---|---|
| Isotonic | Two solutions have equal solute potentials and equal water potentials | No net movement of water — equilibrium |
| Hypotonic | Solution has lower solute concentration (higher water potential) relative to the cell | Water moves into the cell by osmosis |
| Hypertonic | Solution has higher solute concentration (lower water potential) relative to the cell | Water moves out of the cell by osmosis |
Osmosis in Animal Cells
Animal cells have no cell wall — only a flexible cell membrane. This means they are very vulnerable to osmotic changes.
In an isotonic solution: The water potential inside and outside the cell are equal. There is no net movement of water. The red blood cell maintains its normal biconcave disc shape.
In a hypotonic solution (lower solute concentration than the cell): The water potential outside is higher than inside. Water enters the cell by osmosis. Because the cell membrane has no rigid support, the cell swells and eventually bursts — this is called haemolysis (in red blood cells) or lysis (in general).
In a hypertonic solution (higher solute concentration than the cell): The water potential outside is lower than inside. Water leaves the cell by osmosis. The cell shrinks and becomes wrinkled — this is called crenation.
Contractile Vacuoles in Freshwater Organisms
Freshwater unicellular organisms like Paramecium live in a hypotonic environment — the surrounding water has a higher water potential than their cytoplasm. Water constantly enters by osmosis, threatening to burst the cell. To prevent this, these organisms use contractile vacuoles. Ions are actively pumped into the vacuole from the cytoplasm, lowering its water potential. Water enters the vacuole by osmosis. When full, the vacuole fuses with the cell membrane and contracts, expelling the water outside the cell.
Osmosis in Plant Cells
Plant cells differ from animal cells because they have a rigid, inelastic cellulose cell wall surrounding the cell membrane. This changes the behaviour of the cell during osmosis.
The Water Potential Equation
Water Potential of a Plant Cell
Ψ = ΨS + ΨP, where Ψ = water potential, ΨS = solute potential (always negative), and ΨP = pressure potential (zero or positive).
Turgid Cells (in Hypotonic Solution)
When a plant cell is placed in a hypotonic solution, water enters by osmosis. The vacuole expands, pushing the cell contents against the inelastic cell wall. Because the cell wall does not stretch, it exerts an inward pressure potential (ΨP) on the cell contents. When fully turgid, the pressure potential equals the solute potential (but is positive), so: Ψ = 0 (because ΨS + ΨP = 0). At this point, there is no water potential gradient, so no further net entry of water. The cell wall prevents the cell from bursting — this is why plant cells do not lyse.
Plasmolysed Cells (in Hypertonic Solution)
When a plant cell is placed in a hypertonic solution, water leaves by osmosis. The vacuole shrinks and the cell membrane pulls away from the cell wall — this is called plasmolysis. The cell wall, being inelastic, retains its shape. In a fully plasmolysed cell, the pressure potential is zero, so: Ψ = ΨS.
Incipient Plasmolysis
Incipient plasmolysis is the point at which the cell membrane is just starting to pull away from the cell wall. At this point, the pressure potential has just reached zero: ΨP = 0, so Ψ = ΨS. This condition cannot be reliably identified under the microscope for a single cell — it is determined experimentally using the incipient plasmolysis method (see practicals section).
Osmosis Practicals
Practical 1 — Incipient Plasmolysis (Red Onion)
Strips of red onion epidermis are placed in sucrose solutions of increasing concentration. After 20 minutes, cells are observed under the microscope. The red pigment makes the vacuole visible, allowing plasmolysed cells (where the cell contents have visibly shrunk away from the cell wall) to be identified and counted.
The percentage of plasmolysed cells is calculated: (number of plasmolysed cells ÷ total number of cells) × 100. A graph of percentage plasmolysed cells against sucrose concentration is plotted. The sucrose concentration that causes 50% plasmolysis is read from the graph — this is the concentration at which incipient plasmolysis occurs. At this point, the solute potential of the external sucrose solution equals the water potential of the cell (because ΨP = 0).
Practical 2 — Percentage Change in Mass (Potato Cylinders)
Potato cylinders of equal size are weighed and placed in sucrose solutions of different concentrations for 2 hours. They are then removed, dried, and reweighed. The percentage change in mass is calculated: ((final mass − initial mass) ÷ initial mass) × 100.
A graph of percentage change in mass against sucrose concentration is plotted. In hypotonic solutions, the potato gains mass (water enters by osmosis). In hypertonic solutions, it loses mass (water leaves). The concentration at which there is zero percentage change in mass represents the point where the solute potential of the sucrose equals the solute potential of the potato cells — they are isotonic, so there is no net osmosis.
Where Cell Membrane Appears on Your Specification
| Exam Board | Unit / Module | Topic Area |
|---|---|---|
| AQA | Paper 1 (Year 1) | Topic 2: Cells — Cell membrane structure; transport across membranes |
| Edexcel A | Paper 1 (Year 1) | Topic 2: Genes and Health — Membrane structure and transport |
| OCR A | Paper 1 (Year 1) | Module 2: Foundations in Biology — Cell membranes |
| WJEC | Unit 1 | Cell Structure and Organisation — Membrane structure and transport |
| Eduqas | Core Concepts | Cell Structure and Organisation — Membrane structure and transport |
| IB Biology | Topic 1 (SL & HL) | Cell Biology — Membrane structure and transport |
| CIE 9700 | Paper 1 & 2 (AS) | Cell membranes and transport |
Related Resources on This Site
Frequently Asked Questions
It is called “fluid” because the phospholipids are constantly moving within the bilayer — they can move laterally, swap places, and rotate. It is called “mosaic” because the various proteins embedded in and on the surface of the membrane create a pattern resembling a mosaic. The model was first proposed by Singer and Nicholson in 1972.
Both are passive processes (no ATP needed) that move substances down their concentration gradient. Simple diffusion occurs directly through the phospholipid bilayer and only works for lipid-soluble (non-polar) substances like O₂ and CO₂. Facilitated diffusion requires membrane proteins (channel or carrier proteins) and is used by polar, ionic, and hydrophilic substances like glucose and ions that cannot pass through the hydrophobic core of the bilayer.
Both use carrier proteins, but the key differences are: (1) active transport moves substances against their concentration gradient (low to high), while facilitated diffusion moves them down the gradient (high to low); (2) active transport requires energy from ATP, while facilitated diffusion is passive; (3) active transport is stopped by respiratory inhibitors like cyanide or lack of oxygen, while facilitated diffusion is not. Both show the saturation effect.
The saturation effect occurs when all available carrier proteins in a membrane have their binding sites occupied by substrate molecules. There is a limited (finite) number of carrier proteins, so once they are all being used, the rate of transport cannot increase further — it reaches a plateau on a graph. This applies to both facilitated diffusion via carrier proteins and active transport. It does not apply to simple diffusion or facilitated diffusion via channel proteins.
Animal cells have no cell wall, so in a hypotonic solution they swell and burst (lysis/haemolysis), and in a hypertonic solution they shrink (crenation). Plant cells have a rigid cellulose cell wall. In a hypotonic solution, water enters and the cell becomes turgid — the cell wall prevents bursting and creates a pressure potential. In a hypertonic solution, water leaves and the cell membrane pulls away from the cell wall (plasmolysis). The key equation for plant cells is Ψ = ΨS + ΨP.
Two methods are used. The incipient plasmolysis method uses red onion cells in different sucrose concentrations — the concentration causing 50% plasmolysis (read from a graph) equals the point where the external solute potential equals the cell’s water potential (because ΨP = 0 at incipient plasmolysis). The potato cylinder method measures percentage change in mass in different sucrose concentrations — the concentration causing zero change in mass indicates isotonic conditions, where the external solute potential equals the cell’s solute potential.
The Na⁺/K⁺ pump is an antiport carrier protein that uses energy from ATP to transport 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell per cycle, both against their concentration gradients. Na⁺ binds to the intracellular side, ATP is hydrolysed, the pump changes shape and releases Na⁺ outside. K⁺ then binds on the extracellular side, the pump changes shape again, and K⁺ is released inside. It is vital for nerve impulse transmission, kidney function, and intestinal absorption.
Yes — cell membrane structure and transport is a core topic on every A-Level Biology specification. These notes cover the content shared by AQA, Edexcel A and B, OCR A and B, WJEC, Eduqas, IB Biology, and Cambridge International (CIE 9700). The fluid mosaic model, diffusion, facilitated diffusion, active transport, osmosis, and water potential are universal across all boards. Written by a former WJEC/Eduqas and Edexcel examiner.
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