OCR A Level Biology A: Biological membranes (H420) – Revision Notes

These notes condense learning outcomes 2.1.5(a) to (e) of the OCR A Level Biology A specification (H420, Version 4.1, April 2026, first assessment 2025), including the practical work the specification links to PAG5 (colorimeter) and PAG8 (transport in and out of cells). The course sits on the OCR A Level Biology hub; tick off outcomes on the printable checklist. Phospholipids and cholesterol are introduced in the water, carbohydrates and lipids revision notes; membranes inside organelles appear in the cell structure revision notes; and water potential is used again in the exchange and transport revision notes. Colorimeter and dilution technique are in the practical endorsement revision notes, and the free diagnostics show gaps quickly.

Key definitions

Term	Meaning
Partially permeable	Lets some substances cross but not others
Diffusion	Net movement of particles from a region of higher concentration to a region of lower concentration; passive
Facilitated diffusion	Diffusion through channel or carrier proteins, for substances that cannot cross the bilayer directly; passive
Active transport	Movement against a concentration gradient through carrier proteins, using energy from ATP
Endocytosis	Bulk transport into a cell by the membrane enclosing material in a vesicle
Exocytosis	Bulk transport out of a cell by vesicles fusing with the plasma membrane
Water potential (ψ)	A measure of the tendency of water molecules to move, in kPa; pure water at standard temperature and pressure has ψ = 0 kPa
Osmosis	Net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane

Roles of membranes: 2.1.5(a)

Role	Where	Example
Partially permeable barrier between cell and environment	Plasma (cell surface) membrane	Controls what enters and leaves the cell
Barrier between organelles and cytoplasm	Nuclear envelope, membranes of lysosomes, mitochondria, chloroplasts	Lysosome membrane keeps digestive enzymes away from the cytoplasm
Barrier within organelles	Inner mitochondrial membrane, thylakoids	Keeps proton gradients separate; compartments with different conditions
Site of chemical reactions	Cristae of the inner mitochondrial membrane, thylakoid membranes, rough ER	Electron transport chains and ATP synthase are fixed in the membrane
Site of cell communication (cell signalling)	Plasma membrane	Receptors bind hormones; glycoproteins allow cell recognition

• Compartmentalisation: membranes divide the cell into regions where different reactions can run at the right conditions and concentrations.

The fluid mosaic model: 2.1.5(b)

• Fluid: phospholipids and many proteins move about within the layer.
• Mosaic: proteins are scattered through the bilayer, like tiles in a mosaic.

Component	Structure and position	Role
Phospholipids	Bilayer: hydrophilic phosphate heads face the watery cytoplasm and tissue fluid; hydrophobic fatty acid tails point inwards	Barrier to polar molecules and ions; lets small non-polar molecules (O₂, CO₂) and lipid-soluble substances through
Cholesterol	Between phospholipids, in animal cell membranes	Regulates fluidity: reduces movement at higher temperatures and stops the membrane becoming too rigid at low temperatures; adds mechanical stability; reduces leakage of water and ions
Intrinsic (integral) proteins	Span the bilayer	Channel proteins (water-filled pore for ions) and carrier proteins (change shape to move molecules)
Extrinsic (peripheral) proteins	On one surface	Enzymes, receptors, links to the cytoskeleton
Glycoproteins	Proteins with carbohydrate chains, outer surface	Cell recognition and antigens, receptors, cell attachment (adhesion)
Glycolipids	Lipids with carbohydrate chains, outer surface	Cell recognition and antigens, stabilising the membrane

Membrane-bound receptors are proteins or glycoproteins with a shape complementary to a specific signalling molecule.

• Hormones bind them: for example insulin binds receptors on liver and muscle cells.
• Drugs can bind them: some drugs mimic a natural signal and trigger a response; others block the receptor so the natural signal cannot bind (beta blockers block receptors in heart muscle).
• Only cells with the right receptor respond to a given hormone or drug.

Scale (M0.2): a membrane is only a few nanometres thick, so it shows as two dark lines separated by a lighter band in an electron micrograph and cannot be resolved with a light microscope.

Factors affecting membrane structure and permeability: 2.1.5(c)(i)

Temperature

• Low temperature (still above 0 °C): phospholipids have little kinetic energy and pack closely, so the membrane is rigid and permeability is low.
• Below 0 °C: ice crystals can pierce the membrane, so it becomes leaky on thawing.
• Rising temperature: phospholipids gain kinetic energy and move more, the bilayer becomes more fluid and gaps appear, so permeability increases.
• High temperature (about 40 °C and above for many cells): membrane proteins denature, leaving gaps; permeability rises steeply.

Solvents

• Organic solvents such as ethanol and acetone are less polar than water, so they dissolve or disrupt the phospholipid bilayer.
• The higher the solvent concentration, the greater the damage and the more permeable the membrane. Very high concentrations destroy the membrane completely.
• Detergents have a similar effect.

Practical: membrane permeability with beetroot (PAG5, PAG8): 2.1.5(c)(ii)

1. Cut beetroot cylinders with a cork borer; trim them to the same length with a scalpel and tile.
2. Rinse in distilled water to remove the red pigment (betalain) released from cells cut open.
3. Place each cylinder in the same volume of distilled water at a range of temperatures (water baths), or in a range of ethanol concentrations at one temperature, for a fixed time.
4. Remove the cylinders and shake the tubes to mix.
5. Zero a colorimeter with distilled water. Use a blue-green filter (complementary to red).
6. Record the absorbance of each solution; higher absorbance means more pigment leaked, so the membranes were more permeable.
7. Repeat and calculate means; plot absorbance against temperature or concentration.

• Controlled variables: size and surface area of cylinders, volume of water, time, source of beetroot.
• Without a colorimeter: compare colours against a set of standards made by dilution.
• Ethanol series: make concentrations by proportional dilution, for example 20 %, 40 %, 60 % and 80 % ethanol from 100 % ethanol and water.

Movement of molecules across membranes: 2.1.5(d)(i)

Process	Passive or active	Gradient	Route	Examples
Simple diffusion	Passive	Down	Through the phospholipid bilayer	O₂, CO₂, steroid hormones
Facilitated diffusion	Passive	Down	Channel proteins or carrier proteins	Ions through channels; glucose by carriers
Active transport	Needs ATP	Up (against)	Carrier proteins	Na⁺/K⁺ pump; mineral ions into root hair cells
Endocytosis	Needs ATP	Bulk	Vesicle formed from the membrane	Phagocytosis of bacteria by neutrophils
Exocytosis	Needs ATP	Bulk	Vesicle fuses with membrane	Release of digestive enzymes and hormones

Factors affecting the rate of diffusion

• Steeper concentration gradient gives a faster rate.
• Higher temperature gives particles more kinetic energy, so faster diffusion.
• Larger surface area gives a faster rate.
• Thinner exchange surface (shorter diffusion distance) gives a faster rate.
• Smaller and non-polar molecules diffuse faster through the bilayer.

Facilitated diffusion

• Channel proteins: water-filled hydrophilic pores, often gated (open or closed), let specific ions through.
• Carrier proteins: bind a specific molecule and change shape to release it on the other side.
• Rate is limited by the number of channel or carrier proteins as well as by the gradient.

Active transport

1. The molecule or ion binds to a specific carrier protein on one side.
2. ATP is hydrolysed to ADP and phosphate, releasing energy.
3. The carrier changes shape and releases the molecule on the other side.
4. The carrier returns to its original shape.

• The sodium–potassium pump moves 3 Na⁺ out of the cell for every 2 K⁺ moved in, per ATP.
• Cells that carry out much active transport have many mitochondria.

Bulk transport

• Endocytosis: the plasma membrane folds inwards around material and pinches off a vesicle. Phagocytosis takes in solids; pinocytosis takes in liquids.
• Exocytosis: vesicles, often from the Golgi apparatus, move to the plasma membrane, fuse with it and release their contents.
• ATP is the immediate energy source for moving vesicles along the cytoskeleton and for changing the shape of the membrane.

Practical: diffusion in model cells (PAG8): 2.1.5(d)(ii)

• Agar cubes: agar containing an indicator (for example phenolphthalein made pink with dilute sodium hydroxide) is cut into cubes of different sizes and placed in dilute acid. Time how long each cube takes to go colourless, or measure the colourless depth after a set time.
• Visking tubing: acts as a partially permeable membrane. Fill it with a solution, place it in water and sample the outside liquid at intervals, or measure changes in mass.
• Variables to test: surface area to volume ratio, concentration gradient, temperature.
• Smaller cubes have a larger surface area to volume ratio, so a larger proportion of each cube is reached by diffusion in a given time.

Cube side (cm)	Surface area (cm²)	Volume (cm³)	SA : V
0.5	1.5	0.125	12 : 1
1	6	1	6 : 1
2	24	8	3 : 1
3	54	27	2 : 1

Osmosis and water potential: 2.1.5(e)(i)

• Water potential is highest in pure water (0 kPa). Dissolving solutes lowers it, so solutions have negative values. The more concentrated the solution, the more negative ψ is.
• Water moves by osmosis down a water potential gradient, from a less negative to a more negative ψ.
• Water crosses the bilayer directly (slowly) and through channel proteins called aquaporins (quickly).

External solution	Animal cell (for example a red blood cell)	Plant cell
Higher ψ than the cell	Water enters; cell swells and may burst (lysis, called haemolysis for red blood cells)	Water enters; vacuole and cytoplasm push against the cell wall; cell is turgid; the wall stops it bursting
Same ψ as the cell	No net movement; cell unchanged	No net movement; at the point of incipient plasmolysis the membrane is just starting to pull away
Lower ψ than the cell	Water leaves; cell shrinks and the membrane wrinkles (crenation)	Water leaves; cell becomes flaccid, then the membrane pulls away from the wall (plasmolysis)

• Animal cells have no cell wall, so tissue fluid must be kept at a similar water potential to the cells.

Practical: water potential of plant and animal cells (PAG8): 2.1.5(e)(ii)

Potato cylinders in sucrose solutions

1. Make sucrose solutions, for example 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol dm⁻³, by dilution of a 1.0 mol dm⁻³ stock.
2. Cut potato cylinders of the same diameter and length with a cork borer.
3. Blot each one dry and record its initial mass.
4. Leave each in a solution for a fixed time (for example 30 minutes or longer) at the same temperature.
5. Remove, blot dry in the same way and record the final mass.
6. Calculate percentage change in mass = (final − initial) ÷ initial × 100.
7. Plot percentage change against sucrose concentration. Where the line crosses 0 %, the solution has the same water potential as the potato cells.

• Percentage change, not just change, is used because the cylinders start at different masses.
• Blotting removes surface solution so it is not weighed as part of the tissue.

Other investigations

• Red onion or rhubarb epidermis mounted in a range of solutions: count the percentage of plasmolysed cells under the microscope. Where 50 % of cells are plasmolysed is taken as incipient plasmolysis.
• Animal blood in solutions of different concentrations, examined under the microscope for haemolysis or crenation (often using a prepared sample or a model, for safety).

Worked reminders

Percentage change in mass. 2.50 g becomes 2.80 g: (2.80 − 2.50) ÷ 2.50 × 100 = +12 %. A cylinder going from 2.40 g to 2.16 g: (2.16 − 2.40) ÷ 2.40 × 100 = −10 %.

Direction of osmosis. Cell A has ψ = −400 kPa and cell B has ψ = −650 kPa. Water moves from A to B, because −400 kPa is the higher (less negative) water potential.

Dilution. For 10 cm³ of 0.2 mol dm⁻³ sucrose from 1.0 mol dm⁻³ stock: 2 cm³ stock plus 8 cm³ distilled water.

SA : V. A 4 cm cube: surface area 6 × 4² = 96 cm², volume 4³ = 64 cm³, ratio 1.5 : 1.

Writing good answers

• Use water potential language for osmosis, never “water concentration”. Name both regions and the membrane.
• Say passive or active and name the protein when describing transport.
• Link structure to role for each membrane component: “cholesterol sits between phospholipids, restricting their movement, so fluidity is regulated”.
• Explain the colorimeter result: more pigment, higher absorbance, more damaged membranes.
• Read the intercept carefully on a percentage change graph: it gives the concentration with the same water potential as the tissue.

Must-know distinctions

• Diffusion (bilayer) versus facilitated diffusion (channel or carrier protein), both passive.
• Channel protein (pore, ions) versus carrier protein (changes shape).
• Facilitated diffusion (down the gradient, no ATP) versus active transport (against the gradient, ATP).
• Endocytosis (in) versus exocytosis (out); phagocytosis (solids) versus pinocytosis (liquids).
• Glycoprotein (protein + carbohydrate) versus glycolipid (lipid + carbohydrate).
• Lysis and crenation (animal cells) versus turgid, flaccid and plasmolysed (plant cells).

Quick self-test

1. Give three roles of membranes in cells.
2. Why is the model called “fluid mosaic”?
3. How does cholesterol affect membrane fluidity?
4. State one role of glycoproteins.
5. Explain how a drug might reduce a cell’s response to a hormone.
6. Why does more pigment leak from beetroot at 70 °C than at 30 °C?
7. How does ethanol increase membrane permeability?
8. Name two processes that use ATP as the immediate energy source.
9. Why do cells carrying out active transport have many mitochondria?
10. A cube has a side of 5 mm. Calculate its surface area to volume ratio.
11. Define osmosis.
12. What happens to a red blood cell placed in pure water?
13. A potato cylinder goes from 3.20 g to 2.88 g. Calculate the percentage change.
14. On a graph of percentage mass change against sucrose concentration, what does the x-axis intercept show?

Answers

1. Partially permeable barriers (round the cell and organelles, and within organelles), sites of chemical reactions, and sites of cell signalling.
2. Phospholipids and proteins can move within the layer (fluid), and proteins are scattered through the bilayer (mosaic).
3. It sits between phospholipids, restricting movement at higher temperatures and preventing tight packing at low temperatures, so it regulates fluidity.
4. Any one: cell recognition (antigens), acting as receptors, cell attachment.
5. It binds the hormone’s receptor and blocks it, so the hormone cannot bind.
6. At 70 °C membrane proteins are denatured and phospholipids move more, so the membrane has gaps and becomes more permeable.
7. It dissolves or disrupts the phospholipid bilayer.
8. Any two: active transport, endocytosis, exocytosis.
9. Mitochondria release energy in aerobic respiration to make ATP.
10. Surface area 6 × 25 = 150 mm², volume 125 mm³, so 1.2 : 1.
11. 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.
12. Water enters by osmosis; the cell swells and bursts (haemolysis).
13. (2.88 − 3.20) ÷ 3.20 × 100 = −10 %.
14. The sucrose concentration with the same water potential as the potato cells, so there is no net water movement.

Where marks are usually lost

• Writing that water moves “from a high to a low concentration” in osmosis.
• Saying that plant cells burst in pure water.
• Describing facilitated diffusion as needing energy, or active transport as moving down a gradient.
• Calling cholesterol a protein, or saying it only makes membranes more rigid.
• Stating that ψ of a solution is positive, or reading −300 kPa as lower than −500 kPa.
• Forgetting to rinse beetroot cylinders, or to zero the colorimeter.
• Using change in mass instead of percentage change when the starting masses differ.
• Saying glycoproteins “make” hormones rather than act as receptors for them.

Frequently asked questions

How does temperature affect membrane permeability? As temperature rises, phospholipids gain kinetic energy and move more, so the bilayer becomes more fluid and permeability increases. At high temperatures membrane proteins denature, leaving gaps, so permeability rises steeply; below 0 °C ice crystals can pierce the membrane.

What is the difference between channel proteins and carrier proteins? Channel proteins are water-filled hydrophilic pores, often gated, that let specific ions through. Carrier proteins bind a specific molecule and change shape to release it on the other side of the membrane.

How does the sodium–potassium pump work? The sodium–potassium pump is an example of active transport: for each ATP hydrolysed, it moves 3 Na⁺ out of the cell and 2 K⁺ in. Cells that carry out much active transport have many mitochondria.

Why is percentage change in mass used in the potato cylinder practical? The cylinders start at different masses, so percentage change, rather than the raw change, allows a fair comparison between them.

Official syllabus

OCR Level 3 Advanced GCE in Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025, published by OCR. Module 2.1.5 Biological membranes, statements (a) to (e), with PAG5 (colorimeter) and PAG8 (transport in and out of cells).