OCR A Level Biology A: Biological membranes (H420)

Biological membranes are partially permeable phospholipid bilayers with proteins, cholesterol, glycoproteins and glycolipids, described by the fluid mosaic model. For OCR A Level Biology A (H420) section 2.1.5 you must explain the roles of membranes, the function of each component (including membrane-bound receptors for hormones and drugs), how temperature and solvents affect permeability, how molecules cross membranes passively and actively, and how osmosis changes plant and animal cells, along with the practical work in PAG5 and PAG8.

This guide covers learning outcomes 2.1.5(a) to (e) of the OCR A Level Biology A specification, Version 4.1 (April 2026), first assessment 2025, in Module 2, Foundations in biology. Use the biological membranes revision notes for quick recall and the biological membranes practice questions to test yourself. Phospholipid and cholesterol structure is in the water, carbohydrates and lipids revision notes, and cell signalling returns in the cell communication, homeostasis and the liver study guide. Course hub: OCR A Level Biology.

What 2.1.5 asks for

Outcome	What you must be able to do
2.1.5(a)	Explain the roles of membranes as partially permeable barriers (cell and environment, organelles and cytoplasm, within organelles), as sites of chemical reactions and as sites of cell signalling
2.1.5(b)	Describe the fluid mosaic model and the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins, and the role of membrane-bound receptors as sites where hormones and drugs bind
2.1.5(c)	Explain the effects of temperature and solvents on membrane structure and permeability; carry out investigations (PAG5, PAG8)
2.1.5(d)	Explain diffusion and facilitated diffusion as passive processes, and active transport, endocytosis and exocytosis as processes needing ATP; investigate diffusion in model cells (PAG8)
2.1.5(e)	Explain osmosis in terms of water potential and its effects on plant and animal cells; investigate these effects (PAG8)

What do membranes do in cells? (a)

• Partially permeable barriers. The cell-surface membrane separates the cell from its environment; membranes around organelles (such as the nucleus and lysosomes) separate their contents from the cytoplasm; and membranes within organelles (such as the thylakoids in chloroplasts and the cristae in mitochondria) divide them into compartments.
• Sites of chemical reactions. Enzymes and electron carriers are fixed in membranes, for example the inner mitochondrial membrane in respiration and the thylakoid membranes in photosynthesis.
• Sites of cell communication (cell signalling). Receptors on the cell-surface membrane bind signalling molecules such as hormones.

What is the fluid mosaic model? (b)

The membrane is a phospholipid bilayer about 7 nm thick. Hydrophilic phosphate heads face the aqueous cytoplasm and tissue fluid; hydrophobic fatty acid tails face inwards. Phospholipids and many proteins move sideways, so the membrane is fluid; proteins are scattered through it like tiles, so it is a mosaic.

Component	Role
Phospholipids	Form the bilayer; the hydrophobic core is a barrier to ions and polar molecules
Cholesterol	Fits between phospholipids; regulates fluidity and makes the membrane more stable (less fluid at higher temperatures, keeps it fluid at low temperatures)
Channel proteins	Intrinsic proteins with a hydrophilic pore that lets specific ions or polar molecules through
Carrier proteins	Intrinsic proteins that bind a specific molecule and change shape to move it across, in facilitated diffusion or active transport
Glycoproteins	Proteins with carbohydrate chains on the outside; act as receptors and antigens and help cells attach to one another
Glycolipids	Lipids with carbohydrate chains; act as antigens for cell recognition

Why are membrane-bound receptors important for medicine?

Receptors are glycoproteins (or proteins) with a binding site complementary to a specific signalling molecule. Hormones such as insulin bind to receptors on target cells. Many medicinal drugs are designed to have shapes that bind these receptors, either to trigger the same response as the natural molecule or to block it.

What affects membrane structure and permeability? (c)

Factor	Effect
Increasing temperature	Phospholipids gain kinetic energy and move more, so the membrane becomes more fluid and more permeable; at high temperatures membrane proteins denature, leaving gaps, so permeability rises sharply
Very low temperature	Phospholipids pack closely and the membrane is rigid; proteins may be damaged by ice crystals, which can pierce membranes
Organic solvents (such as ethanol)	Dissolve phospholipids and disrupt the bilayer; higher concentration, greater permeability

Practical (PAG5, PAG8). Equal pieces of beetroot are washed to remove pigment from cut cells, then placed in water at different temperatures or in different ethanol concentrations for a set time. The red pigment that leaks from damaged membranes is measured with a colorimeter. Higher absorbance (lower transmission) means more pigment, so greater permeability.

How do molecules move across membranes? (d)

Passive processes

• Diffusion is the net movement of particles from a region of higher concentration to lower concentration, down a concentration gradient, until equilibrium. Small, non-polar molecules (oxygen, carbon dioxide) diffuse through the bilayer.
• Facilitated diffusion is diffusion through channel or carrier proteins, for ions and polar molecules such as glucose. Its rate is limited by the number of proteins.

Rate of diffusion increases with a steeper concentration gradient, larger surface area, higher temperature and a shorter diffusion distance.

Processes needing ATP

• Active transport moves substances against a concentration gradient using carrier proteins. ATP binds to the carrier and is hydrolysed, causing the protein to change shape and move the substance.
• Endocytosis takes in large molecules or particles: the membrane folds around them and pinches off to form a vesicle (phagocytosis of bacteria by neutrophils is an example).
• Exocytosis releases substances: vesicles from the Golgi apparatus fuse with the cell-surface membrane and release their contents (for example digestive enzymes or hormones).

Both endocytosis and exocytosis need ATP, as the immediate source of energy, to move vesicles and change the shape of the membrane.

How do you investigate diffusion in model cells? (PAG8)

Cubes of agar containing an indicator (for example agar made pink with phenolphthalein and sodium hydroxide) are placed in dilute acid. As acid diffuses in, the pink colour disappears. Smaller cubes go colourless sooner because they have a larger surface area to volume ratio.

Worked example 1. A 1 cm cube has surface area 6 x 1 x 1 = 6 cm squared and volume 1 cm cubed: ratio 6 : 1. A 2 cm cube has surface area 6 x 2 x 2 = 24 cm squared and volume 8 cm cubed: ratio 24 : 8 = 3 : 1. Doubling the side halves the ratio.

Worked example 2. Acid diffused 4.5 mm into a cube in 6 minutes. Rate = 4.5 / 6 = 0.75 mm per minute.

Visking (dialysis) tubing filled with a solution can also act as a model cell: change in mass or the appearance of a solute outside the tubing shows diffusion or osmosis.

How does osmosis affect cells? (e)

Osmosis is the net movement of water molecules from a region of higher water potential to lower water potential, across a partially permeable membrane. Water potential is measured in kPa; pure water is 0 kPa, and solutes make it negative.

Solution outside the cell	Animal cell (for example red blood cell)	Plant cell
Higher water potential than the cell	Water enters; cell swells and may burst (haemolysis), as there is no cell wall	Water enters; vacuole swells and the cell becomes turgid; the cell wall prevents bursting
Equal water potential	No net movement; no change	No net movement; cell may be flaccid
Lower water potential than the cell	Water leaves; cell shrinks and the membrane wrinkles (crenation)	Water leaves; the protoplast shrinks and the membrane pulls away from the wall (plasmolysis)

Practical (PAG8). Plant tissue (for example potato) is placed in a range of sucrose solutions and the percentage change in mass is calculated. Worked example 3. Mass falls from 3.20 g to 2.88 g: (2.88 - 3.20) / 3.20 x 100 = -10%. The concentration giving zero change has the same water potential as the tissue. Red onion epidermis under the microscope shows plasmolysis directly; red blood cells can be viewed in different salt solutions.

Frequently asked questions

Why is the membrane described as partially permeable? It lets some substances, such as water and small non-polar molecules, cross freely, but stops ions and large or polar molecules unless there are specific proteins to transport them.

What is the difference between facilitated diffusion and active transport? Both use proteins, but facilitated diffusion is passive and goes down a concentration gradient, whereas active transport goes against the gradient and needs ATP.

Why do plant cells not burst in pure water? The cell wall resists the expanding protoplast, so the cell becomes turgid instead.

Why do drugs often target membrane receptors? Receptors have specific binding sites. A drug with a complementary shape can bind them to trigger or block a response in only the cells that carry the receptor.

Common mistakes

• Saying water moves from high to low concentration of water instead of using water potential.
• Saying endocytosis and exocytosis are passive; both need ATP.
• Describing cholesterol only as “making the membrane rigid”; it regulates fluidity.
• In the beetroot practical, forgetting to wash the pieces first, or not naming the colorimeter.
• Saying plant cells burst in a solution of higher water potential.

Official syllabus

OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025, Module 2 Foundations in biology, section 2.1.5 Biological membranes, learning outcomes (a) to (e), with PAG5 and PAG8.