OCR A Level Biology A: Biological membranes (H420) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover 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 PAG5 and PAG8 practical work. Questions 6, 8 and 10 practise the calculations the specification links to this section (surface area to volume ratio, rates and percentage change). Every answer gives the marking points.

Related: Biological membranes study guide and biological membranes revision notes. Course hub: OCR A Level Biology.

How should you use these questions?

Answer in full before checking. In osmosis answers, compare water potentials using “higher” and “lower” and name the membrane as partially permeable.

Questions

1. Give three roles of membranes within and at the surface of cells, with an example of each. [3]

2. Explain why the model of membrane structure is called the fluid mosaic model. [2]

3. State the role of each of the following in a cell-surface membrane: (a) cholesterol; (b) glycoproteins; (c) channel proteins. [3]

4. Many drugs are designed to bind to membrane-bound receptors. Explain why a drug of this kind affects only some cells in the body. [2]

5. A student placed beetroot pieces in water at 20, 40, 60 and 80 degrees Celsius for 30 minutes and measured the absorbance of the water with a colorimeter.

(a) Explain why the pieces were washed in water before the experiment. [1] (b) Predict and explain the results. [3] (c) Suggest how the experiment could be changed to investigate the effect of a solvent. [1]

6. Agar cubes containing an indicator were placed in acid. The pink colour disappears as acid diffuses in.

Cube side (cm)	Surface area (cm squared)	Volume (cm cubed)
0.5
1.0	6.0	1.0
3.0

(a) Complete the table and calculate the surface area to volume ratio for the 0.5 cm and 3.0 cm cubes. [3] (b) Explain which cube becomes colourless first. [2]

7. Compare diffusion and facilitated diffusion. [3]

8. In a 4-minute period, acid diffused 3.4 mm into an agar block. Calculate the rate of diffusion in mm per minute. [1]

9. Describe how a cell releases a protein hormone by exocytosis, and explain why this process needs ATP. [3]

10. Potato cylinders were placed in sucrose solutions. One cylinder’s mass increased from 2.40 g to 2.58 g.

(a) Calculate the percentage change in mass. [1] (b) Explain this change in terms of water potential. [2]

11. Describe and explain what happens to a red blood cell and to a plant cell placed in pure water. [4]

12. Explain why a plant cell becomes plasmolysed in a concentrated salt solution. [2]

Answers

1. Any three [1 each, with example]: partially permeable barrier between the cell and its environment (cell-surface membrane); barrier between organelles and cytoplasm (for example nuclear envelope, lysosome membrane); compartments within organelles (for example thylakoids, cristae); site of chemical reactions (for example inner mitochondrial membrane in respiration); site of cell signalling (for example hormone receptors).

2. Fluid: phospholipids and proteins can move sideways within the bilayer [1]. Mosaic: proteins are scattered through the phospholipid bilayer in a pattern [1].

3. (a) Regulates fluidity / stabilises the membrane [1]. (b) Act as receptors / antigens for cell recognition / cell attachment [1]. (c) Allow specific ions or polar molecules to cross through a hydrophilic pore [1].

4. The drug binds only to receptors with a complementary binding site [1]; only cells with that receptor on their membrane respond [1].

5. (a) To remove pigment released from cells damaged by cutting, so only pigment leaking through membranes is measured [1]. (b) Absorbance increases as temperature increases [1], because phospholipids gain kinetic energy and the membrane becomes more fluid and permeable [1]; above about 40 to 60 degrees Celsius it rises sharply because membrane proteins denature, leaving gaps [1]. (c) Keep the temperature constant and use different concentrations of ethanol (or another solvent) [1].

6. (a) 0.5 cm cube: surface area = 6 x 0.5 x 0.5 = 1.5 cm squared; volume = 0.125 cm cubed; ratio = 12 : 1 [1]. 3.0 cm cube: surface area = 6 x 3 x 3 = 54 cm squared; volume = 27 cm cubed [1]; ratio = 2 : 1 [1]. (b) The 0.5 cm cube [1]: it has the largest surface area to volume ratio and the shortest distance to the centre, so acid reaches the centre fastest [1].

7. Both are passive and move substances down a concentration gradient [1]. Diffusion is directly through the phospholipid bilayer, for small non-polar molecules [1]. Facilitated diffusion uses channel or carrier proteins, for ions and polar molecules, and its rate is limited by the number of proteins [1].

8. 3.4 / 4 = 0.85 mm per minute [1].

9. The hormone is packaged into vesicles by the Golgi apparatus [1]; vesicles move to and fuse with the cell-surface membrane, releasing the hormone outside the cell [1]. ATP provides the energy to move the vesicles and for the membrane to fuse / change shape [1].

10. (a) (2.58 - 2.40) / 2.40 x 100 = +7.5% [1]. (b) The solution had a higher water potential than the potato cells [1], so there was a net movement of water into the cells by osmosis through the partially permeable membranes [1].

11. Red blood cell: water enters by osmosis because pure water has a higher water potential than the cell [1]; there is no cell wall, so it swells and bursts (haemolysis) [1]. Plant cell: water enters by osmosis [1]; the cell wall resists expansion, so the cell becomes turgid and does not burst [1].

12. The solution has a lower water potential than the cell, so water leaves by osmosis [1]; the protoplast / vacuole shrinks and the cell-surface membrane pulls away from the cell wall [1].

Where marks are usually lost

• Calling the membrane “semi-permeable” without explaining which substances cross.
• Missing that endocytosis and exocytosis need ATP.
• Getting the surface area of a cube wrong: it is 6 x side squared.
• Saying plant cells burst in pure water.
• Forgetting the plus sign for a percentage increase.

Frequently asked questions

Why does a smaller agar cube lose its colour sooner? It has a larger surface area to volume ratio and a shorter distance from the surface to the centre, so the acid reaches every part of it sooner.

How do I describe absorbance results? More pigment in the solution means higher absorbance, which shows the membranes were more permeable.

Which processes need ATP? Active transport, endocytosis and exocytosis. Diffusion, facilitated diffusion and osmosis are passive.

Official syllabus

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