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A-Level Biology May/June 2025 Q3(b): Suggest and explain how the protease produced by S. epidermidis cells prevents the grow…
A-Level Biology · Paper 9700/23 · May/June 2025 · Question 3(b) · [3 marks]
Suggest and explain how the protease produced by S. epidermidis cells prevents the growth of an S. aureus population on human skin.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The protease produced by S. epidermidis is an extracellular enzyme, meaning it is secreted by the bacterial cells onto the skin surface.
The active site of this protease is complementary to the specific three-dimensional shape of proteins that are essential for S. aureus survival, such as those forming its biofilm. The protease binds to these proteins to form enzyme-substrate complexes.
This enzymatic action breaks down the protein matrix of the S. aureus biofilm. Without a functional biofilm, the S. aureus cells cannot adhere to the surface of human skin cells, preventing them from establishing a population and growing.
How the marks are awarded
- B1 — The answer correctly identifies that the protease is secreted or is an extracellular enzyme, which is necessary for it to act on a separate organism. This corresponds to the first marking point.
- B1 — The answer explains the principle of enzyme specificity, stating that the protease's active site is complementary to a specific substrate protein on S. aureus, leading to the formation of an enzyme-substrate complex. This corresponds to the second marking point.
- B1 — The answer correctly identifies a specific target (biofilm proteins) and explains the consequence of its breakdown – that S. aureus can no longer attach to the skin, thus inhibiting its growth. This corresponds to the third marking point.
Common mistakes
- Simply stating 'the protease digests S. aureus' without specifying that it breaks down proteins.
- Forgetting to mention that the protease must be secreted or released from the S. epidermidis cell to have an effect.
- Giving a vague explanation of enzyme action, such as 'the enzyme damages the cell', without referring to specific protein targets like those in the biofilm or cell membrane.
- Confusing the roles of the two bacteria, for example by stating that S. aureus produces the protease.
Examiner tip: When explaining a biological process, always link the function of a molecule (like an enzyme's specific active site) to its direct consequence in the given context (like breaking down a biofilm to prevent attachment).
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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