Explain how pH affects the rate of an enzyme-catalysed reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Each enzyme has an optimum pH at which the rate of reaction is greatest.
The shape of the active site depends on the pattern of ionic and hydrogen bonds that maintain the tertiary structure of the enzyme; changes in pH alter the ionisation of R‑groups (amino acid side chains).
At pH values above or below the optimum, the active site changes shape so the substrate no longer fits – enzyme–substrate complementarity is reduced, lowering the rate of reaction.
At extreme pH values, denaturation occurs – the tertiary structure is permanently disrupted so the enzyme cannot function even if pH is restored.
The shape of the active site depends on the pattern of ionic and hydrogen bonds that maintain the tertiary structure of the enzyme; changes in pH alter the ionisation of R‑groups (amino acid side chains).
At pH values above or below the optimum, the active site changes shape so the substrate no longer fits – enzyme–substrate complementarity is reduced, lowering the rate of reaction.
At extreme pH values, denaturation occurs – the tertiary structure is permanently disrupted so the enzyme cannot function even if pH is restored.
Examiner tips
- Use the word ‘optimum’ and mention the active site shape; link pH to ionisation of side chains; include the idea of denaturation for extremes.
Common mistakes
- Confusing pH with temperature; not mentioning ionisation of side chains; saying the enzyme is ‘inactive’ without explaining denaturation.
Mark scheme (4 marks)
- Each enzyme has an optimum pH at which the rate of reaction is greatest.
- The shape of the active site depends on the pattern of ionic / hydrogen bonds that maintain the tertiary structure of the enzyme; changes in pH alter the ionisation of R-groups (amino acid side chains).
- At pH values above or below the optimum, the active site changes shape so the substrate no longer fits / enzyme–substrate complementarity is reduced, lowering the rate of reaction.
- At extreme pH values, denaturation occurs — the tertiary structure is permanently disrupted so the enzyme cannot function even if pH is restored.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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