Explain how a change in pH away from the optimum affects the ability of an enzyme to break down its substrate. Include reference to the active site in your answer.

Eduqas A-Level Biology — 1.1 Chemical elements and biological molecules · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Enzymes have an optimum pH at which the active site has the correct shape for the substrate. As the pH moves away from this optimum, the charge on amino‑acid side chains changes, altering the shape of the active site. The substrate can no longer fit into the active site because it is no longer complementary. Consequently, no enzyme‑substrate complexes form. This loss of structure is called denaturation, so the rate of reaction decreases.

Examiner tips

  • Use the word ‘optimum pH’ first, then explain shape change and loss of fit, finish with denaturation and rate decrease.
  • Show the chain: pH change → shape change → no complex → denaturation → ↓ rate.
  • Keep the answer concise – 5 marks, so one sentence per point.

Common mistakes

  • Confusing pH with temperature effects, or saying the enzyme is ‘inactive’ instead of ‘denatured’.
  • Omitting the link between shape change and loss of substrate fit.

Mark scheme (5 marks)

  1. Enzymes have an optimum pH at which the active site has the correct/complementary shape for the substrate
  2. As pH moves away from the optimum, the shape of the active site begins to change
  3. The substrate can no longer fit into the active site / the active site is no longer complementary to the substrate
  4. No enzyme-substrate complexes can form
  5. This is called denaturation / the enzyme is denatured, so the rate of reaction decreases

Key terms in this question

active site

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