Explain why enzymes are described as biological catalysts, and explain how a change in pH can prevent an enzyme from carrying out its function.

Pearson Edexcel International GCSE Biology (4BI1) — 2.3 Biological molecules · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Enzymes are biological catalysts because they increase the rate of biochemical reactions without being consumed or altered in the process.

A change in pH can disturb the enzyme’s structure. Extreme pH alters the shape of the active site, so the substrate can no longer fit and an enzyme‑substrate complex cannot form, preventing the enzyme from functioning.

Examiner tips

  • Use the term ‘biological catalyst’ and mention rate increase and no consumption; link pH change to active‑site shape and substrate fit; keep answer concise to fit 4 marks.

Common mistakes

  • Confusing enzyme with substrate; describing pH effect as only changing activity level rather than active‑site shape; omitting that the enzyme remains unchanged after the reaction.

Mark scheme (4 marks)

  1. Enzymes increase the rate of reactions in living organisms
  2. Enzymes are not used up / remain unchanged after the reaction
  3. The shape of the active site changes / is altered at extreme pH
  4. The substrate can no longer fit the active site / enzyme-substrate complex cannot form, so the enzyme cannot function

Key terms in this question

enzyme · biological catalyst

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