Explain why an enzyme stops working when the pH moves far from its optimum.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
An enzyme stops working when the pH moves far from its optimum because the change in pH alters the ionisation of amino‑acid side chains in the active site, which distorts its shape. This distortion prevents the substrate from fitting into or binding to the active site, so no enzyme‑substrate complexes can form. If the pH change is extreme, the enzyme can become permanently denatured, further preventing activity.
Examiner tips
- Use the term "active site" and explain how pH changes its shape; link this to loss of substrate binding; mention permanent denaturation for extreme pH; keep answer concise and use correct terminology
Common mistakes
- Confusing pH with temperature effects; describing only a temporary loss of activity without mentioning permanent denaturation; using vague phrases like "enzyme stops" without explaining the mechanism
Mark scheme (4 marks)
- Change in pH alters the shape of the active site
- Substrate can no longer fit into / bind to the active site
- No enzyme-substrate complexes can form
- The enzyme has been denatured / the change is permanent
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →