A student places pieces of fresh liver into solutions of different pH values and measures how quickly the enzyme catalase breaks down hydrogen peroxide. Explain how changing the pH away from the optimum affects the ability of catalase to break down hydrogen peroxide.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Catalase is an enzyme found in liver cells. It breaks down hydrogen peroxide into water and oxygen. It has an optimum pH at which it works most effectively.
Model answer (5 marks)
As the pH moves away from the optimum, the enzyme’s active site changes shape, becoming denatured. The altered shape means hydrogen peroxide can no longer fit into the active site. Consequently, fewer or no enzyme‑substrate complexes form. The rate of the reaction therefore decreases. Because the denaturation is permanent, the enzyme cannot return to its original shape even if the pH is later restored to optimum.
Examiner tips
- Use the term ‘denatured’ and explain loss of complementarity with substrate
- Show the logical sequence: pH change → shape change → fewer complexes → lower rate
- Mention that denaturation is irreversible
Common mistakes
- Saying the enzyme is ‘less active’ without explaining shape change
- Confusing ‘optimum pH’ with ‘pH of the solution’
- Assuming the enzyme can recover after pH change
Mark scheme (5 marks)
- As pH moves away from the optimum, the active site begins to change shape / becomes denatured
- The substrate (hydrogen peroxide) is no longer complementary to / can no longer fit into the active site
- Fewer / no enzyme-substrate complexes can form
- The rate of reaction decreases
- Denaturation is permanent / the enzyme cannot return to its original shape even if pH is restored to optimum
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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