# 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.

> WJEC A-Level Biology (Wales) — 1.3 Cell membranes and transport · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. As pH moves away from the optimum, the active site begins to change shape / becomes denatured
2. The substrate (hydrogen peroxide) is no longer complementary to / can no longer fit into the active site
3. Fewer / no enzyme-substrate complexes can form
4. The rate of reaction decreases
5. Denaturation is permanent / the enzyme cannot return to its original shape even if pH is restored to optimum

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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