# Explain how the structure of an enzyme allows it to break down only one specific type of substrate. In your answer, refer to the active site and the 'lock and key' hypothesis.

> WJEC A-Level Biology (Wales) — 1.1 Chemical elements and biological molecules · Explain · 5 marks

## Mark scheme (5 marks)

1. The enzyme has an active site with a specific shape
2. The active site is complementary to the shape of one specific substrate
3. The substrate enters the active site to form an enzyme-substrate complex
4. Only one substrate can fit the active site, so the enzyme cannot break down other substrates — linked to the 'lock and key' hypothesis
5. The substrate is broken down and the product is released, and the enzyme is unchanged / can be reused

## Key terms

- [active site](https://www.gradenine.co.uk/glossary/active-site)
- [substrate](https://www.gradenine.co.uk/glossary/substrate)

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-an-db5879e4) · Published by Druglandscape Ltd.