# Explain why enzyme-controlled reactions have an optimum substrate concentration, and describe what happens to the rate of reaction when substrate concentration continues to increase beyond this optimum.

> AQA A-Level Biology (7402) — 3.1.4 Proteins and Enzymes · Explain · 5 marks

> Enzymes catalyse chemical reactions in living organisms. The rate of an enzyme-controlled reaction is affected by substrate concentration.

## Mark scheme (5 marks)

1. Enzymes have an active site that substrate molecules bind to / enzyme and substrate form an enzyme-substrate complex
2. As substrate concentration increases, more enzyme-substrate complexes form, so the rate of reaction increases
3. At the optimum substrate concentration, all active sites are occupied / enzymes are working at maximum rate
4. Beyond the optimum, the rate of reaction does not increase / levels off / plateaus
5. Because there are no free active sites available for additional substrate molecules to bind to

## Key terms

- [substrate concentration](https://www.gradenine.co.uk/glossary/substrate-concentration)
- [optimum](https://www.gradenine.co.uk/glossary/optimum)
- [rate of reaction](https://www.gradenine.co.uk/glossary/rate-of-reaction)

## Related

- [Revision notes for AQA A-Level Biology (7402)](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-why-enzyme-controlled-reactions-have-an-26e4738c) · Published by Druglandscape Ltd.