# Explain how substrate concentration affects the rate of an enzyme-controlled reaction. Include in your answer what happens to the rate when substrate concentration continues to increase beyond a certain point.

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

## Mark scheme (5 marks)

1. As substrate concentration increases, the rate of reaction increases
2. Because there are more substrate molecules available to collide with / bind to the active site of the enzyme
3. Above a certain substrate concentration, the rate levels off / plateaus / stays constant
4. Because all active sites are occupied / all enzymes are working at maximum capacity
5. Adding more substrate cannot increase the rate further because there are no free active sites available

## Key terms

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

## 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-how-substrate-concentration-affects-the-6f72daa5) · Published by Druglandscape Ltd.