# Explain why increasing the concentration of an enzyme increases the rate of a reaction, and describe what happens to the rate when the substrate concentration becomes the limiting factor.

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

> A student investigates how enzyme concentration affects the breakdown of hydrogen peroxide by catalase. She keeps substrate concentration constant throughout.

## Mark scheme (5 marks)

1. More enzyme molecules means more active sites available
2. Substrate molecules collide with / bind to active sites more frequently / faster
3. So the rate of reaction increases (as enzyme concentration increases)
4. When substrate concentration is limiting, all active sites are occupied / enzyme molecules are saturated
5. Rate no longer increases / rate levels off / rate stays constant (because adding more enzyme has no effect)

## Key terms

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

## 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-increasing-the-concentration-of-0dea4008) · Published by Druglandscape Ltd.