# A student claims that increasing substrate concentration will always increase the rate of an enzyme-controlled reaction. Explain why this claim is not fully correct.

> OCR A-Level Biology A (H420) — 2.4 Enzymes · Explain · 4 marks

> Enzymes are biological catalysts that bind substrate molecules at their active sites to form enzyme-substrate complexes. The rate of an enzyme-controlled reaction can be affected by several factors, including substrate concentration.

## Mark scheme (4 marks)

1. Rate increases as substrate concentration increases initially because more enzyme-substrate complexes can form
2. At high substrate concentrations, all active sites are occupied / enzymes are saturated
3. Beyond saturation, rate reaches a maximum (Vmax) and cannot increase further
4. The rate is now limited by the number of enzyme molecules / enzyme concentration, not substrate concentration

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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/a-student-claims-that-increasing-substrate-3e2d4d92) · Published by Druglandscape Ltd.