# A student claims that competitive inhibitors always reduce the rate of an enzyme-controlled reaction to the same extent, regardless of substrate concentration. Explain why this claim is incorrect.

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

> Competitive inhibitors bind to the active site of an enzyme. They are structurally similar to the substrate and compete with it for access to the active site.

## Mark scheme (4 marks)

1. Competitive inhibitor and substrate molecules compete for the same active site
2. Increasing substrate concentration increases the probability of substrate (rather than inhibitor) binding to the active site
3. Therefore the degree of inhibition decreases as substrate concentration increases
4. At sufficiently high substrate concentration, the rate approaches that of the uninhibited enzyme (Vmax is unchanged)

## Key terms

- [competitive inhibitor](https://www.gradenine.co.uk/glossary/competitive-inhibitor)
- [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-competitive-inhibitors-1feca3e4) · Published by Druglandscape Ltd.