# A student investigates the effect of increasing substrate concentration on the rate of an enzyme-controlled reaction. At low substrate concentrations, the rate increases proportionally. However, beyond a certain substrate concentration, the rate plateaus and adding more substrate has no further effect. Explain why the rate of reaction plateaus at high substrate concentrations.

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

> Enzymes catalyse reactions by binding substrate molecules at the active site to form an enzyme-substrate complex. The rate of an enzyme-controlled reaction can be affected by substrate concentration.

## Mark scheme (4 marks)

1. At high substrate concentrations, all active sites are occupied / saturated
2. Enzyme-substrate complexes are forming at the maximum possible rate
3. Adding more substrate does not increase the rate because there are no free active sites available for additional substrate molecules to bind to
4. The rate can only increase if more enzyme is added, as this would provide more active sites

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-investigates-the-effect-of-d82f24d4) · Published by Druglandscape Ltd.