# Explain how the concentration of substrate affects the rate of an enzyme-catalysed reaction, with reference to enzyme–substrate complex formation and the concept of enzyme saturation.

> IB DP Biology Higher Level (2023 syllabus) — C1.1 Enzymes and metabolism · Explain · 4 marks

## Mark scheme (4 marks)

1. As substrate concentration increases, the rate of reaction increases because more enzyme–substrate complexes form per unit time.
2. At low substrate concentrations, many active sites are unoccupied, so rate is proportional to substrate concentration.
3. At high substrate concentrations, all active sites become occupied (enzyme saturation), so further increases in substrate concentration do not increase the rate.
4. The maximum rate (Vmax) is reached at saturation and can only be increased by adding more enzyme.

## Key terms

- [enzyme–substrate complex](https://www.gradenine.co.uk/glossary/enzyme-substrate-complex)
- [saturation](https://www.gradenine.co.uk/glossary/saturation)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](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/explain-how-the-concentration-of-substrate-64e509a2) · Published by Druglandscape Ltd.