# Explain how substrate concentration affects the rate of an enzyme-catalysed reaction, including what happens when substrate concentration is increased beyond a certain point.

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

## Mark scheme (4 marks)

1. At low substrate concentrations, rate increases as substrate concentration increases because more enzyme–substrate complexes form per unit time.
2. Rate increases because more active sites are occupied / frequency of enzyme–substrate complex formation increases.
3. Beyond a certain substrate concentration, the rate no longer increases / the rate plateaus / maximum rate (Vmax) is reached.
4. This plateau occurs because all active sites are saturated / occupied at any given moment, so no free active sites are available to bind additional substrate molecules.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-substrate-concentration-affects-the-3d60a5c0) · Published by Druglandscape Ltd.