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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
As the concentration of substrate increases, more enzyme–substrate complexes are formed per unit time, so the reaction rate rises.
At low substrate concentrations, many active sites are free; the rate is therefore proportional to substrate concentration.
When substrate concentration becomes high, all active sites are occupied (enzyme saturation); further increases in substrate do not raise the rate.
The maximum rate (Vmax) is reached at saturation and can only be increased by adding more enzyme.
At low substrate concentrations, many active sites are free; the rate is therefore proportional to substrate concentration.
When substrate concentration becomes high, all active sites are occupied (enzyme saturation); further increases in substrate do not raise the rate.
The maximum rate (Vmax) is reached at saturation and can only be increased by adding more enzyme.
Examiner tips
- Use the term ‘enzyme–substrate complex’ and ‘saturation’
- Show the proportional relationship at low [S] and the plateau at high [S]
- Mention Vmax and that it is limited by enzyme amount
Common mistakes
- Confusing substrate concentration with enzyme concentration
- Saying the rate continues to rise indefinitely
- Forgetting to mention that Vmax is reached when all sites are occupied
Mark scheme (4 marks)
- As substrate concentration increases, the rate of reaction increases because more enzyme–substrate complexes form per unit time.
- At low substrate concentrations, many active sites are unoccupied, so rate is proportional to substrate concentration.
- At high substrate concentrations, all active sites become occupied (enzyme saturation), so further increases in substrate concentration do not increase the rate.
- The maximum rate (Vmax) is reached at saturation and can only be increased by adding more enzyme.
Key terms in this question
enzyme–substrate complex · saturation
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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