A student claims that increasing substrate concentration will always increase the rate of an enzyme-controlled reaction. Explain why this claim is not fully correct.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes are biological catalysts that bind substrate molecules at their active sites to form enzyme-substrate complexes. The rate of an enzyme-controlled reaction can be affected by several factors, including substrate concentration.
Model answer (4 marks)
1. At low substrate concentrations the rate rises because more enzyme‑substrate complexes can form.
2. When substrate concentration becomes very high, all active sites are occupied and the enzymes are saturated.
3. Once saturation is reached the rate plateaus at a maximum (Vmax) and cannot increase further.
4. Beyond this point the rate is limited by the number of enzyme molecules, not by substrate concentration.
2. When substrate concentration becomes very high, all active sites are occupied and the enzymes are saturated.
3. Once saturation is reached the rate plateaus at a maximum (Vmax) and cannot increase further.
4. Beyond this point the rate is limited by the number of enzyme molecules, not by substrate concentration.
Examiner tips
- Use the word ‘saturation’ to show understanding of Vmax.
- Mention that the plateau occurs because all active sites are occupied.
- Show the progression from increasing rate to a maximum rate.
- Link the limitation to enzyme concentration rather than substrate.
Common mistakes
- Claiming the rate continues to rise indefinitely with substrate.
- Forgetting to mention that the plateau is due to all active sites being occupied.
- Confusing enzyme concentration with substrate concentration as the limiting factor.
Mark scheme (4 marks)
- Rate increases as substrate concentration increases initially because more enzyme-substrate complexes can form
- At high substrate concentrations, all active sites are occupied / enzymes are saturated
- Beyond saturation, rate reaches a maximum (Vmax) and cannot increase further
- The rate is now limited by the number of enzyme molecules / enzyme concentration, not substrate concentration
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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