Explain why enzyme-controlled reactions have an optimum substrate concentration, and describe what happens to the rate of reaction when substrate concentration continues to increase beyond this optimum.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes catalyse chemical reactions in living organisms. The rate of an enzyme-controlled reaction is affected by substrate concentration.
Model answer (5 marks)
Enzymes have an active site that binds substrate molecules, forming an enzyme‑substrate complex.
As substrate concentration rises, more complexes are formed, so the reaction rate increases.
At the optimum concentration all active sites are occupied and the enzyme works at maximum rate.
If substrate concentration is increased further, the rate no longer rises – it levels off or plateaus.
This is because there are no free active sites left for additional substrate molecules to bind to.
As substrate concentration rises, more complexes are formed, so the reaction rate increases.
At the optimum concentration all active sites are occupied and the enzyme works at maximum rate.
If substrate concentration is increased further, the rate no longer rises – it levels off or plateaus.
This is because there are no free active sites left for additional substrate molecules to bind to.
Examiner tips
- Use the term ‘active site’ and ‘enzyme‑substrate complex’.
- Show the sequence: more substrate → more complexes → higher rate → saturation point → plateau.
- Mention that the plateau occurs because all sites are occupied.
Common mistakes
- Confusing ‘substrate’ with ‘product’ in the explanation.
- Saying the rate keeps increasing indefinitely instead of plateauing.
- Using vague terms like ‘maximum’ without linking to active site occupancy.
Mark scheme (5 marks)
- Enzymes have an active site that substrate molecules bind to / enzyme and substrate form an enzyme-substrate complex
- As substrate concentration increases, more enzyme-substrate complexes form, so the rate of reaction increases
- At the optimum substrate concentration, all active sites are occupied / enzymes are working at maximum rate
- Beyond the optimum, the rate of reaction does not increase / levels off / plateaus
- Because there are no free active sites available for additional substrate molecules to bind to
Key terms in this question
substrate concentration · optimum · rate of reaction
Related
- Revision notes: Proteins and Enzymes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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