A student investigates the effect of increasing substrate concentration on the rate of an enzyme-controlled reaction. At low substrate concentrations, the rate increases proportionally. However, beyond a certain substrate concentration, the rate plateaus and adding more substrate has no further effect. Explain why the rate of reaction plateaus at high substrate concentrations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes catalyse reactions by binding substrate molecules at the active site to form an enzyme-substrate complex. The rate of an enzyme-controlled reaction can be affected by substrate concentration.
Model answer (4 marks)
At high substrate concentrations all enzyme active sites are occupied, i.e. the enzyme is saturated. Once every enzyme molecule is bound to substrate, the enzyme‑substrate complexes are forming at the maximum possible rate. Adding more substrate therefore cannot increase the rate because there are no free active sites left for additional substrate molecules to bind. The only way to raise the rate further is to add more enzyme, which provides more active sites.
Examiner tips
- Use the term ‘saturation’ and mention ‘maximum rate’ (Vmax).
- Explain that no free sites remain, linking to the plateau.
- Show that adding enzyme would increase rate, not substrate.
Common mistakes
- Confusing saturation with inhibition; stating that high substrate slows the reaction.
- Using vague phrases like ‘too much substrate’ without explaining site occupation.
Mark scheme (4 marks)
- At high substrate concentrations, all active sites are occupied / saturated
- Enzyme-substrate complexes are forming at the maximum possible rate
- Adding more substrate does not increase the rate because there are no free active sites available for additional substrate molecules to bind to
- The rate can only increase if more enzyme is added, as this would provide more active sites
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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