Explain how substrate concentration affects the rate of an enzyme-catalysed reaction, including what happens when substrate concentration is increased beyond a certain point.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
At low substrate concentrations the reaction rate increases as the concentration of substrate rises because more enzyme–substrate complexes are formed per unit time. This occurs because more active sites are occupied and the frequency of enzyme–substrate complex formation increases. When substrate concentration is increased beyond a certain point the rate no longer increases; it reaches a plateau known as the maximum rate (Vmax). This plateau happens because all active sites are saturated or occupied at any given moment, so no free active sites are available to bind additional substrate molecules.
Examiner tips
- Use the term ‘substrate–enzyme complex’ and ‘active sites’. Show the two regimes: linear increase at low [S] and plateau at high [S]. Mention Vmax and saturation. Keep the answer concise and use correct terminology.
Common mistakes
- Confusing the increase in rate with an increase in enzyme concentration. Forgetting to mention that the plateau is due to saturation of active sites. Using vague phrases like ‘more substrate’ instead of ‘more enzyme–substrate complexes’.
Mark scheme (4 marks)
- At low substrate concentrations, rate increases as substrate concentration increases because more enzyme–substrate complexes form per unit time.
- Rate increases because more active sites are occupied / frequency of enzyme–substrate complex formation increases.
- Beyond a certain substrate concentration, the rate no longer increases / the rate plateaus / maximum rate (Vmax) is reached.
- 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 in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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