Explain how temperature affects the rate of an enzyme-catalysed reaction, including the effect of temperatures both below and above the optimum.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Below the optimum temperature, lower kinetic energy means fewer enzyme–substrate collisions, reducing the rate of reaction.
At the optimum temperature, the rate of reaction is greatest because the frequency of successful enzyme–substrate collisions is maximised.
Above the optimum temperature, the enzyme begins to denature – hydrogen bonds (and other bonds) maintaining the tertiary structure break.
Denaturation alters the shape of the active site so the substrate can no longer bind, reducing (or stopping) enzyme activity – this change is permanent.
At the optimum temperature, the rate of reaction is greatest because the frequency of successful enzyme–substrate collisions is maximised.
Above the optimum temperature, the enzyme begins to denature – hydrogen bonds (and other bonds) maintaining the tertiary structure break.
Denaturation alters the shape of the active site so the substrate can no longer bind, reducing (or stopping) enzyme activity – this change is permanent.
Examiner tips
- Use the word ‘optimum’ and explain both sides of it; include ‘denaturation’ for high temperatures.
- Show the link between kinetic energy, collision frequency and enzyme activity.
- Mention that denaturation is irreversible for the exam context.
Common mistakes
- Confusing ‘optimal’ with ‘maximum’ temperature; not explaining why collisions are fewer below optimum.
- Saying the enzyme is ‘inactivated’ without mentioning denaturation and loss of active‑site shape.
- Using vague terms like ‘too hot’ without describing bond breakage.
Mark scheme (4 marks)
- Below the optimum temperature, lower kinetic energy means fewer enzyme–substrate collisions, reducing the rate of reaction.
- At the optimum temperature, the rate of reaction is greatest because the frequency of successful enzyme–substrate collisions is maximised.
- Above the optimum temperature, the enzyme begins to denature — hydrogen bonds (and other bonds) maintaining the tertiary structure break.
- Denaturation alters the shape of the active site so the substrate can no longer bind, reducing (or stopping) enzyme activity — this change is permanent.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Enzymes and metabolism questions
- Explain how the structure of an enzyme's active site determines its specificity …
- Explain how competitive inhibitors affect enzyme activity, including the effect …
- Explain how the concentration of an enzyme affects the rate of an enzyme-catalys…
- Explain how pH affects the rate of an enzyme-catalysed reaction.
- Explain how the products of enzyme-catalysed reactions accumulate in metabolic p…
- Explain how substrate concentration affects the rate of an enzyme-catalysed reac…