Explain how enzymes lower the activation energy of a reaction, and outline why this is important for metabolic reactions in living cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Enzymes bind substrate(s) at the active site to form an enzyme–substrate complex.
The binding stresses or distorts bonds in the substrate (induced‑fit/transition‑state stabilisation), lowering the energy required to reach the transition state.
A lower activation energy means a greater proportion of substrate molecules have sufficient energy to react at a given temperature, so the reaction proceeds faster.
This allows metabolic reactions to occur rapidly at the mild temperatures and near‑neutral pH of living cells, which could not sustain the high temperatures otherwise needed to overcome activation energy barriers.
The binding stresses or distorts bonds in the substrate (induced‑fit/transition‑state stabilisation), lowering the energy required to reach the transition state.
A lower activation energy means a greater proportion of substrate molecules have sufficient energy to react at a given temperature, so the reaction proceeds faster.
This allows metabolic reactions to occur rapidly at the mild temperatures and near‑neutral pH of living cells, which could not sustain the high temperatures otherwise needed to overcome activation energy barriers.
Examiner tips
- Show the four key points in order; use terms ‘active site’, ‘induced fit’, ‘transition state’, ‘activation energy’ and ‘metabolic rate’.
- Explain the link between lower activation energy and faster reaction rate, then relate this to cellular conditions.
- Use concise, exam‑style sentences – avoid unnecessary words.
- Mention the temperature and pH context to demonstrate relevance.
Common mistakes
- Failing to mention the active site or enzyme–substrate complex.
- Using vague terms like ‘speed up’ without linking to activation energy.
- Omitting the cellular temperature/pH context.
Mark scheme (4 marks)
- Enzymes bind substrate(s) at the active site to form an enzyme–substrate complex
- Binding at the active site stresses/distorts bonds in the substrate (induced fit / transition state stabilisation), reducing the energy needed to reach the transition state
- Lower activation energy means a greater proportion of substrate molecules have sufficient energy to react at a given temperature, so the reaction proceeds faster
- This allows metabolic reactions to occur rapidly at the mild temperatures and near-neutral pH of living cells, which could not sustain the high temperatures otherwise needed to overcome activation energy barriers
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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