Explain why enzymes are described as specific, and describe what happens to enzyme activity when the substrate concentration is increased from a very low level.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes are biological catalysts that speed up chemical reactions in living organisms. Each enzyme catalyses only one type of reaction.
Model answer (4 marks)
Enzymes are specific because each enzyme has an active site with a unique or complementary shape. Only a substrate that matches this shape can bind to the active site, so the enzyme catalyses only that reaction.
When substrate concentration is increased from a very low level, the rate of reaction rises because more enzyme molecules bind substrate. As the concentration continues to rise, the rate eventually levels off when all active sites are occupied; the enzyme becomes the limiting factor and the reaction reaches a maximum rate.
When substrate concentration is increased from a very low level, the rate of reaction rises because more enzyme molecules bind substrate. As the concentration continues to rise, the rate eventually levels off when all active sites are occupied; the enzyme becomes the limiting factor and the reaction reaches a maximum rate.
Examiner tips
- Use the terms ‘active site’ and ‘complementary shape’ to show understanding of specificity.
- Explain the change in rate with increasing substrate and mention the maximum rate (Vmax).
- Show the two stages: rising rate and plateau due to saturation.
Common mistakes
- Claiming enzymes are non‑specific or that all substrates can bind.
- Forgetting to mention that the plateau occurs when all active sites are occupied.
- Using vague phrases like ‘more enzymes’ instead of ‘more substrate binding’.”
Mark scheme (4 marks)
- Enzymes are specific because each enzyme has an active site with a unique/complementary shape
- Only a substrate with a complementary shape can fit into / bind to the active site
- As substrate concentration increases from a low level, the rate of reaction increases
- Rate of reaction levels off / reaches a maximum because all active sites are occupied / enzyme molecules become the limiting factor
Key terms in this question
enzyme · substrate · substrate concentration · specific
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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