Explain why an enzyme can be described as a biological catalyst, and explain how the 'lock and key' hypothesis accounts for enzyme specificity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes are proteins made from chains of amino acids. They are found in all living cells and are essential for controlling metabolic reactions.
Model answer (5 marks)
An enzyme is a biological catalyst because it increases the rate of a chemical reaction without being consumed or permanently altered, allowing it to be reused.
The lock‑and‑key hypothesis explains enzyme specificity: each enzyme has an active site with a unique shape. The substrate has a complementary shape that fits precisely into this active site, forming an enzyme‑substrate complex. Because the active site is specific, only substrates with the correct shape can bind, so the enzyme cannot act on other substrates.
The lock‑and‑key hypothesis explains enzyme specificity: each enzyme has an active site with a unique shape. The substrate has a complementary shape that fits precisely into this active site, forming an enzyme‑substrate complex. Because the active site is specific, only substrates with the correct shape can bind, so the enzyme cannot act on other substrates.
Examiner tips
- Use the term ‘biological catalyst’ and mention speed, no consumption, reuse. Show the lock‑and‑key idea: unique active site, complementary substrate shape, exclusive binding. Keep answer concise, use correct terminology.
- common_mistakes
- :
- Confusing enzyme with substrate; saying the enzyme is changed. Using vague terms like ‘fast’ instead of ‘increases rate’. Mentioning general ‘binding’ without linking to lock‑and‑key specificity.
Mark scheme (5 marks)
- An enzyme speeds up the rate of a chemical reaction
- The enzyme is not used up / not permanently changed by the reaction and can be reused
- Each enzyme has an active site with a specific / unique shape
- The substrate has a complementary shape to the active site, allowing only that substrate to fit / form an enzyme-substrate complex
- Because the active site shape is specific, the enzyme cannot act on other substrates / other substrates cannot fit the active site
Key terms in this question
biological catalyst · enzyme specificity
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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