Proteases and lipases are enzymes found in the digestive system. Explain how the structure of an enzyme allows it to act as a biological catalyst, and why a protease cannot break down lipids.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Enzymes are proteins made of chains of amino acids that fold to give a specific three‑dimensional shape. The folded shape creates an active site that has a complementary shape to its substrate. When the substrate fits into the active site an enzyme‑substrate complex is formed; the reaction then occurs and the product is released. The enzyme itself is not altered by the reaction and can be reused, acting as a catalyst.
A protease cannot break down lipids because the lipid molecule does not fit the complementary shape of the protease’s active site. Enzyme specificity means each enzyme only acts on one particular substrate, so a protease will not act on a lipid substrate.
A protease cannot break down lipids because the lipid molecule does not fit the complementary shape of the protease’s active site. Enzyme specificity means each enzyme only acts on one particular substrate, so a protease will not act on a lipid substrate.
Examiner tips
- Use the term ‘active site’ and ‘substrate specificity’
- Show the enzyme‑substrate complex and that the enzyme is unchanged
- Explain that the shape of the active site determines which substrate can bind
Common mistakes
- Confusing the enzyme with the substrate as the catalyst
- Claiming the enzyme changes during the reaction
- Saying the protease can act on lipids because it is an enzyme
Mark scheme (5 marks)
- Enzymes are proteins made of chains of amino acids that fold to form a specific shape
- Each enzyme has an active site with a specific/complementary shape to its substrate
- The substrate fits into the active site to form an enzyme-substrate complex, and the product is released
- The enzyme is not changed by the reaction and can be reused, acting as a catalyst
- A protease cannot break down lipids because the lipid molecule does not have a complementary shape to the active site of the protease / enzyme specificity means each enzyme only acts on one specific substrate
Key terms in this question
enzyme · biological catalyst · protease
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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