This free quiz has 12 real AQA A-level Biology exam questions on enzymes (26 marks, about 35 minutes). It marks itself: tick-box and graph questions are marked for you, and for written answers you check your work against the real AQA mark scheme.
What the quiz covers (AQA 3.1.4.2)
activation energy and how enzymes lower it
the induced-fit and lock and key models
why forming enzyme-substrate complexes increases the rate
working out a rate from a graph with a tangent
competitive and non-competitive inhibitors
how enzymes with different amino acid sequences can catalyse the same reaction
explaining results with a competitive inhibitor (4 marks)
temperature and denaturation (4 marks)
How to use it
Write each answer in full before you open the mark scheme. Most marks here come from linking ideas in a chain: active site shape, then complementary to the substrate, then enzyme-substrate complexes, then rate.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
Induced fit vs lock and key. Many students wrote that the enzyme changes shape, rather than the active site, or said the substrate changes shape.
✔ Gains credit: in the induced-fit model the active site changes shape so it becomes complementary to the substrate; in the lock and key model the active site is rigid and already complementary.
Lowering activation energy. Many knew enzymes lower activation energy but couldn't explain how. In one question only 18% linked it to bending bonds in the substrate.
✔ Gains credit: as the active site changes shape around the substrate it puts strain on bonds in the substrate, so less energy is needed to break them.
Active site and tertiary structure. Vague statements such as “the enzyme is complementary to the substrate”, with no mention of the active site, lost marks. “3°” isn't accepted for tertiary.
✔ Gains credit: the active site has a specific tertiary structure, so it is complementary only to its substrate and an enzyme-substrate complex can form.
Competitive inhibitors. Some said the inhibitor has the same shape as the substrate, or the same shape as the active site, or described non-competitive inhibition instead.
✔ Gains credit: a competitive inhibitor has a similar shape to the substrate, so it binds to the active site and reduces enzyme-substrate complex formation.
Non-competitive inhibitors. Some said the inhibitor binds to the enzyme without saying where, or wrote about a vague “loss of specificity”.
✔ Gains credit: the inhibitor binds away from the active site, changing the enzyme's tertiary structure and active site shape, so the active site is no longer complementary.
Enzyme-substrate complexes. Students often stopped at a factor, such as a larger surface area, without saying what it does. In one question only about 5% made the link. “E-S” alone wasn't accepted unless the full term appeared elsewhere.
✔ Gains credit: more enzyme-substrate complexes form, so the rate of reaction increases.
Temperature and denaturation. Some wrote that a higher temperature lowers activation energy, or that heating “kills” enzymes.
✔ Gains credit: a higher temperature gives more kinetic energy so more enzyme-substrate complexes form; a very high temperature denatures the enzyme by changing its tertiary structure.
Keep practising
The other AQA 3.1 self-marking quizzes: Carbohydrates · Lipids · Proteins · Nucleic acids · ATP, water and ions
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Questions © AQA, from past A-level and AS Biology papers, with their mark schemes and examiners’ reports.