Enzymes: AQA A Level Biology Exam Questions by Topic

AQAEnzymesExam Questions
Written by Tom Whitburn

Enzymes (AQA 3.1.4.2) are tested on every AS and A-level paper, often in an unfamiliar context such as ATP synthase, a cancer drug or a bacterial toxin. You need the induced-fit model and how enzymes lower activation energy, how the tertiary structure of the active site makes an enzyme specific, how enzyme and substrate concentration, pH, temperature and competitive and non-competitive inhibitors change the rate, and how models of enzyme action have changed over time.

The examiners' reports show the marks go on precise wording. Students say the 'enzyme' changes shape instead of the active site, write 'specific' or 'same shape' instead of complementary, abbreviate enzyme-substrate complex to 'E-S' without ever writing it in full, leave out tertiary structure, or describe a non-competitive inhibitor without saying where it binds.

Start with how enzymes work, then active sites and specificity, then rates and inhibitors. Answer each on paper before you open the mark scheme. If you'd like help with this topic, my AQA masterclasses run online on Tuesdays at 5.15pm for Year 12 and on Wednesdays or Thursdays at 6.30pm for Year 13.

What you need from GCSE and Year 12

Each question on this page assumes you're secure on these GCSE and Year 12 ideas. If any of them feels shaky, revise it before you start.

  • GCSE: enzymes are biological catalysts that speed up reactions without being used up.
  • GCSE: the lock and key model: the substrate fits the active site, so each enzyme is specific.
  • GCSE: temperature and pH have an optimum; too high a temperature or the wrong pH denatures the enzyme.
  • Year 12 (AQA 3.1.4.1): enzymes are proteins, and their active site depends on their tertiary structure, held by hydrogen bonds, ionic bonds and disulfide bridges. See the Proteins page.

Key terms

These words come up in the questions and mark schemes. Examiners expect you to use them exactly.

  • Catalyst: a substance that speeds up a reaction without being used up.
  • Activation energy: the minimum energy needed for a reaction to start. Enzymes lower it.
  • Active site: the part of an enzyme, with a specific tertiary structure, where the substrate binds.
  • Substrate: the molecule an enzyme acts on.
  • Enzyme-substrate complex: the enzyme with its substrate bound to the active site. Write it in full: examiners don't accept 'E-S' on its own.
  • Induced fit: the model in which the active site changes shape slightly as the substrate binds, putting strain on the substrate's bonds.
  • Specificity: an enzyme usually catalyses only one type of reaction, because only a substrate with a complementary shape can bind to its active site.
  • Denatured: the tertiary structure, and so the active site, has changed shape so the substrate can no longer bind. It is not 'killed'.
  • Competitive inhibitor: a molecule with a similar shape to the substrate that binds to the active site; adding more substrate reduces its effect.
  • Non-competitive inhibitor: a molecule that binds somewhere other than the active site (an allosteric site), changing the active site's shape; more substrate doesn't reduce its effect.

What these questions cover (AQA 3.1.4.2)

The questions test these statements from the AQA AS and A-level Biology specification (7401/7402):

  • Activation energy and induced fit: Each enzyme lowers the activation energy of the reaction it catalyses. The induced-fit model of enzyme action.
  • Active site: The properties of an enzyme relate to the tertiary structure of its active site and its ability to combine with complementary substrate(s) to form an enzyme-substrate complex.
  • Specificity: The specificity of enzymes
  • Factors affecting rate: The effects of the following factors on the rate of enzyme-controlled reactions – enzyme concentration, substrate concentration, concentration of competitive and of non-competitive inhibitors, pH and temperature. (the practical side of this is on the Required practical 1 page)
  • Students should be able to appreciate how models of enzyme action have changed over time
  • Students should be able to appreciate that enzymes catalyse a wide range of intracellular and extracellular reactions that determine structures and functions from cellular to whole-organism level.

How to use this page

Write your answer on paper before you open the mark scheme. Allow about 1.3 minutes per mark: the 18 questions are worth 44 marks, so about 57 minutes in total. Then mark your answer strictly against the mark scheme and read the examiners' tip underneath it. If you'd rather print the questions, download the PDF pack.

How enzymes work (3.1.4.2)

Question 1

AS Paper 1 · June 2022 · Q05.2 · 2 marks · Medium

Give one similarity and one difference between the lock and key and induced-fit models.

Figure 5: lock and key model, with a substrate fitting an enzyme's active site and then leaving as two products.
Show the mark scheme Mark scheme: similarity, substrate binds to the active site (enzyme-substrate complex); difference, in induced fit the active site changes shape (and isn't complementary at first), but not in lock and key.

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From the examiners' report: Most got the similarity. For the difference, say the active site changes shape, not 'the enzyme'.

Question 2

AS Paper 1 · June 2022 · Q05.3 · 1 mark

State how enzymes let reactions go quickly at lower temperatures.

Question text: state how enzymes help reactions to proceed quickly at lower temperatures; do not write about active sites.
Show the mark scheme Mark scheme: lower or reduce the activation energy.

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From the examiners' report: 60% got this. 'Alternative pathway' or just 'catalysing the reaction' didn't score.

Question 3

A level Paper 1 · June 2018 · Q04.1 · 2 marks

Explain how forming an enzyme-substrate complex increases the rate of reaction.

Question text: formation of an enzyme-substrate complex increases the rate of reaction; explain how, 2 marks.
Show the mark scheme Mark scheme: reduces activation energy; by bending (stressing) bonds, or without the enzyme few substrates have enough energy.

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From the examiners' report: Many knew activation energy is lowered, but only 18% explained it by bending the bonds in the substrate.

Question 4

AS Paper 2 · June 2025 · Q06.1 · 3 marks · Medium

Use the induced-fit model to explain how catalase breaks down hydrogen peroxide.

Question text: catalase breaks down hydrogen peroxide into water and oxygen; use the induced-fit model to explain how, 3 marks.
Show the mark scheme Mark scheme: the active site becomes complementary to the substrate; strain or bending on the substrate's bonds; less energy needed, lowering activation energy.

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From the examiners' report: Good answers described induced fit, the strain on bonds and the lower activation energy. Weaker ones just rewrote the equation.

Question 5

A level Paper 1 · June 2025 · Q02.4 · 4 marks · Medium

Compare a 1914 description of enzymes with the induced-fit model.

Question text: in 1914 scientists concluded an enzyme causes a reaction at ordinary temperatures by being present but not by combining with another substance; describe similarities and differences with our current understanding.
Show the mark scheme Mark scheme: similarities, enzymes are not changed or used up, and speed up reactions at ordinary temperatures (lower activation energy); differences, the active site changes shape, and an enzyme-substrate complex forms.

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From the examiners' report: Fewer than 10% got all four. Be precise about the active site changing shape, and say substrate, not 'substance'.

Question 6

A level Paper 1 · June 2017 · Q04.3 · 2 marks

Suggest an advantage to a bacterium of secreting an extracellular protease.

Question text: some bacteria secrete proteases as extracellular enzymes; suggest one advantage in its natural environment and explain it.
Show the mark scheme Mark scheme: to digest protein; so it can absorb amino acids for growth or protein synthesis, or destroy a toxic protein.

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From the examiners' report: There's no examiners' report for this paper, so this tip comes from the mark scheme. Say what the enzyme digests and what the bacterium then gains.

Active sites and specificity (3.1.4.2)

Question 7

A level Paper 3 · June 2017 · Q05.2 · 2 marks · Medium

Suggest why phosphorylation lets enzymes bind to their substrates.

Question text: ATR phosphorylates other DNA-repair enzymes, which then become able to bind to their substrates; use your knowledge of enzyme structure to suggest why.
Show the mark scheme Mark scheme: changes the tertiary structure of the enzyme; the active site forms or becomes complementary.

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From the examiners' report: About 38% got both marks. Link the change in tertiary structure to the active site becoming complementary.

Question 8

A level Paper 1 · June 2022 · Q05.2 · 2 marks

Explain how two enzymes with different amino acid sequences can catalyse the same reaction.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question text: explain how two enzymes with different amino acid sequences can catalyse the same reaction.
Show the mark scheme Mark scheme: both active sites have a similar tertiary structure; so both form enzyme-substrate complexes with the same substrate.

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From the examiners' report: Many scored nothing, writing about the degenerate code or mutations. Talk about the active sites' tertiary structure, and write enzyme-substrate complex in full.

Question 9

AS Paper 2 · June 2023 · Q03.3 · 2 marks

Explain why human amylase does not digest GOS.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question text: galacto-oligosaccharides (GOS) are polymers of galactose; explain why amylase produced in the human digestive system does not digest GOS.
Show the mark scheme Mark scheme: the active site is only complementary to starch (not GOS); because of its tertiary structure.

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From the examiners' report: Marks were lost for vague claims that 'amylase is not complementary to GOS'. 'Same shape' and 'binding sites' got no credit.

Question 10

AS Paper 2 · June 2023 · Q02.2 · 4 marks · Hard

Suggest how the shape of ATP synthase lets it make ATP and let H+ ions through.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Figure 1: ATP synthase spanning the inner mitochondrial membrane, with H+ ions passing through it and ADP + Pi forming ATP.
Show the mark scheme Mark scheme: active site complementary to ADP + Pi; enzyme-substrate complex forms; a channel; facilitated diffusion of H+ (or a tertiary structure specific for H+).

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From the examiners' report: Only about 25% got 2 or more. Too many said 'enzymes are complementary to substrates' with no active site, and 'E-S' alone wasn't accepted.

Factors affecting the rate and inhibitors (3.1.4.2)

Question 11

A level Paper 1 · November 2020 · Q02.2 · 2 marks · Easy

Explain why gut samples were heated to 70 °C straight after collection.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question text: a scientist collected samples from a man's gut at intervals after a meal of triglycerides, then immediately heated them to 70 °C for 10 minutes; explain why.
Show the mark scheme Mark scheme: to denature the enzymes (lipase); so no further digestion happened.

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From the examiners' report: 53% got both marks. Relate it to this investigation: no more lipid is digested. Enzymes are denatured, not 'killed'.

Question 12

A level Paper 1 · November 2021 · Q01.4 · 2 marks

Explain how ATP production changes as phosphate concentration rises.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Figure 1: concentration of ATP produced rising with inorganic phosphate concentration from 0 to 40 mmol dm⁻³ and then levelling off at about 13 units.
Show the mark scheme Mark scheme: more Pi gives more enzyme-substrate complexes; at 40 mmol dm⁻³ and above all active sites are occupied (enzyme concentration is limiting).

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From the examiners' report: Answers naming ADP as the limiting factor didn't score: in this investigation all the other substrates were in excess.

Question 13

AS Paper 2 · November 2020 · Q05.3 · 3 marks

Explain how a competitive inhibitor decreases the rate of reaction.

Question text: a competitive inhibitor decreases the rate of an enzyme-controlled reaction; explain how, 3 marks.
Show the mark scheme Mark scheme: similar shape to the substrate; binds to the active site; prevents or reduces enzyme-substrate complex formation. Reject 'same shape'.

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From the examiners' report: Many good answers. Marks were lost for saying 'identical' or 'same shape', or for describing non-competitive inhibition.

Question 14

A level Paper 1 · June 2023 · Q08.2 · 3 marks · Easy

Suggest how a non-competitive inhibitor makes dynamin inactive.

Question text: MiTMAB acts as a non-competitive inhibitor of the enzyme dynamin; suggest how MiTMAB can cause dynamin to become inactive.
Show the mark scheme Mark scheme: binds other than at the active site; changes the active site or tertiary structure; no longer complementary, so no enzyme-substrate complexes.

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From the examiners' report: Over 50% got full marks. Mark point 3 was missed by not saying the substrate can no longer bind because the active site isn't complementary.

Question 15

A level Paper 1 · June 2025 · Q03.3 · 4 marks · Medium

Use results with lactose, galactose and lactase to explain competitive inhibition.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Table 2: glucose concentration after 10 minutes is 100 for lactose + galactose + lactase, 180 for lactose + water + lactase, and 0 for galactose + water + lactase.
Show the mark scheme Mark scheme: (X) galactose binds to the active site; so less lactose hydrolysis; (Y) more lactose hydrolysed so more glucose; (Z) no substrate, or galactose isn't a lactase substrate. Reject 'stops'.

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From the examiners' report: About 15% got all four. Say galactose binds to the active site (not just 'a site'), and use 'hydrolysis'.

Question 16

A level Paper 1 · June 2025 · Q03.4 · 2 marks · Hard

Give a change to the experiment that would confirm galactose is a competitive inhibitor.

Question text: give one change the student could make in tube X to confirm galactose is a competitive inhibitor, and the expected result.
Show the mark scheme Mark scheme: increase the lactose (substrate) concentration; more product (glucose) or a faster rate.

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From the examiners' report: About 25% got both. Changing only the galactose concentration changes the amount of inhibition, not its type.

Question 17

AS Paper 2 · November 2020 · Q05.4 · 3 marks · Hard

Suggest how a competitive inhibitor of amylase in stale bread could reduce weight gain.

Question text: retrograded starch in stale bread may be a competitive inhibitor of amylase in the small intestine; suggest how eating stale bread could help reduce weight gain.
Show the mark scheme Mark scheme: less hydrolysis of starch; to maltose; so less glucose absorbed, or more starch egested.

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From the examiners' report: Over 50% scored, usually for less absorption. Few named maltose as the product of amylase, or used the word hydrolysis.

Question 18

AS Paper 2 · June 2025 · Q06.5 · 1 mark · Hard

Explain why potato discs release oxygen faster than a whole cylinder.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Earlier part of the same exam question, giving the information this part uses. Question text: the rate of reaction is faster with five discs of potato than with the uncut potato cylinder; explain why.
Show the mark scheme Mark scheme: greater total surface area, so more enzyme-substrate complexes.

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From the examiners' report: Only about 5% got this. Most just said the surface area was bigger; link it to more enzyme-substrate complexes forming.

Where students lose marks (from AQA examiners' reports and mark schemes)

Each point below is followed by wording that gained marks.

  • The active site changes shape: Many wrote that in induced fit 'the enzyme changes shape', or said the active site is complementary in both models, which isn't true of induced fit.

    ✔ Gains credit: "In induced fit the active site changes shape as the substrate binds; in lock and key the active site is already complementary and doesn't change."

  • Bend the bonds, lower the energy: Many said enzymes lower activation energy, but only 18% explained it by the enzyme putting strain on the substrate's bonds.

    ✔ Gains credit: "Reduces activation energy, by bending/stressing the bonds in the substrate."

  • Write enzyme-substrate complex in full: Examiners repeatedly penalised 'E-S' used on its own: the abbreviation isn't in the specification.

    ✔ Gains credit: "…so more enzyme-substrate complexes form" (write it in full at least once).

  • Complementary because of tertiary structure: Answers made vague claims that amylase is 'not complementary to GOS', or used 'same shape' and 'binding sites', without linking the active site to tertiary structure.

    ✔ Gains credit: "The active site is only complementary to starch, because of its tertiary structure."

  • Similar shape, not the same shape: Some said a competitive inhibitor has the same shape as the substrate, or the same shape as the active site, and some described non-competitive inhibition instead.

    ✔ Gains credit: "The inhibitor has a similar shape to the substrate, binds to the active site and reduces enzyme-substrate complex formation."

  • Say where a non-competitive inhibitor binds: Errors included the inhibitor binding to the enzyme without saying where, or the enzyme changing shape without mentioning its active site, or a vague 'loss of specificity'.

    ✔ Gains credit: "Binds at a site other than the active site, changes the tertiary structure so the active site is no longer complementary, so fewer enzyme-substrate complexes."

  • More substrate tests competitive inhibition: To confirm competitive inhibition, some only changed the inhibitor concentration, which changes the amount of inhibition but not its type.

    ✔ Gains credit: "Increase the lactose (substrate) concentration; more glucose is produced."

  • Surface area means more complexes: Only about 5% explained why potato discs worked faster than a cylinder; most just repeated that the surface area was bigger.

    ✔ Gains credit: "Greater total surface area, so more enzyme-substrate complexes form."

Download the questions

Download the PDF pack (18 questions, mark schemes at the end)

Keep practising

Questions, mark schemes and examiners' report extracts © AQA.

Tom Whitburn
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