AQA A-Level Biology Paper 1, November 2021: Question 1

8 marks · Medium difficulty · Short Answer

Describe the induced-fit model of enzyme action, identify the substrate for ATP synthase to produce ATP, suggest a procedure to stop an enzyme reaction, and explain the effect of inorganic phosphate concentration on ATP concentration.

Practise this question

Question

A four-part exam question about enzymes, ATP synthase, and inorganic phosphate concentration. Question 01.1 asks to describe the induced-fit model and how an enzyme acts as a catalyst (3 marks). Question 01.2 presents multiple-choice checkboxes for Adenine, Adenosine diphosphate, Glucose, and Ribose to identify the substrate needed alongside inorganic phosphate to produce ATP (1 mark). Question 01.3 includes Figure 1, a line graph showing the concentration of ATP produced against the concentration of inorganic phosphate (mmol dm-3), and asks to suggest and explain a procedure to stop the reaction after 2 minutes (2 marks). Question 01.4 asks to explain the change in ATP concentration with increasing inorganic phosphate concentration based on the graph (2 marks).
Question text

01.1 Describe the induced-fit model of enzyme action and how an enzyme acts as a

catalyst.

[3 marks]

01.2 Scientists investigated the action of the enzyme ATP synthase. They made reaction

mixtures each containing:

• ATP synthase

• buffer (to control pH)

• substrates.

One of the substrates required in these reaction mixtures is inorganic phosphate (Pi).

Tick ( ) one box to show which other substrate the scientists must add to the reaction

mixtures to produce ATP.

[1 mark]

Adenine

Adenosine diphosphate

Glucose

Ribose 3

01.3 The scientists investigated the effect of concentration of inorganic phosphate (Pi) on

ATP synthase activity.

After 2 minutes, they stopped each reaction and then measured the concentration of

ATP.

Figure 1 shows the scientists’ results.

Figure 1

Suggest and explain a procedure the scientists could have used to stop each reaction.

[2 marks]

01.4 Explain the change in ATP concentration with increasing inorganic phosphate

concentration.

[2 marks]

Mark scheme

Show the mark scheme The mark scheme providing answers for questions 01.1 to 01.4. For 01.1, points include substrate binding to the active site forming an enzyme-substrate complex, active site changing shape to become complementary or to distort/break/form bonds, and lowering activation energy. For 01.2, the correct answer is adenosine diphosphate. For 01.3, accepted methods to stop the reaction include boiling/adding acid to denature the enzyme, cooling to lower kinetic energy, or adding a high concentration of an inhibitor. For 01.4, points include increasing Pi concentration forming more enzyme-substrate complexes, and active sites becoming saturated or enzyme concentration becoming a limiting factor at or above 40 mmol dm-3.

Question Marking Guidance Mark Comments

01.1 1. Substrate binds to the active site/enzyme 1. Accept for ‘binds’,

OR fits

Enzyme-substrate complex forms;

2. Active site changes shape (slightly) so it is

complementary to substrate

OR

Active site changes shape (slightly) so

distorting/breaking/forming bonds in the

substrate;

3. Reduces activation energy;

01.2 Adenosine diphosphate; 1

01.3 Mark in pairs, 1 and 2 OR 3 and 4 OR 5 and 6 1. Accept heat at >

50oC OR at very

1. Boil high temperatures

OR

Add (strong) acid/alkali;

2. Accept for

2. Denatures the enzyme/ATP synthase; 'denatures', a

description of

OR denaturation

3. Put in ice/fridge/freezer;

4. Lower kinetic energy so no enzyme-substrate 4. Accept ES for

complexes form; enzyme substrate

complex

OR

5. Add high concentration of inhibitor;

6. Enzyme-substrate complexes do not form;

– A-LEVEL BIOLOGY – –

01.4 1. (With) increasing Pi concentration, more

enzyme-substrate complexes are formed;

2. At or above 40 (mmol dm-3) all active sites

occupied 2

OR

At or above 40 (mmol dm-3) enzyme

concentration is a limiting factor;

How to answer it

Enzymes and ATP Synthase Study Guide

What this question tests

This exam sequence assesses your core knowledge of enzyme kinetics and mechanisms (specifically the induced-fit model), practical techniques regarding enzyme control and inhibition, and your ability to interpret enzyme-substrate concentration graphs. You will need to use precise biological terminology to describe enzyme action and limiting factors.

Question 01.1 (3 marks)

Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst.

✅ Correct Answer Structure

  • Point 1: The substrate binds to the active site / forms an enzyme-substrate complex.
  • Point 2: The active site changes shape slightly so that it becomes complementary to the substrate (or moulds around the substrate / distorts/breaks/forms bonds).
  • Point 3: This lowers the activation energy of the reaction.

💡 Key Knowledge

Unlike the older "lock and key" model, the induced-fit model recognises that the active site is flexible. The interaction between the substrate and enzyme induces a conformational change in the tertiary structure of the enzyme's active site.

🧠 Exam Technique

Ensure you explicitly state that the active site changes shape after or as the substrate binds. Merely stating "the substrate fits into the active site" is insufficient for top-level credit as it describes lock-and-key.

❌ Common Errors

  • Stating that the substrate changes shape to fit the enzyme (it is the enzyme's active site that changes shape).
  • Vague references to "lowering energy" without specifying activation energy.
Question 01.2 (1 mark)

Substrate required for ATP synthesis alongside inorganic phosphate (Pi)

✅ Correct Answer

Tick the box for: Adenosine diphosphate (ADP)

💡 Key Knowledge

ATP (Adenosine triphosphate) is synthesised via a condensation reaction joining Adenosine diphosphate (ADP) and inorganic phosphate ( Pi ), catalysed by the enzyme ATP synthase.

Question 01.3 (2 marks)

Suggest and explain a procedure the scientists could have used to stop each reaction.

✅ Correct Answers (Marked in Pairs)

  • Pair 1 & 2: Boil (or add strong acid/alkali) which denatures the enzyme / ATP synthase.
  • Pair 3 & 4: Put in ice/fridge/freezer to lower kinetic energy so no enzyme-substrate complexes form.
  • Pair 5 & 6: Add a high concentration of inhibitor so enzyme-substrate complexes do not form.

🧠 Exam Technique

This is a marked-in-pairs question. You must give a valid action (e.g., boiling) paired directly with its correct scientific explanation (e.g., causes denaturation of the enzyme). Mentioning temperature reduction without linking it to kinetic energy or collision frequency loses the second mark.

❌ Common Errors

  • Stating "put in the freezer" without explaining that it reduces kinetic energy/enzyme activity.
  • Saying that boiling "kills" the enzyme (enzymes are molecules, not living things; you must use the term denatured).
Question 01.4 (2 marks)

Explain the change in ATP concentration with increasing inorganic phosphate concentration.

✅ Correct Answer

  • 1. With increasing Pi concentration, more enzyme-substrate complexes are formed (driving the initial upward curve).
  • 2. At or above 40 mmol dm⁻³ , all active sites are occupied (or enzyme concentration becomes the limiting factor), explaining the plateau.

💡 Key Knowledge

In enzyme kinetics graphs, an initial linear/curved rise occurs because substrate concentration is limiting. Once the curve levels off (plateau), further increases in substrate have no effect because all active sites are saturated at any given moment.

🧠 Exam Technique

Read graph axes carefully. Quote the threshold value from the x-axis ( 40 mmol dm⁻³ ) to earn the explanation mark for saturation/limiting factors.

Topics

Biology · Practical skills · 3.1 Biological molecules · Experimental design

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.