AQA AS Level Biology Paper 2, June 2025: Question 6

10 marks · Medium difficulty · Practical Techniques & Data Analysis

Explain enzyme action using the induced-fit model, suggest safety precautions, calculate the surface area comparison of potato discs, and explain the resulting rate of reaction.

Practise this question

Question

Question text

06 The reaction catalysed by the enzyme catalase is shown below.

06.1 Use your knowledge of the induced-fit model to explain how catalase breaks down

hydrogen peroxide.

[3 marks]

Potato cells contain catalase.

A student investigates the effect of the surface area of potato pieces on the rate of the

reaction of catalase.

06.2 Before starting the investigation, the student reads the following information.

Hydrogen peroxide:

• is a clear, colourless solution

• is harmful if swallowed

• causes serious eye damage

• may cause damage or irritation to the skin

• may cause respiratory irritation.

Use this information to suggest two safety precautions the student should take.

Give one reason for each safety precaution.

[2 marks]

Precaution 1

Reason

Precaution 2

Reason

The student uses the following method.

1. Cut a 75 mm long cylinder of potato into 5 discs of equal size.

2. Arrange the discs on a large pin in a boiling tube.

3. Add 40 cm3 of 1.0 mol dm–3 hydrogen peroxide.

4. Record the volume of oxygen collected in 2 minutes.

5. Repeat steps 1 to 4 with an uncut cylinder of potato 75 mm long.

Figure 10 shows the equipment the student uses.

Figure 10

06.3 Suggest why the student arranges the potato discs on the pin as shown in Figure 10.

[1 mark]

Table 3 gives measurements for one potato disc and the uncut potato cylinder used in

the investigation.

Table 3

Length (L) / mm Diameter / mm

One potato disc 15 10

Uncut potato cylinder 75 10

06.4 Calculate how many times greater the total surface area of the five discs of potato is

compared with the surface area of the uncut cylinder.

Use the information in Table 3 and the formula below.

The formula for the surface area of a disc or cylinder = 2πr(r+L)

where

r = radius

*22* L = length

π = 3.14

[3 marks]

times greater

06.5 The rate of reaction is faster with the five discs of potato than with the uncut

potato cylinder.

Explain why.

[1 mark]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. The shape of the active site becomes

1. Reject catalase has

complementary (to the hydrogen peroxide/

similar/same shape as

substrate);

hydrogen peroxide

2. (So) tension/strain/twisting/bending of the

3 2. Ignore ‘weakens

bond(s) (in hydrogen peroxide);

06.1 (3 x bonds’

AO1)

3. (So) less energy needed to break the bond(s)

Lowers activation energy;

1. Label glassware/beaker/flasks/boiling Each mark needs

tubes/hydrogen peroxide precaution and reason

because hydrogen peroxide could be mistaken

for water;

2. Accept lab coat or

2. Wear gloves

mask/visor but only if

idea of covering the

to prevent hydrogen peroxide/solution making

skin is clear

contact with skin/damaging skin;

3. Allow mask

3. (Work in) a well-ventilated room or fume

2 max

cupboard

06.2 (2 x

AO2)

to prevent inhaling hydrogen peroxide

(fumes)

to prevent respiratory damage;

4. Wear (safety) glasses/goggles

to prevent hydrogen peroxide entering the

eyes/damaging the eyes;

Ignore increasing the

To expose all of the surface (to the hydrogen

surface area

peroxide/substrate)

06.3 OR (1 x

AO2)

To maximise the surface exposed (to the hydrogen

peroxide);

Question Marking Guidance Mark Comments 13

Correct answer of 1.3/1.25 = 3 marks;;; Surface areas 3140

and 2512 if 3.14 used

Incorrect answer but shows 3140 and 2512 = for π

2 marks Surface areas 3142

and 2513 if calculator

OR used for π

30.8 if surface area of

06.4 Incorrect answer but shows 3142 and 2513 = (3 x cylinder divided by

2 marks AO2) surface area of discs

1.5/1.47 if diameter

OR used instead of radius

628 for surface area

Incorrect answer of 1.5/1.47 = 2 marks;; of one disc

Incorrect answer but shows 628/628.3 = 1 mark;

(Total surface area greater so) more enzyme- 1 Accept E-S

06.5 substrate complexes; (1 x complexes

AO2)

How to answer it

Enzyme Action, Safety & Surface Area: Catalase Investigation

WHAT THIS QUESTION TESTS

This multi-part practical and theoretical question tests core skills across enzyme kinetics, experimental design, and mathematical analysis:

Induced-Fit Mechanism: How active site conformational changes put strain on substrate bonds to lower activation energy.
Practical Risk Assessment: Identifying specific laboratory hazards from raw data and prescribing targeted safety controls.
Apparatus Setup: Rationalising experimental configurations designed to maximise surface contact with substrate solutions.
Quantitative Geometry: Handling cylinder/disc surface area formulae, resolving radius vs diameter, and calculating ratios accurately.
Collision Theory & Initial Rates: Explaining kinetic differences in terms of effective enzyme-substrate complex ( E-S complex ) formation.

Question 06.1 • 3 Marks

The Induced-Fit Model of Catalase Action

Explaining catalytic breakdown of hydrogen peroxide via active site conformational change

✅ Mark Scheme Criteria

The shape of the active site changes / becomes complementary to the substrate (hydrogen peroxide).
This causes bending, twisting, tension, or strain on the chemical bond(s) in hydrogen peroxide.
Consequently, less energy is needed to break the bonds (or lowers activation energy).

🧠 Exam Technique

Contextualise: Name the substrate ( hydrogen peroxide ) directly—do not speak purely in abstract enzyme terms.
Be precise with change: The active site is not fully complementary initially; it becomes complementary upon binding.
Explain how activation energy falls: Do not just state "it lowers activation energy"; explicitly explain that putting strain/stress on the bonds lowers the energy needed to break them.

❌ Common Errors & Pitfalls

Describing lock-and-key: Saying the active site has a fixed, rigid shape that is already complementary before binding.
Shape confusion: Stating that catalase has the same shape as hydrogen peroxide (the substrate must fit into the active site).
Vague descriptions: Writing that the enzyme simply "weakens bonds" without explaining the physical strain or tension induced.

💡 Examiner Insight

Top-tier answers clearly sequenced the events: initial non-perfect fit → conformational change upon binding leading to induced fit → mechanical strain on substrate bonds → lower activation energy ( Eₐ ) → product formation.

Mark allocation: 3 marks (AO1). 1 mark per bullet point in the mark scheme.

Question 06.2 • 2 Marks

Safety Precautions & Risk Management

Formulating risk controls based on specific hazardous properties of hydrogen peroxide

✅ Acceptable Precautions & Paired Reasons

Precaution: Wear eye protection / safety goggles.
Reason: Prevents hydrogen peroxide entering/splashing into eyes and causing damage.
Precaution: Wear protective gloves (or lab coat).
Reason: Prevents contact with skin / avoids skin irritation or chemical burns.
Precaution: Work in a well-ventilated room or fume cupboard (or wear a mask).
Reason: Avoids inhaling fumes / prevents respiratory irritation or damage.
Precaution: Clearly label beakers / glassware.
Reason: It is colourless and could be mistaken for water and swallowed.

❌ Common Errors

Mismatched precaution and reason: Suggesting gloves to stop ingestion, or a fume cupboard to prevent eye splashes. Each mark requires a coherent, matched pair.
Vague precautions: Writing "be careful", "handle with care", or "wash hands after" without a specific preventative control measure.
Ignoring the prompt: Proposing generic lab safety rules (e.g., tying hair back) that do not directly address the hazards listed in the stimulus.

Mark allocation: 2 marks (AO2). 1 mark awarded for each distinct precaution paired correctly with its corresponding reason.

Question 06.3 • 1 Mark

Experimental Apparatus Design

Why potato discs are mounted separately on a large pin

✅ Mark Scheme Answer

To expose all of the surface of the potato discs to the hydrogen peroxide solution / substrate.

OR: To maximise the surface area in contact with the liquid and prevent discs sticking together or lying flat on the tube floor.

🧠 Exam Technique & Trap Alert

Do NOT just say: "To increase the surface area." Cutting the cylinder into discs already increased the total surface area!
Do say: The pin ensures that all surfaces are exposed to the surrounding liquid, preventing discs from resting flat on the bottom or clumping against each other, which would shield active sites.

Mark allocation: 1 mark (AO2). Mark scheme explicitly notes: "Ignore increasing the surface area". Must mention exposure to substrate.

Question 06.4 • 3 Marks

Surface Area Calculation: Discs vs Cylinder

Quantitative comparison using the geometric formula: Surface Area = 2πr(r + L)

📐 Step-by-Step Calculation

Step 1: Identify key dimensions
Diameter = 10 mm → Radius (r) = 5 mm
Discs: Length (L) = 15 mm, Quantity = 5
Uncut cylinder: Length (L) = 75 mm, Quantity = 1
Formula: Area = 2πr(r + L) (where π = 3.14)

Step 2: Surface area of ONE disc
SA = 2 × 3.14 × 5 × (5 + 15)
SA = 31.4 × 20 = 628 mm² (or 628.3 mm² if using full π)

Step 3: Total surface area of FIVE discs
Total SA = 5 × 628 = 3140 mm² (or 3141.6 mm² using full π)

Step 4: Surface area of UNCUT cylinder
SA = 2 × 3.14 × 5 × (5 + 75)
SA = 31.4 × 80 = 2512 mm² (or 2513.3 mm² using full π)

Step 5: Calculate ratio (how many times greater)
Ratio = 3140 ÷ 2512 = 1.25 (or 1.3 times greater)

❌ Calculation Traps & Partial Credit

Diameter vs Radius Trap: Using d = 10 instead of r = 5 gives 1.5 or 1.47 (scores 2 marks for working).
Forgetting 5 Discs: Dividing 628 by 2512 gives 0.25 (confusing 1 disc with all 5 discs).
Inverting the Ratio: Dividing cylinder by discs (2512 ÷ 3140 = 0.8) gives how much smaller the cylinder is, not how many times greater the discs are.
Partial Marks:
- Showing 3140 and 2512 = 2 marks
- Showing 628 (one disc SA) = 1 mark

Mark allocation: 3 marks (AO2). Correct final answer of 1.25 or 1.3 gains full 3 marks automatically.

Question 06.5 • 1 Mark

Explaining the Increased Rate of Reaction

Relating surface area to enzyme-substrate collision rates

✅ Mark Scheme Answer

More enzyme-substrate complexes (or E-S complexes) are formed (per unit time).

🧠 Exam Technique: The Golden Phrase

Whenever an AQA Biology question asks why an enzyme-catalysed reaction is faster due to higher substrate concentration, enzyme concentration, or surface area, your immediate automatic response must be:

"More enzyme-substrate complexes (E-S complexes) formed"

Do not simply write "more collisions occur". At AS Level, collisions are only effective if they produce enzyme-substrate complexes.

Mark allocation: 1 mark (AO2). Accept abbreviations such as E-S complexes .

Topics

Biology · Practical skills · Required Practicals · 3.1 Biological molecules · Experimental design · Data analysis · AS practicals (1–6)

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.