AQA AS Level Biology Paper 1, June 2025: Question 8

11 marks · Medium difficulty · Practical Techniques & Data Analysis

Identify membrane components and investigate the effect of temperature on membrane permeability in beetroot using a dilution series and colorimeter.

Practise this question

Question

Question 8 across two pages. Figure 10 shows a diagram of a cell-surface membrane with molecule A (the phospholipid bilayer) and molecule B (an intrinsic protein). Parts 08.1 asks for names and functions of A and B (2 marks). Part 08.2 describes an experiment on beetroot membrane permeability at different temperatures, asking for a controlled variable and explanation (2 marks). Figure 11 shows test tubes in a dilution series (20%, 40%, 60%, 80%, 100% pigment). Figure 12 shows a bar graph of estimated percentage concentration of pigment vs temperature of water bath (0% at 20 °C, 10% at 40 °C, 80% at 60 °C, 90% at 80 °C, and 90% at 100 °C). Part 08.3 asks to describe and explain the general trend (4 marks). Part 08.4 asks how to use the dilution series and a colorimeter to accurately measure pigment concentration (3 marks).
Question text

08.1 Figure 10 shows a diagram of a cell-surface membrane.

Figure 10

Name and give the functions of the molecules labelled A and B.

[2 marks]

Molecule A name

Molecule A function

Molecule B name

Molecule B function

08.2 A student investigated the effect of temperature on the permeability of cell-surface

membranes in beetroot cells. Beetroot cells contain a red pigment that is not

transported across intact cell-surface membranes.

The student:

• cut discs of beetroot tissue

• took five boiling tubes and placed two discs of beetroot into each boiling tube

• added water to the boiling tubes

• placed each boiling tube into a water bath at a different temperature for 30 minutes

• removed the discs and examined the water in each boiling tube.

Give one variable the student should have controlled in this investigation and explain

why.

Do not refer to number of discs or length of time.

[2 marks]

Controlled variable

Explanation

Another student carried out the same investigation, ensuring that all variables were

controlled.

Figure 11 shows a dilution series the student prepared from 100% red beetroot

pigment solution.

Figure 11

This student used the colours of the dilution series to estimate the percentage

concentration of pigment in the boiling tubes at each temperature.

*21* 23

Figure 12 shows the student’s results.

Figure 12

08.3 Describe and explain the general trend of the results in Figure 12.

[4 marks]

08.4 Describe how the student could use the dilution series (Figure 11, on page 22) and a

colorimeter to determine an accurate measurement of the pigment concentration in

the tubes at different temperatures.

*23* [3 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 8. 08.1 awards marks for naming A as phospholipid and its function (barrier to water-soluble/polar substances or allowing lipid-soluble substances to pass) and B as protein and its function (facilitated diffusion/active transport or allowing large/charged/polar substances to pass). 08.2 awards 2 marks for a controlled variable (e.g., volume of water or disc dimensions/mass/surface area) and its paired explanation. 08.3 awards up to 4 marks for noting increasing concentration leveling off at 80 °C, increased membrane permeability/damage, denaturing of membrane proteins, increased phospholipid fluidity, and maximum membrane damage at 80 °C. 08.4 awards 3 marks for measuring absorbance/transmission of standards, plotting a calibration curve, and measuring samples to determine concentration from the curve.

Question Marking Guidance Mark Comments

1. A = phospholipid and form a barrier to Both name and

water-soluble/charged/ions/polar substances function should be

correct for 1 mark

OR

1 and 2. Accept

hydrophilic for polar

A = phospholipid and allows lipid soluble/non-

polar/hydrophobic substances to pass (through

the cell membrane) Accept for ‘water-

soluble’ non-lipid

OR soluble

A = phospholipid and simple diffusion;

2. B = protein and allow large/charged/ions/polar 2. Ignore the type of

molecules to pass (through the cell membrane) protein

2 max

08.1 OR (2 x

AO1)

B = protein and facilitated diffusion

OR

B = protein and active transport

OR

B = protein and maintain different concentrations

(either side)

OR

B = protein and providing support/shape;

Mark in pairs: 1 and 2 OR 3 and 4.

1. Ignore amount of

1. Volume of water;

water

2. (Varying volumes of water) affects the

concentration/colour (of the resulting solution);

3. Diameter/thickness of disc

3. Allow length for

thickness

OR

3. Ignore shape/size

3. Allow weight for

Surface area of disc

mass

3. Accept ‘beetroot’

14 OR

2 max for disc

08.2 Volume of disc (2 x

AO2)

OR

Mass of disc

OR

Same beetroot;

4. Affects quantity/amount of pigment

OR

Affects rate of release of pigment;

MP1 must be credited to achieve maximum 4

marks.

1. As temperature increases, (percentage)

concentration of pigment increases but levels off

at 80°C;

2. As temperature increases, the membrane

becomes damaged/more permeable; 4 max

(2 x

08.3 3. Because the membrane proteins denature at

AO1, 2

high temperatures; x AO2)

4. As temperature increases, fluidity of the cell

membrane/phospholipids increases;

5. At 80°C the membrane is fully permeable

OR 15

At 80°C maximum damage to membrane;

1. (Use colorimeter to) measure 1. Accept description

absorbance/transmission of diluted of absorbance or

solutions/series/standards; transmission

08.4 (3 x

2. Produce a calibration curve; 2. Accept descriptions

AO3)

of calibration curve

3. Measure absorbance/transmission of sample

and find concentration (on calibration curve);

How to answer it

Membrane Permeability, Transport & Calibration Curves

What this question tests

This question integrates specification sections 3.2.3 (Transport across cell membranes) and Required Practical 2 (investigating factors affecting membrane permeability):

  • Fluid Mosaic Model: Identifying phospholipids and intrinsic proteins and explaining their permeability roles.
  • Experimental Design & Control Variables: Identifying key variables that must remain constant to ensure valid measurements of leakage.
  • Data Analysis & Biological Explanations: Describing non-linear trends in quantitative data and linking them to phospholipid fluidity and protein denaturation.
  • Quantitative Apparatus (Colorimetry): Constructing and using calibration curves from known dilution standards.
Question 08.1 • 2 Marks

Membrane Components and Their Specific Functions

Identifying molecules A and B and linking structure to transport

✅ Mark Scheme Requirements

  • Molecule A: Phospholipid (or phospholipid bilayer)
    Function: Forms a barrier to water-soluble / polar substances / ions OR allows lipid-soluble / non-polar molecules to pass by simple diffusion.
  • Molecule B: Protein (intrinsic / transport / channel / carrier protein)
    Function: Allows polar / charged / ions / large water-soluble molecules to cross (via facilitated diffusion or active transport).
Both name AND function must be correct to score each mark (1 mark for A, 1 mark for B).

❌ Common Student Errors

  • Writing only "head and tail" or "lipid" instead of naming phospholipid.
  • Vague functions like "lets things in and out" without specifying what type of substances (lipid-soluble vs polar/charged).
  • Confusing the roles: claiming phospholipids transport ions or that proteins block water-soluble substances.

🧠 Exam Technique: Name & Function Pairing

When a question requires both a name and a function for a single mark, ensure you qualify the chemical nature of the molecules passing through: mention polarity (non-polar vs polar), solubility (lipid-soluble vs water-soluble), or the exact transport mechanism (simple diffusion vs facilitated diffusion / active transport).

Question 08.2 • 2 Marks

Controlling Variables in Beetroot Permeability Investigations

Identifying an uncontrolled factor and explaining its effect on validity

✅ Accepted Pairs (1 mark variable + 1 mark explanation)

Option 1:

  • Variable: Volume of water added to boiling tubes.
  • Explanation: Different volumes would dilute the pigment to different extents, directly altering the pigment concentration and color intensity independently of membrane damage.

Option 2:

  • Variable: Surface area / thickness / diameter / mass of beetroot discs.
  • Explanation: Affects the rate of diffusion or the total amount of pigment available to diffuse out.

Option 3:

  • Variable: Beetroot source / same beetroot.
  • Explanation: Different beetroots (or different ages/parts) contain different concentrations of pigment.

❌ Restrictions & Disallowed Answers

  • Directly excluded by the question: Do not state "number of discs" or "length of time".
  • Saying "amount of water" instead of volume of water loses the mark. Always use scientific quantities.
  • Saying "size of discs" without stating surface area, diameter, thickness, or mass is too vague.
Question 08.3 • 4 Marks

Describing and Explaining Temperature vs. Pigment Leakage

Linking visual graph trends to molecular changes in membrane structure

✅ Mark Scheme Breakdown (Max 4 marks)

  • MP1 (Compulsory): As temperature increases, the concentration of pigment increases and levels off / plateaus at 80°C (must state 80°C).
  • MP2: As temperature increases, the cell-surface membrane becomes damaged / more permeable.
  • MP3: Membrane proteins denature at high temperatures (tertiary structure altered, disrupting membrane integrity).
  • MP4: Phospholipids gain kinetic energy and membrane fluidity increases (gaps form between fatty acid tails).
  • MP5: At 80°C, the membrane is fully permeable / maximum damage has occurred.
⚠️ Critical Rule: MP1 is mandatory. Without stating that the increase levels off / plateaus at 80°C, candidates could not score the full 4 marks.

💡 Key Knowledge: Why Pigment Leaks

  • Below 40°C: Phospholipids have low kinetic energy; membrane is intact and relatively impermeable to betalain pigment (0–10%).
  • 40°C to 60°C: Phospholipids gain kinetic energy and move more; membrane fluidity increases significantly.
  • Above 60°C: Hydrogen and ionic bonds in membrane transport proteins break, causing proteins to denature. This tears holes in the bilayer, making it fully permeable (plateau at 80–100°C).

🧠 Exam Technique: "Describe and Explain" Commands

Always divide your answer clearly:
1. Describe (MP1): Quote values from the axes! "Increases up to 80°C and then plateaus / remains constant at 90% between 80°C and 100°C."
2. Explain (MP2–MP5): Provide biological causes at the molecular level: kinetic energy of phospholipids + denaturation of membrane proteins.

Question 08.4 • 3 Marks

Using a Dilution Series and Colorimeter (Calibration Curve)

Transitioning from qualitative visual estimates to precise quantitative data

📐 Step-by-Step Practical Method

  1. Step 1 (Measure standards): Use the colorimeter to measure the absorbance (or light transmission) of each known dilution in the series (e.g. 0%, 20%, 40%, 60%, 80%, 100%).
  2. Step 2 (Plot calibration curve): Plot a graph of absorbance against known concentration of pigment and draw a line of best fit to create a calibration curve.
  3. Step 3 (Determine unknown): Measure the absorbance of the beetroot water samples from each temperature, and read across from absorbance to determine the exact pigment concentration on the calibration curve.

💡 Key Laboratory Context

  • A dilution series visually compares colours, which is subjective and imprecise (human eye cannot detect fine differences).
  • A colorimeter provides quantitative, objective values by measuring how much light of a specific wavelength passes through the solution.

❌ Common Errors in Calibration Curve Questions

  • Forgetting to state that you measure the absorbance of both the dilution series (standards) and the experimental samples.
  • Saying "measure concentration using the colorimeter" — a colorimeter measures absorbance or transmission, never concentration directly!
  • Failing to mention plotting or creating a calibration curve / graph.

Topics

Biology · Practical skills · Required Practicals · 3.1 Biological molecules · 3.2 Cells · Experimental design · Data analysis · AS practicals (1–6)

Question and mark scheme from the AQA AS Level Biology examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.