AQA AS Level Biology Paper 2, November 2020: Question 4

9 marks · Medium difficulty · Practical Techniques & Data Analysis

Analyze an experiment investigating the effect of ethanol, hydrochloric acid, and temperature on beetroot cell membrane permeability, including control of variables, measurement uncertainty, and conclusions from pigment release.

Practise this question

Question

An exam question with multiple sub-parts (04.1 to 04.4) describing an investigation into the effect of ethanol, hydrochloric acid, and temperature on beetroot cell membrane permeability. Figure 2 shows part of a measuring cylinder scale with 10 and 30 cm3 markings. Figure 3 illustrates four test tubes labeled E, F, G, and H with varying intensities of red pigment shading, corresponding to different conditions (water/ethanol/hydrochloric acid at 25 degrees C, and water at 70 degrees C).
Question text

04 A student investigated the effect of ethanol, hydrochloric acid and temperature on the

loss of red pigment from beetroot cells.

During the procedure, the student:

• added 10 cm3 water into one test tube

• added 10 cm3 ethanol into a second test tube

• added 10 cm3 hydrochloric acid into a third test tube

• put the three tubes into a 25 oC water bath

• cut four cylinders of tissue from a beetroot

• put a cylinder into each tube and fitted bungs

• added 10 cm3 water into a fourth test tube and put this tube into a

70 oC water bath

• placed the fourth cylinder into this tube and fitted a bung

• later removed the cylinders from the tubes

• estimated the intensity of red pigment in each solution by eyesight.

04.1 Give one way in which the student could ensure the first three beetroot cylinders were

kept at 25 oC throughout her experiment.

[1 mark]

04.2 Give two variables that the student did not control in her procedure.

[2 marks]

04.3 The student used a measuring cylinder to obtain 10 cm3 of each solution.

Figure 2 shows some of the scale graduations on the side of this measuring cylinder.

Figure 2

What is the uncertainty of taking a reading of 10 cm3 with this measuring cylinder?

Suggest how you could reduce the uncertainty calculated.

[2 marks]

Uncertainty ± cm3

Reducing uncertainty

A different student used the same procedure and she controlled all variables

appropriately. Her results are shown in Figure 3.

Figure 3

04.4 Using Figure 3, what can you conclude about the damage caused to beetroot cells by

water, ethanol, hydrochloric acid and different temperatures?

Provide explanations for your conclusions.

[4 marks]

Mark scheme

Show the mark scheme The mark scheme table detailing acceptable answers for questions 04.1 through 04.4, providing marking guidance, mark allocations, and specific comments for each part of the question.

Question Marking Guidance Mark Comments

04.1 1 Accept use

Measure temperature (in tube) at intervals and use

thermometer/probe/

appropriate corrective measure (if temperature has

equivalent device for

fluctuated);

measure temperature

04.2 1. Length and diameter 2 1. Ignore shape/size

OR 2. Ignore ‘time’ if

Surface area unqualified

OR

Volume

OR

Mass/weight (of cylinders);

2. Time in solution;

04.3 2 2. Accept correct

1. 1; numerical figure, eg

<2 (cm3)

2. Use instrument with closer/finer/smaller

intervals/graduations/scale; 2. Ignore ‘higher

resolution’

04.4 4 max 1. Accept no

colour/intensity

1. Water/25oC caused no damage/no pigment change for ‘pigment’

release (in E); Accept ‘high

temperature’ for 70oC

2. (Damage to) cell(-surface) membrane;

3. Ethanol/acid caused some/similar/identical 1. and 3. Accept

damage description of

‘pigment release’ for

OR ‘damage’

70oC caused most damage;

2. Accept description

4. (By) ethanol dissolving phospholipid bilayer of membrane, eg

phospholipid

OR

bilayer/bilayer

(By acid) altering membrane protein;

4. and 5. Accept

5. (By) 70oC denaturing/altering membrane protein

carrier OR channel

OR OR intrinsic OR

extrinsic protein for

(By) 70oC increasing fluidity/permeability of

membrane protein

membrane;

5. Ignore reference to

enzymes unless

– LOGYqualified as– – JUNE 2020

membrane bound

8 TOTAL 9

How to answer it

Investigation into Beetroot Membrane Permeability

What this question tests

This question assesses your practical design skills, understanding of experimental controls, calculation of measurement uncertainties, and application of biological knowledge regarding cell membrane structure (phospholipid bilayers and membrane proteins) and how temperature, organic solvents (ethanol), and acids disrupt them.

Question 04.1 [1 mark]

Maintaining Constant Temperature

✅ Correct Answer

Measure the temperature (in the tube) at intervals and use an appropriate corrective measure if the temperature has fluctuated.

🧠 Exam Technique

To secure this 1 mark, your answer must mention two distinct actions: monitoring the temperature (using a thermometer or temperature probe) and acting to correct any drift (e.g., adjusting the water bath or adding hot/cold water).

Mark scheme guidance: Accept use of thermometer / probe / equivalent device to measure temperature.
Question 04.2 [2 marks]

Identifying Uncontrolled Variables

✅ Correct Answers (Choose any two)

  • Length and diameter (or surface area, volume, mass/weight) of the beetroot cylinders
  • Time in solution
  • Age / part / variety / source of the beetroot

❌ Common Errors

Students often lose marks by being too vague. Saying just "size" is penalised unless qualified by surface area, volume, length, or diameter. Similarly, just writing "time" without stating "time in solution" is too vague.

Mark scheme guidance: 2 marks total (1 mark per variable). Ignore shape/size if unqualified. Ignore 'time' if unqualified.
Question 04.3 [2 marks]

Measurement Uncertainty and Resolution

📐 Calculations & Answers

Uncertainty: ± 1 cm³

Reducing uncertainty: Use an instrument with closer, finer, or smaller intervals/graduations/scale (e.g., a pipette, burette, or smaller measuring cylinder).

💡 Key Knowledge

The uncertainty of a graduated scale is typically half of the smallest division (or equal to the smallest division depending on whether readings require both a start and end point, but standard AQA accepted convention for a single scale reading is ± half the smallest scale interval, though here the mark scheme accepts 1 based on the graduated scale intervals shown).

Mark scheme guidance: 1 mark for uncertainty value. 1 mark for suggesting a finer/closer scale instrument. Do not credit the word "higher resolution" without explaining it.
Question 04.4 [4 marks max]

Conclusions and Explanations from Figure 3

✅ Correct Marking Points

  • Water at 25 °C (Tube E): Caused no damage / no pigment release.
  • Membrane damage: Damage occurred to the cell surface membrane.
  • Ethanol & Acid vs 70 °C: Ethanol and acid caused similar/some damage, whereas 70 °C caused the most damage (darkest intensity).
  • Ethanol mechanism: Ethanol dissolves the phospholipid bilayer.
  • Acid / Temperature mechanism: Acid and 70 °C denature/alter membrane proteins (carrier/channel/intrinsic/extrinsic proteins). Alternatively, 70 °C increases membrane fluidity/permeability.

🧠 Top-Level Exam Strategy

To hit the maximum 4 marks, structure your answer systematically: state what happened to the pigment intensity in each tube, link it directly to membrane damage, and provide the underlying cellular/molecular mechanism (lipids dissolving or proteins denaturing).

Mark scheme guidance: 4 marks max. Accept alternative descriptions of pigment release for damage, and accept descriptions of bilayer/membrane structure for points 2 and 4. Ignore references to enzymes unless qualified as membrane-bound.

Topics

Biology · Practical skills · Required Practicals · 3.2 Cells · Experimental design · Uncertainty and evaluation · Data analysis · AS practicals (1–6)

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.