AQA AS Level Biology Paper 1, June 2023: Question 3

9 marks · Medium difficulty · Short Answer

Compare the structure of triglycerides and phospholipids, explain bilayer formation, and interpret data on fatty acid composition and membrane fluidity.

Practise this question

Question

A multipart question about lipids and cell membranes. Figure 2 shows a fatty acid with 20 carbon atoms and 4 double bonds, requiring a box drawn around the R group. Figure 3 shows a triglyceride structure with glycerol, ester bonds, and three fatty acids, followed by a request to describe two differences between triglycerides and phospholipids. Figure 3 is followed by a question explaining why phospholipids form a bilayer but triglycerides cannot. Figure 4 shows two fatty acids, A (saturated) and B (unsaturated with one double bond), alongside an experimental study on rats fed fish oil over 4 months, with Table 1 presenting data on the mean percentages of fatty acids A and B and membrane fluidity.
Question text

03.1 Figure 2 shows a fatty acid that contains 20 carbon atoms and four double bonds.

Figure 2

On Figure 2, draw a box around the R group of the fatty acid.

[1 mark]

03.2 Figure 3 shows a triglyceride.

Figure 3

Describe two differences between the structure of the triglyceride shown in Figure 3

and a phospholipid.

[2 marks]

03.3 Explain why phospholipids can form a bilayer but triglycerides cannot.

[3 marks]

03.4 Figure 4 shows two fatty acids, A and B.

Figure 4

A

B

Scientists fed rats a diet with added fish oil for 4 months.

They obtained samples of red blood cells from the rats before starting this diet

(0 months) and after 4 months on this diet.

For each red blood cell sample, they separated the cell-surface membranes and

measured:

• the percentage of phospholipids containing each of the fatty acids7 A and B

• the fluidity of the membrane.

Table 1 shows the scientists’ results.

Table 1

Time sample of Mean percentage Mean percentage

*06* Mean fluidity of

red blood cells of phospholipids of phospholipids

the membrane /

obtained / containing fatty containing fatty

arbitrary units

months acid A acid B

0 19.8 1.7 31

4 11.7 9.0 97

Suggest why the fluidity of the membrane was higher after 4 months.

Use all the information provided in the question.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme provides answers for four parts. Question 03.1 awards 1 mark for boxing everything except the COOH group. Question 03.2 awards up to 2 marks for stating triglycerides have 3 fatty acids, 3 ester bonds, and no phosphate group. Question 03.3 awards 3 marks for noting that phospholipids are polar/have a charged hydrophilic phosphate group and are amphipathic, whereas triglycerides are entirely hydrophobic/non-polar, allowing hydrophilic groups to interact with water. Question 03.4 awards up to 3 marks for identifying fatty acid A as saturated and B as unsaturated, noting the increased proportion of fatty acid B from fish oil after 4 months, and explaining that unsaturated fatty acids have bends/kinks that increase membrane fluidity.

Question Marking Guidance Mark Comments

03.1 Everything other than the COOH inside drawn box;

(AO1)

(Triglyceride) 1, 2 and 3 Accept

converse

1. 3 fatty acids rather than 2; 2 max

1. Accept ‘only 2 fatty

03.2 (2 x

2. 3 ester bonds rather than 2; acids’

AO1)

3. No phosphate group;

1. Phospholipid both hydrophobic and hydrophilic

OR

Phospholipid polar

OR

2. Accept

Phosphate group is charged; 3

‘Triglycerides not

03.3 (3 x hydrophilic’

2. Triglycerides only hydrophobic AO1)

OR

3. Accept ‘faces

Fatty acid/triglyceride is non-polar; water’ for ‘attracts

water’

3. Ignore ‘fatty acids

3. Hydrophilic/phosphate group attracts water (to repel water’

either side of bilayer); – LOGY – – JUNE 2023

1. Fatty acid A is saturated

OR

1.and 2. Accept

Fatty acid B is unsaturated; identification of A (as

saturated) or B (as

unsaturated) using

2. (At 4 months) less fatty acid A and more fatty

numbers from table 1

acid B

OR

Fish oil has more fatty acid B than fatty acid A;

3 max

(2 x 7

03.4 3. Increase in fluidity caused by increased

AO2, 1

unsaturated fatty acids

x AO3)

OR

Increase in fluidity caused by increased fatty acid

B (from the fish oil);

4. Double bonds/unsaturated fatty acids cause

bends/kinks in fatty acid tail

OR

(Membrane more fluid because) phospholipids

further apart;

How to answer it

AQA AS Level Biology: Lipids and Membrane Structure Study Guide

What this question tests

This question assesses your understanding of biological molecules (specifically the structure of triglycerides, fatty acids, and phospholipids), how molecular structure determines properties like polarity and membrane bilayer formation, and your ability to apply data analysis skills to explain changes in cell-surface membrane fluidity.

Question 03.1

Identifying R Groups in Fatty Acids

✅ Correct Answer

A box drawn around everything in Figure 2 except the COOH (carboxyl) group.

💡 Key Knowledge

  • Fatty acids consist of a long hydrocarbon chain (the R group) and a acidic carboxyl group ( COOH ).
  • The hydrocarbon tail can be saturated (only single C-C bonds) or unsaturated (containing one or more double C=C bonds).

❌ Common Errors

Students often accidentally include the carbon or oxygen atoms of the COOH group inside the box, or fail to enclose the entire hydrocarbon chain.

Marks available: 1 mark (AO1)
Question 03.2

Comparing Triglycerides and Phospholipids

✅ Correct Answers (Any 2 max)

  1. Triglycerides have 3 fatty acids, whereas phospholipids have 2.
  2. Triglycerides have 3 ester bonds, whereas phospholipids have 2.
  3. Triglycerides do not have a phosphate group, whereas phospholipids do.

🧠 Exam Technique

When a question asks for "differences," ensure you make comparative points referencing both molecules clearly (e.g., use words like "whereas" or explicitly contrast the two). Bullet points or numbered lists help keep your answer concise and structured.

Marks available: 2 marks (2 x AO1)
Question 03.3

Why Phospholipids Form Bilayers

✅ Correct Answer Structure (3 marks)

  1. Phospholipids are both hydrophobic and hydrophilic (or polar / have a charged phosphate group).
  2. Triglycerides are only hydrophobic / non-polar.
  3. The hydrophilic phosphate group / head attracts water, orienting outwards toward the aqueous environment on either side of the bilayer.

❌ Common Errors

Avoid saying that "fatty acids repel water" — examiners penalize this phrasing. Instead, state that hydrophobic fatty acid tails are repelled by water or simply face inwards away from water.

Marks available: 3 marks (3 x AO1)
Question 03.4

Explaining Membrane Fluidity Changes

✅ Correct Answer Points (3 max)

  1. Fatty acid A is saturated, whereas fatty acid B is unsaturated.
  2. After 4 months, there is less fatty acid A and more fatty acid B in the red blood cell membranes (or fish oil is richer in fatty acid B).
  3. Unsaturated fatty acid B has double bonds that cause bends or kinks in the hydrocarbon tail.
  4. These kinks prevent close packing of phospholipid molecules, pushing them further apart and increasing membrane fluidity.

🧠 Exam Technique

Make sure to use all information provided by referencing the data trends from Table 1 alongside structural details from Figure 4 to secure full application (AO2) and analysis (AO3) marks.

Marks available: 3 marks (2 x AO2, 1 x AO3)

Topics

Biology · Practical skills · 3.1 Biological molecules · 3.2 Cells · Data analysis

Question and mark scheme from the AQA AS Level Biology examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.