AQA A-Level Biology Paper 1, June 2024: Question 1

8 marks · Medium difficulty · Short Answer

Identify the bond and reaction involved in triglyceride formation, name a saturated fatty acid and describe its relationship with melting point using Table 1, and explain the effect of unsaturated fatty acids on membrane fluidity and phagocytosis.

Practise this question

Question

Figure 1 shows chemical structures of a fatty acid and a glycerol molecule, asking students to circle the atoms removed during bond formation, name the bond (ester), and name the reaction (condensation). Table 1 gives the chemical structures and melting points for stearic acid, oleic acid, and linoleic acid, followed by questions identifying saturated fatty acids, describing the relationship between melting point and structure, and explaining why an increase in unsaturated fatty acids leads to increased phagocytosis.
Question text

01.1 Figure 1 shows the structure of a fatty acid molecule and the structure of a glycerol

molecule.

Figure 1

On Figure 1, draw a circle around the part of the fatty acid molecule and the part of

the glycerol molecule that is removed to form a bond in a triglyceride molecule.

Name the bond formed between a fatty acid and glycerol in a triglyceride molecule.

Name the reaction involved in forming a bond between a fatty acid and glycerol in a

triglyceride molecule.

[3 marks]

Bond

Reaction

Table 1 shows information about three fatty acids.

Table 1

01.2 Name the fatty acid shown in Table 1 that is a saturated fatty acid.

*02* [1 mark]

01.3 The melting point is the temperature at which a solid changes state to be a liquid.

Use Table 1 to describe the relationship between fatty acid structure and fatty acid

melting point.

[1 mark]

01.4 The ratio of saturated to unsaturated fatty acids in a cell-surface membrane

determines the extent of the membrane’s fluidity.

Scientists provided a cell culture of mouse phagocytes with liquid broth rich in

unsaturated fatty acids.

The scientists observed:

• an increase in the proportion of phospholipids in the phagocytes containing

unsaturated fatty acids

• more phagocytosis.

Suggest and explain why there was more phagocytosis.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme details the answers for parts 01.1 through 01.4. For 01.1, it awards 3 marks for circling the OH from the fatty acid and H from glycerol, naming the ester bond, and condensation reaction. For 01.2, 1 mark for Stearic acid. For 01.3, 1 mark for describing that as double bonds or unsaturation increases, melting point decreases. For 01.4, 3 marks for explaining that unsaturated fatty acids increase membrane fluidity, making the membrane more flexible so cells are easier to engulf.

Marking Guidance Mark Comments

Question

1. Circle(s)/shape(s) drawn around H in one of the 1. Accept

HO groups of the glycerol and the OH group of circle(s)/shape(s)

the fatty acid; 3 drawn around the OH

01.1 (3 x group of one molecule

2. Ester (bond);

AO1) and the H of an OH

3. Condensation (reaction); group of the other

molecule

Question Marking Guidance Mark Comments

Stearic (acid); 1

01.2 (1 x

AO1)

As (number of C) double bonds increases, melting Accept converse

point decreases

Accept C=C for

OR 1 double bonds

01.3 (1 x

As unsaturation increases, melting point decreases Accept as (number of)

AO2) hydrogen/H (atoms)

OR

increases, melting

As saturation increases, melting point increases; point increases

1. More unsaturated fatty acids increases fluidity (in Accept for 2 marks,

(cell-surface membrane); more unsaturated

fatty acids increases

2. (Making cell-surface) membrane more fluid/

3 membrane fluidity

flexible;

01.4 (3 x 3. Accept endocytosis

3. Easy to engulf; for engulf

AO2)

3. Accept more/easier

for phagosomes to

form

How to answer it

Lipid Structure, Triglycerides, and Membrane Fluidity

What this question tests

This question assesses your understanding of biological molecules (specifically the structure of triglycerides, fatty acids, and glycerol), ester bond formation via condensation reactions, the distinction between saturated and unsaturated fatty acids, and how fatty acid composition affects cell-surface membrane fluidity and cellular processes like phagocytosis.

Question 01.1 (3 Marks)

Triglyceride Formation & Bonding

✅ Correct Answer

  • Drawing: A circle drawn around the -OH (hydroxyl) group of the glycerol molecule AND the -H of the hydroxyl group on the fatty acid (or vice versa, capturing the elements lost as water during condensation).
  • Bond: Ester (bond)
  • Reaction: Condensation

💡 Key Knowledge

  • Triglycerides consist of one glycerol molecule chemically bonded to three fatty acid molecules.
  • Each ester bond is formed by a condensation reaction, eliminating one molecule of water H₂O per bond formed.

🧠 Exam Technique

When asked to circle parts removed during bond formation, ensure you clearly isolate the -H from one molecule and the -OH from the other. Circling entire functional groups incorrectly will lose you the drawing mark.

❌ Common Errors

Students frequently confuse condensation with hydrolysis, or name the resulting bond a "peptide" or "glycosidic" bond instead of an ester bond.

Mark Breakdown: 1 mark for the correct drawing, 1 mark for naming "Ester", 1 mark for naming "Condensation".
Question 01.2 (1 Mark)

Identifying Saturated Fatty Acids

✅ Correct Answer

Stearic acid

💡 Key Knowledge

A saturated fatty acid contains only single carbon-carbon ( C-C ) bonds and no carbon-carbon double bonds ( C=C ). Looking at Table 1, Stearic acid contains exclusively single bonds, whereas Oleic and Linoleic acids contain double bonds.

🧠 Exam Technique

Scan the hydrocarbon chain for double lines ( C=C ). If none are present, the fatty acid is saturated.

Mark Breakdown: 1 mark for correctly identifying Stearic acid.
Question 01.3 (1 Mark)

Relationship Between Structure and Melting Point

✅ Correct Answer

As the number of carbon-carbon double bonds increases, the melting point decreases (or accept converse: as unsaturation increases, melting point decreases).

💡 Key Knowledge

Double bonds introduce kinks into the hydrocarbon tail, preventing molecules from packing closely together. This reduces intermolecular forces, lowering the melting point.

❌ Common Errors

Failing to state the direction of the relationship clearly. Vague answers like "they are linked" or just stating differences without describing the trend will not score.

Mark Breakdown: 1 mark for correctly stating the inverse relationship between double bonds/unsaturation and melting point.
Question 01.4 (3 Marks)

Membrane Fluidity and Phagocytosis

✅ Correct Answer

  1. More unsaturated fatty acids increase membrane fluidity.
  2. Making the cell-surface membrane more fluid / flexible.
  3. Making it easier to engulf (pathogens/materials) / easier to form phagosomes / endocytosis.

💡 Key Knowledge

Phagocytosis requires the cell-surface membrane to change shape and invaginate to engulf particles. Higher proportions of unsaturated fatty acids prevent tight packing of phospholipids, enhancing membrane fluidity and flexibility required for bulk transport.

🧠 Exam Technique

Link questions require a logical chain of reasoning. Follow this formula: Biological molecule property → Cellular/membrane effect → Physiological consequence (More unsaturated → more fluid membrane → easier engulfment/phagocytosis).

Mark Breakdown: 3 marks total (3 × AO2). Awarded for linking unsaturated fatty acids to increased membrane fluidity/flexibility, and explaining how this facilitates the physical process of engulfment during phagocytosis.

Topics

Biology · 3.1 Biological molecules · 3.2 Cells

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.