AQA AS Level Biology Paper 2, June 2023: Question 6

8 marks · Medium difficulty · Short Answer

Describe the hydrolysis of triglycerides, the functions of bile salts in lipid digestion, and how natural selection led to all mammals producing the lipase CEL.

Practise this question

Question

Three-part exam question. Question 06.1 asks to describe the hydrolysis reactions involved in the digestion of triglycerides excluding lipase, worth 2 marks. Question 06.2 asks to describe two other functions of bile salts given that CEL is activated by them, worth 2 marks. Question 06.3 asks to describe how natural selection may have led to all mammals in a population producing CEL due to a gene mutation, worth 4 marks.
Question text

06.1 Describe the hydrolysis reactions involved in the digestion of triglycerides.

Do not write about the activity of lipase.

[2 marks]

06.2 All mammals produce a lipase called CEL.

CEL digests triglycerides.

CEL is activated by bile salts binding to the enzyme.

Describe two other functions of bile salts.

[2 marks]

06.3 Mammals feed their young on milk. CEL digests the triglycerides in milk. The ability

to produce CEL occurred due to a gene mutation.

Describe how natural selection may have led to all mammals in a population

producing CEL.

[4 marks]

Mark scheme

Show the mark scheme Mark scheme tables for questions 06.1, 06.2, and 06.3. Question 06.1 awards marks for breaking ester bonds and addition of water. Question 06.2 awards marks max 2 for emulsifying lipids, increasing surface area for lipase, and forming micelles. Question 06.3 awards marks max 4 for mutation resulting in a new allele, individuals with the allele being able to digest milk triglycerides and surviving/reproducing, directional selection, and increase in allele frequency.

Question Marking Guidance Mark Comments

1. Breaking of ester bonds; 2

2. Accept ‘using’,

06.1 2. By addition of water; (2 x

‘with’ for addition

AO1)

1. Emulsify lipids/fats; 1. Allow descriptions

2 max

2. Increases surface area (of lipid/fat) for Ignore ‘neutralise’/

06.2 (2 x

(increased) lipase activity; ‘increase the pH’

AO1)

3. Form micelles;

1. Mutation results in (a new) allele; Allow ECF for use of

gene rather than allele

2. Those with the (new) allele able to digest

after not awarding

milk/triglycerides;

mark in the first

3. Individuals with CEL/allele more likely to (survive instance but max 3

and) reproduce; overall.

4. Directional selection; 3. Accept ‘pass on

5. Increase in frequency of (this) allele in allele/characteristic’

population; 4 max for reproduce

06.3 (4 x 5. Accept description

AO2) of increasing

frequency, eg ‘more

common’, ‘higher

proportion’ but ignore

increase in number of

allele

2. 3. 5. Accept

‘mutation’ for allele

How to answer it

Lipid Digestion and Natural Selection

What this question tests

This section assesses your understanding of biological molecules (specifically triglyceride structure and hydrolysis), physiological processes involving bile salts in lipid digestion, and evolutionary mechanisms (specifically natural selection and directional selection applied to gene mutations).

Question 0.6.1 (2 marks)

Describing Triglyceride Hydrolysis

✅ Correct Answers

  • Breaking of ester bonds
  • By the addition of water (using/with water)

💡 Key Knowledge

Triglycerides are formed by condensation reactions between one molecule of glycerol and three fatty acids, creating ester bonds. Hydrolysis is the reverse process, breaking these bonds.

🧠 Exam Technique

Read the stem carefully! The question explicitly states: "Do not write about the activity of lipase." Mentioning lipase will cost you marking points even if your chemistry is correct.

❌ Common Errors

  • Stating that "glycosidic" or "peptide" bonds are broken instead of ester bonds.
  • Describing the removal of water (condensation) instead of the addition of water.
  • Discussing enzyme action (lipase) despite the negative constraint in the prompt.
Mark scheme guidance: 2 marks total (2 × AO1). Point 2 accepts 'using' or 'with' for addition.
Question 0.6.2 (2 marks)

Functions of Bile Salts

✅ Correct Answers (Any 2)

  • Emulsify lipids / fats
  • Increases surface area (of lipid/fat) for (increased) lipase activity
  • Form micelles

💡 Key Knowledge

Bile salts have a hydrophilic (water-loving) and hydrophobic (lipid-loving) region. They break large fat globules down into tiny droplets (emulsification), massively increasing surface area for rapid enzyme digestion.

🧠 Exam Technique

This is a "state two" question requiring two distinct bullet points. Ensure you explicitly link surface area to lipase activity if you choose the second marking point.

❌ Common Errors

  • Writing that bile salts "neutralise stomach acid" or "increase pH". Bile salts do not act as buffers; that is the role of sodium hydrogen carbonate secreted by the pancreas.
  • Confusing emulsification with chemical digestion (emulsification is physical breakdown).
Mark scheme guidance: 2 marks max (2 × AO1). Ignore references to neutralisation or pH changes.
Question 0.6.3 (4 marks)

Natural Selection of CEL Production

✅ Correct Answers (Up to 4)

  • Mutation results in a (new) allele
  • Those with the (new) allele are able to digest milk/triglycerides
  • Individuals with CEL/allele are more likely to survive and reproduce
  • Directional selection occurs
  • Increase in frequency of this allele in the population

💡 Key Knowledge

Standard natural selection framework: Mutation → Advantage/Selection Pressure → Survival → Reproduction → Allele Frequency Increase. Contextualise every step directly to milk digestion and the CEL enzyme.

🧠 Exam Technique

Top-level responses avoid vague terminology like "genes are passed on". Use accurate language: "alleles are passed on to offspring" or individuals with the advantageous allele are more likely to reproduce.

❌ Common Errors

  • Suggesting that the organism "creates" a mutation because it needs to digest milk (Lamarckian evolution trap).
  • Stating that the number of individuals changes rather than the allele frequency or proportion.
  • Failing to link the presence of CEL directly to survival advantage (getting more nutrients from milk).
Mark scheme guidance: 4 marks max (4 × AO2). Allow ECF if student writes "gene" instead of "allele" after the first instance. Accept 'mutation' used interchangeably for allele in points 2, 3, and 5.

Topics

Biology · 3.1 Biological molecules · 3.3 Organisms exchange substances with their environment · 3.4 Genetic information, variation and relationships between organisms

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.