AQA AS Level Biology Paper 2, June 2023: Question 3

9 marks · Medium difficulty · Short Answer

Explain the structure, digestion, and prebiotic properties of galacto-oligosaccharides (GOS) compared to lactose.

Practise this question

Question

Four-part exam question about galacto-oligosaccharides (GOS). Question 03.1 asks to explain why GOS are described as polysaccharides (2 marks). Question 03.2 asks to give two differences between the structures of GOS and lactose (2 marks). Question 03.3 asks to explain why amylase does not digest GOS (2 marks). Question 03.4 asks to suggest how GOS can work as a prebiotic stimulating healthy bacterial growth (3 marks).
Question text

03 Galacto-oligosaccharides (GOS) are polymers of galactose.

03.1 Explain why GOS are described as polysaccharides.

[2 marks]

03.2 Give two differences between the structures of GOS and lactose.

[2 marks]

03.3 Explain why amylase produced in the human digestive system does not digest GOS.

[2 marks]

03.4 Prebiotics are foods used to promote good health in humans.

Prebiotics stimulate the growth of ‘healthy’ bacterial populations in the human

digestive system.

The bacteria in these ‘healthy’ populations produce enzymes that hydrolyse GOS.

*06* Suggest how GOS can work as a prebiotic.

[3 marks]

Mark scheme

Show the mark scheme Mark scheme tables for questions 03.1 to 03.4. For 03.1, accepts points on galactose being a monomer, carbohydrate polymer, and monomers joined by condensation or glycosidic bonds (2 max). For 03.2, accepts differences in glucose/galactose presence, disaccharide vs polysaccharide status, and number of glycosidic bonds (2 max). For 03.3, accepts active site not being complementary to GOS due to tertiary structure (2 marks). For 03.4, accepts providing galactose for bacterial respiration and binary fission (3 marks).

Question Marking Guidance Mark Comments

Any two from:

1. Galactose is a monosaccharide/monomer;

2. (Polysaccharide is a) carbohydrate polymer;

3. (Several) monosaccharides/monomers/galactose 2 max

03.1 joined by condensation reactions (2 x

AO2)

OR

(Several) monosaccharides/monomers/galactose

joined by glycosidic bonds;

1. Lactose contains (alpha) glucose and GOS does

not

OR

2 max

Lactose contains (alpha)glucose + galactose

03.2 (2 x

and GOS contains only galactose;

AO2)

2. Lactose is a disaccharide and GOS is a

polysaccharide; 3. Accept ‘more than

3. Lactose has one glycosidic bond and GOS has one’ for many

many glycosidic bonds;

1. Ignore ‘hydrolysis’

1. Active site (only) complementary to starch

OR

03.3 (2 x

Active site not complementary to GOS; AO1)

2. (Due to) tertiary structure;

1. Provides galactose/sugar/monosaccharide;

Reject ‘glucose’ for

galactose/sugar once

2. (Bacteria use the galactose/sugar) for

3 only

respiration;

03.4 (3 x 3. Accept (cell) division

3. (Bacteria use the galactose/sugar) for binary

AO3) /replication/reproduction

fission;

for binary fission

3. Reject mitosis;

How to answer it

Carbohydrates, Enzymes & Prebiotics Study Guide

What this question tests

This question assesses your understanding of biological molecules (specifically carbohydrate structures like monomers, polymers, and glycosidic bonds), enzyme specificity involving tertiary structure and active sites, and the application of biological knowledge to gut health and bacterial metabolism.

Question 0.3.1

Explaining Polysaccharide Structure

Explain why GOS are described as polysaccharides. [2 marks]

✅ Correct Answer

  • Galactose is a monosaccharide / monomer.
  • GOS are carbohydrate polymers.
  • They consist of several monosaccharides/galactose units joined by condensation reactions / glycosidic bonds.
Any 2 points from the above for 2 marks (2 × AO2).

💡 Key Knowledge

  • Polysaccharide definition: A polymer formed by many monosaccharide units joined together.
  • The stem states that GOS are "polymers of galactose", making galactose the repeating monomer unit.

❌ Common Errors

  • Vaguely stating "they are made of many sugars" without specifying that galactose is the monomer.
  • Confusing condensation with hydrolysis when describing bond formation.
Question 0.3.2

Comparing Carbohydrate Structures

Give two differences between the structures of GOS and lactose. [2 marks]

✅ Correct Answer

Any two clear structural differences, such as:

  • Lactose contains (alpha) glucose AND GOS does not (GOS contains only galactose).
  • Lactose is a disaccharide AND GOS is a polysaccharide.
  • Lactose has one glycosidic bond AND GOS has many / more than one glycosidic bond.
2 max (2 × AO2). Must give a direct comparison between the two molecules.

🧠 Exam Technique

When a question asks for differences, ensure your answer explicitly references both substances in each point (e.g., writing "Lactose is a disaccharide whereas GOS is a polysaccharide"). Comparative language secures the marks.

Question 0.3.3

Enzyme Specificity

Explain why amylase produced in the human digestive system does not digest GOS. [2 marks]

✅ Correct Answer

  • Amylase active site is only complementary to starch (or: active site is not complementary to GOS).
  • Due to its specific tertiary structure.
2 marks total (2 × AO1).

💡 Key Knowledge

Enzymes are highly specific. Amylase breaks down starch (alpha-1,4 glycosidic bonds in amylose/amylopectin). Because GOS consists of galactose polymers with different bond arrangements, its shape does not fit into amylase's active site.

❌ Common Errors

  • Saying the enzyme is "denatured" by GOS—this is incorrect; the enzyme just cannot bind because the shapes don't match.
  • Forgetting to mention tertiary structure when explaining why the active site shape is unique.
Question 0.3.4

Applied Biological Reasoning

Suggest how GOS can work as a prebiotic. [3 marks]

✅ Correct Answer

  • Provides galactose / sugar / monosaccharide for the 'healthy' bacteria.
  • Bacteria use the galactose/sugar for respiration.
  • Bacteria use the galactose/sugar for binary fission / cell replication / reproduction.
3 marks total (3 × AO3).

🧠 Exam Technique & Examiner Guidance

This is an AO3 (Application) question requiring you to link the breakdown products of GOS to bacterial survival and growth.

  • Reject: Using 'glucose' instead of galactose (the stem states GOS are polymers of galactose).
  • Reject: Mentioning 'mitosis' or 'meiosis' for bacteria—bacteria reproduce via binary fission.

Topics

Biology · 3.1 Biological molecules

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.