AQA A-Level Biology Paper 1, June 2023: Question 4

9 marks · Medium difficulty · Short Answer

Explain how Clostridium difficile toxins damage ileum cells leading to reduced absorption and diarrhoea, analyze anti-toxin antibody levels in hospital patients, and describe protein digestion of antibodies.

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Question text

04.1 Some hospital patients suffer from diarrhoea caused by infection with the bacterium

Clostridium difficile. The C. difficile bacteria release toxins. These toxins cause the

diarrhoea.

The toxins damage the cells lining the ileum, causing them to lose their microvilli.

The damage to the cells reduces the absorption of the products of digestion and

reduces the absorption of water, resulting in diarrhoea.

Explain why the damage to the cells lining the ileum reduces absorption of the

products of digestion and why this reduces absorption of water.

[3 marks]

Not all patients in hospital with C. difficile develop diarrhoea.

Scientists measured the anti-toxin antibody concentration in hospital patients with and

without C. difficile infection.

They measured the anti-toxin antibody concentration four times:

• on admission to hospital (day 0)

• on day 3

• on day 6

• on the day the patient left the hospital.

Figure 4 shows the scientists’ results.

Figure 4

04.2 The scientists suggest that the anti-toxin antibody could be given to some patients as

a form of passive immunity.

Use Figure 4 to suggest how this passive immunity would work and which patients

should be offered this anti-toxin antibody.

[3 marks]

04.3 To be used as passive immunity treatment, the anti-toxin antibody would be injected.

If it was given by mouth, it would be digested.

Describe how the anti-toxin antibody would be digested.

[3 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. Reduced surface area 1. Ignore references

to diffusion OR

OR facilitated diffusion

Fewer co-transport/carrier/channel proteins; OR active transport

1. Ignore SA

2. Decreases water potential in ileum/lumen Accept gut for ileum

2. Accept Ψ for water

OR 3

potential

04.1 (3 x

Increases water potential in cells; 2. Ignore WP

AO2)

2. Accept reduces

3. (So) water moves out of cells/into ileum water potential

by osmosis gradient

OR 3. Accept lumen for

ileum

(So) less/no water moves into cells/out of ileum

by osmosis; 3. Accept absorbed

for moves

1. Anti-toxins/antibodies cause phagocytosis/ 1. For ‘neutralised’,

destruction/agglutination/neutralisation (of accept idea of

toxin); preventing toxin

binding/damaging

cells lining the

2. Anti-toxin/antibody prevents/reduces (chance of)

ileum.

diarrhoea

(C difficile) patients with no diarrhoea have 2. and 3 Accept

high(est) (concentration of) anti-toxin/antibody people for patients

04.2 (1 x

OR AO1, 2 2. Ignore symptoms

x AO2) for diarrhoea

(C difficile) patients with diarrhoea have low(est)

(concentration of) anti-toxin/antibody;

3. Accept ‘passive

3. (Offered to C. difficile) patients with diarrhoea immunity offered’ for

‘antibody offered’

(Offered to) patients with low (concentrations of)

anti-toxin/antibody;

1. Peptide bonds hydrolysed; Ignore named

structures in the

2. Endopeptidase(s) break internal (peptide) bonds;

digestive system

3. Exopeptidase(s) break terminal (peptide) bonds;

2. Accept ‘bonds 11

4. (Membrane-bound) dipeptidase(s) break within’ OR ‘bonds in

dipeptides to amino acids; middle’ for internal

3. Accept ‘external

3 max bonds’ OR ‘bonds at

04.3 ends’ OR ‘penultimate

(3 x

bonds’ for terminal

AO1)

2., 3. and 4. Accept

‘act on’ OR ‘affect’ OR

‘hydrolyse’ for break

4. Accept between 2

amino acids for

dipeptides

Ignore stomach acid

How to answer it

Clostridium difficile, Digestion, and Passive Immunity

What this question tests

This question assesses your understanding of cell ultrastructure and absorption mechanisms in the ileum (co-transport, water potential, and osmosis), interpretation of graphical immunological data, and the enzymatic hydrolysis of proteins during digestion. It bridges topics across cells, organisms responding to changes in their environment, and biochemical molecules.

Question 04.1 (3 marks)

Ileum Damage and Water Absorption

✅ Correct Answer Structure

Mark 1: Reduced surface area OR fewer co-transport / carrier / channel proteins.
Mark 2: Decreases water potential in the ileum / lumen OR increases water potential in the cells.
Mark 3: Water moves out of cells / into the ileum by osmosis (OR less water moves into cells).

💡 Key Knowledge

Microvilli provide a massive surface area packed with membrane-bound carrier proteins for absorbing digestion products (like monosaccharides and amino acids). As these solutes enter the cells, they lower water potential, driving water absorption via osmosis. Disrupting this transport stops the osmotic gradient.

🧠 Exam Technique

Ensure you explicitly link why products of digestion are not absorbed (loss of surface area/proteins) to how this impacts water movement (altering water potential gradients and stopping osmosis).

❌ Common Errors

Vaguely stating "diffusion stops" without specifying transport proteins or surface area.
Using incorrect terminology like "water potential gradient" written backwards, or saying "water moves down a concentration gradient" instead of water potential.

Question 04.2 (3 marks)

Interpreting Graphical Data & Passive Immunity

✅ Correct Answer Structure

Mark 1: Anti-toxins / antibodies cause phagocytosis, destruction, agglutination, or neutralisation of the toxin.
Mark 2: Patients with C. difficile and diarrhoea have the lowest anti-toxin concentration (whereas patients without diarrhoea have the highest).
Mark 3: Therefore, offer the anti-toxin to patients with C. difficile infection / low antibody concentration.

💡 Key Knowledge

Passive immunity involves the introduction of antibodies formed elsewhere (in this case, anti-toxins). Because antibodies are specific, they bind to bacterial toxins and neutralise them before they can damage ileal cells.

🧠 Exam Technique

Read graph legends carefully. Note that the top line represents infected patients without diarrhoea (high antibodies), while the bottom line represents infected patients with diarrhoea (low antibodies).

❌ Common Errors

Confusing active and passive immunity principles.
Failing to explicitly link who should receive the treatment based on data lines from Figure 4.

Question 04.3 (3 marks - max 3)

Protein Digestion Mechanisms

✅ Correct Answer Structure

Mark 1: Peptide bonds are hydrolysed.
Mark 2: Endopeptidases break internal peptide bonds.
Mark 3: Exopeptidases break terminal (external/end) peptide bonds.
Mark 4: Membrane-bound dipeptidases break dipeptides into amino acids.

💡 Key Knowledge

Proteins are large polymers. Complete digestion requires sequential enzyme action: endopeptidases first create smaller peptide fragments to increase the available ends for exopeptidases, followed by membrane-bound dipeptidases on the epithelial lining.

🧠 Exam Technique

Memorise the precise locations of enzyme action: endopeptidases = internal bonds; exopeptidases = terminal bonds; dipeptidases = dipeptides into amino acids. Mentioning hydrolysis of peptide bonds is essential.

❌ Common Errors

Stating that "bonds are broken" without specifying peptide bonds .
Confusing the roles of endopeptidases and exopeptidases, or omitting membrane-bound enzymes altogether.

Topics

Biology · 3.1 Biological molecules · 3.2 Cells · 3.3 Organisms exchange substances with their environment · 3.4 Genetic information, variation and relationships between organisms

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