AQA AS Level Biology Paper 1, June 2023: Question 2

8 marks · Medium difficulty · Short Answer

Compare prokaryotic and eukaryotic cell structures regarding DNA and membranes, describe the mechanism of breathing, and explain how bacterial capsule thickness affects survival against phagocytes.

Practise this question

Question

Figure 1 shows a transmission electron microscope image of a Streptococcus bacterium. Sub-question 02.1 asks to describe how the appearance of the area containing DNA in a eukaryotic cell differs from Figure 1 (2 marks). Sub-question 02.2 asks for one difference between the structure of DNA in a prokaryotic cell and in a eukaryotic cell (1 mark). Sub-question 02.3 asks to describe the mechanism of breathing that causes air to enter the lungs (3 marks). Sub-question 02.4 asks to suggest how Streptococcus bacteria with a thicker capsule are more likely to survive and cause lung disease (2 marks).
Question text

02 Figure 1 shows a transmission electron microscope (TEM) image of a Streptococcus

bacterium.

Figure 1

02.1 Describe how the appearance of the area containing DNA in a TEM image of a

eukaryotic cell would differ from that shown in Figure 1.

[2 marks]

02.2 Describe one difference between the structure of DNA in a prokaryotic cell and in a

eukaryotic cell.

[1 mark]

02.3 Streptococcus bacteria can infect the lungs when air is breathed in and cause lung

disease.

Describe the mechanism of breathing that causes air to enter the lungs.

[3 marks]

02.4 Some strains of Streptococcus bacteria are more likely to cause lung disease than

other strains.

Strains that do not cause lung disease are quickly destroyed by phagocytes.

Phagocytes are stimulated when they bind to murein on Streptococcus bacteria.

Each strain of Streptococcus bacteria has a capsule of different thickness from the

others.

Suggest how Streptococcus bacteria with a thicker capsule are more likely to survive

and so cause lung disease.

[2 marks]

Mark scheme

Show the mark scheme The mark scheme provides answers for questions 02.1 through 02.4. For 02.1: nucleus, nucleolus, or nuclear envelope (2 max). For 02.2: circular vs linear DNA, not associated with proteins/histones, or no introns (1 mark). For 02.3: diaphragm contracts/flattens, external intercostal muscles contract/ribcage moves up and out, volume increases and pressure decreases in thoracic cavity (3 marks). For 02.4: thicker capsule makes it less likely to bind to murein/be stimulated by phagocytes, leading to reduced phagocytosis and increased bacterial growth (2 marks).

Question Marking Guidance Mark Comments

1. Nucleus;

2. Nucleolus/nucleoli 2 max

Accept ‘membrane

(1 x

02.1 OR bound nucleus’ = 2

AO1, 1

marks

Nuclear membrane/envelope; x AO2)

3. Mitochondria/chloroplast contain DNA;

(In prokaryotes) Ignore ‘loop’

Circular not linear

Ignore ‘plasmid’

OR

02.2

Not associated with proteins/histones (AO1)

OR

No introns;

1. Diaphragm (muscles)

contract and diaphragm flattens/pulled down;

2. External intercostal muscles

contract and ribcage pulled up/out; 3

02.3 (3 x

3. (Causes) volume increase and pressure AO1) 3. Accept lungs or

decrease in thoracic cavity (to below atmospheric thorax for ‘thoracic

pressure); cavity’

1. (Thicker capsule so phagocytes) less likely to

bind to murein (in cell wall)

OR 2

02.4 (2 x

(Thicker capsule so phagocytes) less likely to be AO2)

stimulated by murein (in cell wall); 2. Accept replicate/

2. Reduced phagocytosis so more bacterial multiplication but

growth/division/reproduction/binary fission; reject mitosis

How to answer it

Cell Structure, Gas Exchange, and Immunity

What this question tests

This multi-topic AQA AS Biology question assesses your knowledge of prokaryotic vs. eukaryotic cell ultrastructure (specifically DNA and nuclear organization), the mechanics of pulmonary ventilation (breathing), and cellular immune responses involving phagocytosis and bacterial capsules.

Question 02.1 (2 marks)

Comparing Eukaryotic and Prokaryotic DNA Appearance

✅ Correct Answer

  • Reference to a nucleus, nucleolus, or nuclear membrane/envelope.
  • Mention that eukaryotic DNA is contained within membrane-bound organelles (e.g., mitochondria or chloroplasts).

💡 Key Knowledge

  • Prokaryotes (like Streptococcus) lack a true nucleus; their DNA forms a nucleoid region that is free in the cytoplasm without a membrane.
  • Eukaryotes possess a distinct, membrane-bound nucleus and linear chromosomes associated with histones.
Mark breakdown: Max 2 marks (1 mark for AO1 knowledge of eukaryotic structures, 1 mark for AO2 applying this to contrast with the TEM image). Accepting 'membrane bound nucleus' gains both marks.
Question 02.2 (1 mark)

Structural Differences in DNA

✅ Correct Answer

Any one of the following structural differences:

  • Circular (in prokaryotes) vs linear.
  • Not associated with proteins/histones (prokaryote DNA is 'naked').
  • No introns present in prokaryotic DNA.

❌ Common Errors

Students frequently lose this mark by mentioning plasmids or loops. Plasmids are extra-chromosomal DNA and not the main bacterial DNA strand. Examiners explicitly ignore references to 'loops' and 'plasmids' for this specific marking point.

Mark breakdown: 1 mark for AO1 accurate recall.
Question 02.3 (3 marks)

Mechanism of Breathing (Inhalation)

✅ Correct Answer

  • 1. Diaphragm muscles contract and the diaphragm flattens / is pulled down.
  • 2. External intercostal muscles contract, pulling the ribcage up and out.
  • 3. This causes an increase in thoracic volume and a decrease in pressure below atmospheric pressure, forcing air in.

🧠 Exam Technique

To secure all 3 marks, you must link muscle action (contract), structural change (flatten/up and out), and the resulting pressure/volume physics. Vague statements like "chest expands" or "diaphragm moves" without mentioning contraction and volume/pressure changes will fail to score.

Mark breakdown: 3 marks total (3 × AO1). Note: Examiners accept 'lungs' or 'thorax' interchangeably for 'thoracic cavity'.
Question 02.4 (2 marks)

Bacterial Capsules and Phagocytosis

✅ Correct Answer

  • 1. A thicker capsule means phagocytes are less likely to bind to or be stimulated by murein (in the cell wall).
  • 2. This results in reduced phagocytosis, allowing for increased bacterial growth, division, or reproduction (binary fission).

❌ Common Errors

Be careful with biological terminology! While you can accept terms like 'replicate' or 'multiplication', examiners will strictly reject 'mitosis' since bacteria divide via binary fission, not mitosis.

Mark breakdown: 2 marks (2 × AO2 application). Point 1 requires linking the capsule to masking or preventing binding to murein; point 2 links this directly to survival and reproduction.

Topics

Biology · 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 AS Level Biology examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.