AQA A-Level Biology Paper 1, June 2022: Question 8

9 marks · Medium difficulty · Short Answer

Analyze fish gill structure, surface area-to-volume calculations, gas exchange effects of cell division, and differences between single and double circulatory systems.

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Question

Question text

08 Figure 11 shows images of gills from two fish as seen through an optical microscope.

Image C shows gills from a fish with healthy gills.

Image D shows gills from a fish with damaged gills.

Figure 11

Figure 11 not reproduced here due to third-party copyright restrictions

08.1 To observe the fish gills with the optical microscope, the scientists used two different

stains. The first stain binds to DNA; the second stain binds to the red blood cells.

Explain why a second stain would be needed to stain the red blood cells.

Suggest which molecule the stain could bind to in the red blood cells.

[2 marks]

Explanation

Molecule

08.2 Using Figure 11, the scientists calculated the surface area to volume ratios for each

gill filament in these two fish. Some of their results are shown in Table 4.

Complete Table 4. State your calculated volume and surface area:volume ratio to

2 significant figures.

[2 marks]

*24* Table 4

Surface

Surface area / 3

Fish gill 2 Volume / μm area:volume

μm

ratio

Healthy 7.4 × 103 2.3 × 104

Damaged 1.1 × 104 0.13:1

08.3 The damage to the gills causes uncontrolled cell division in the cells around the

capillaries in the gill filaments.

Other than surface area:volume ratio, describe one way this uncontrolled cell division

changes the gills, as shown in Figure 11.

Explain how this difference would affect gas exchange.

[3 marks]

Difference

Explanation

Figure 12 shows the general pattern of blood circulation in fish.

Figure 12

08.4 Use Figure 12 to complete Table 5 to show two differences between the circulation

of blood in fish and the circulation of blood in a mammal.

[2 marks]

Table 5

Difference Circulation of blood in Circulation of blood in

fish mammal

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. Accept (To

1. (Red blood cells) do not have a nucleus/DNA; 2

distinguish

2. Haemoglobin; (2 x RBCs from

AO3) other cells as)

fish red blood

08.1 cells have a

nucleus

Accept haem

OR globin

Ignore Hb

1 mark for each correct row 2

Volume / Surface (2 x

μm3 area:volume AO2)

ratio

0.32:1

8.5 x 104

08.2 85 000

If no marks awarded, accept for 1 mark,

0.3217 (correct ratio calculation not given to 2 significant

figures)

Number that can be rounded to 85 000 eg 84615–A-(correctLEVEL BIOLOGY – –

calculation not given to 2 significant figures)

(Difference) 3 Accept a correct

1. More cells (between water and capillary/ blood) (3 x difference and a

AO2) correct

OR explanation in

either section

Wider/thicker filament/lamella;

1. Accept

(Explanation) thicker

2. Longer diffusion pathway epithelium for

more cells

08.3 1. Accept gill

Longer diffusion distance; plate for lamella

3. (So) slower gas exchange

OR 3 Ignore

(So) slower absorption of oxygen efficiency

3. Ignore less

(So) slower release of carbon dioxide

– A-LEVEL BIOLOGY – –

(So) slower rate of diffusion;

1 mark for each correct row 2 max

(2 x

Difference Circulation of Circulation of AO2)

blood in fish blood in 3 and 4 Accept

mammal; name of

relevant blood

12 chambers 4 chambers vessels eg

aorta,

OR OR pulmonary

1 ventricle 2 ventricles artery/vein,

vena cava

OR OR

1 atrium 2 atria

OR OR

2 valves 4 valves;

2 Blood does not Blood returns

return to heart to heart after

after being being

oxygenated oxygenated

OR OR

Blood does not Blood returns

return to heart to heart after

08.4 after passing passing

through gills through lungs 23

OR OR

Heart contains Heart contains

deoxygenated oxygenated

blood and

deoxygenated

blood;

3 One vein Two veins

(carrying blood (carrying blood

towards the towards heart) 5 Accept

heart) descriptions of

single

4 One artery Two arteries circulation and

(carrying blood (carrying blood double

away) away) circulation

5 Single Double 5 Accept

circulation circulation; ‘system’ for

circulatory

6 Blood reaching Blood reaching system

body capillaries body capillaries

at low(er) at high(er)

pressure pressure;

How to answer it

Fish Gills, Gas Exchange, and Circulatory Systems Study Guide

What this question tests

This exam question assesses your understanding of cell ultrastructure, microscopy staining techniques, surface area-to-volume ratio calculations, Fick's Law of diffusion in gas exchange surfaces, and comparative cardiovascular anatomy (single vs. double circulation systems).

Question 08.1

Microscopy Stains and Red Blood Cells

Explain why a second stain would be needed to stain the red blood cells. Suggest which molecule the stain could bind to in the red blood cells. (2 marks)

✅ Correct Answers

Explanation: Red blood cells do not have a nucleus or DNA (so the first DNA stain will not bind to them).
Molecule: Haemoglobin (or haem / globin).

💡 Key Knowledge

Mammalian and fish mature erythrocytes (red blood cells) lack nuclei and DNA. When using a general DNA stain (like methylene blue or acetocarmine), red blood cells remain unstained and invisible unless a secondary cytoplasmic or protein-specific stain is applied.

❌ Common Errors

Stating that red blood cells "have no cells" or "no organelles" too vaguely.
Writing "Hb" instead of naming the full molecule (haemoglobin). The mark scheme accepts haem or globin, but abbreviations like "Hb" are often penalised depending on the strictness of the paper.

Mark breakdown: 1 mark for explaining the lack of nucleus/DNA in red blood cells, and 1 mark for naming haemoglobin.

Question 08.2

Surface Area to Volume Ratio Calculations

Complete Table 4. State your calculated volume and surface area:volume ratio to 2 significant figures. (2 marks)

📐 Step-by-Step Calculation

Row 1 (Healthy): Given Surface Area = 7.4 × 10³ and Volume = 2.3 × 10⁴ .
Ratio = Surface Area ÷ Volume = (7.4 × 10³) / (2.3 × 10⁴) = 0.3217...
Rounded to 2 s.f. = 0.32:1
Row 2 (Damaged): Given Surface Area = 1.1 × 10⁴ and Ratio = 0.13:1 .
Volume = Surface Area ÷ Ratio = (1.1 × 10⁴) / 0.13 = 84615.38...
Rounded to 2 s.f. = 8.5 × 10⁴ (or 85000 )

🧠 Exam Technique & Traps

Significant Figures: The question explicitly asks for 2 significant figures. Failing to round costs you marks even if your initial number-crunching is correct.
Consequential Marking: If you miss the rounding rule, the mark scheme allows 1 mark for a correct unrounded calculation (e.g., writing 0.3217 or 84615 ). Always show your working!

Mark breakdown: 1 mark for the correct ratio (0.32:1), 1 mark for the correct volume (8.5 × 10⁴ or 85000), both formatted to 2 s.f.

Question 08.3

Uncontrolled Cell Division and Gas Exchange

Other than surface area:volume ratio, describe one way this uncontrolled cell division changes the gills, and explain how this difference would affect gas exchange. (3 marks)

✅ Correct Marking Points

Difference (1 mark): More cells between water and capillary/blood OR wider/thicker filament/lamella.
Explanation (2 marks): Longer diffusion pathway / longer diffusion distance (1 mark), which results in a slower rate of gas exchange / slower absorption of oxygen (1 mark).

🧠 Exam Technique

This is a classic Fick's Law application question. Whenever a barrier thickens (increasing diffusion distance), link it directly to the rate of diffusion. Always pair your structural description with a direct functional consequence.

❌ Common Errors

Using vague terms like "less efficient" instead of describing a slower rate of gas exchange or diffusion.
Confusing surface area points with diffusion distance (the question explicitly asks for changes other than surface area:volume ratio).

Mark breakdown: 1 mark for describing the structural change (more cells / thicker lamellae), 1 mark for linking to increased diffusion pathway, and 1 mark for stating the resulting slower rate of gas exchange.

Question 08.4

Comparative Circulation: Fish vs. Mammals

Use Figure 12 to complete Table 5 to show two differences between the circulation of blood in fish and the circulation of blood in a mammal. (2 marks)

✅ Valid Differences (Any Two)

Fish Circulation	Mammalian Circulation
2 chambers (1 atrium, 1 ventricle)	4 chambers (2 atria, 2 ventricles)
Blood does not return to heart after oxygenation	Blood returns to heart after oxygenation (lungs)
Single circulation system	Double circulation system
Blood reaches body capillaries at low pressure	Blood reaches body capillaries at high pressure

💡 Key Knowledge

Fish possess a single circulatory system where blood passes through the heart once for every complete circuit of the body. Mammals have a double circulatory system, allowing blood to be re-pressurised after passing through respiratory surfaces (lungs), maintaining a high pressure gradient for systemic delivery.

Mark breakdown: 1 mark per correct, comparative row in the table (maximum 2 marks). Both columns must match correctly.

Topics

Biology · 3.2 Cells · 3.3 Organisms exchange substances with their environment

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.