AQA A-Level Biology Paper 1, November 2021: Question 2

11 marks · Medium difficulty · Short Answer

Explain gas exchange mechanisms and adaptations in fish and mammals, including surface area to volume ratios, counter-current flow, and lung volume calculations.

Practise this question

Question

Question text

02.1 Explain the advantage for larger animals of having a specialised system that

facilitates oxygen uptake.

[2 marks]

Figure 2 shows two models of oxygen uptake found in animals.

Figure 2

02.2 Suggest how the environmental conditions have resulted in adaptations of systems

using Model A rather than Model B.

[2 marks]

02.3 Figure 3 shows changes in concentration of oxygen in two gas exchange systems.

Figure 3

A student studied Figure 3 and concluded that the fish gas exchange system is more

efficient than the human gas exchange system.

Use Figure 3 to justify this conclusion.

[2 marks]

02.4 Explain how the counter-current principle allows efficient oxygen uptake in the

fish gas exchange system.

[2 marks]

02.5 Table 1 shows features of two mammals.

*06* Bats are flying mammals; shrews are ground-living mammals.

Table 1

Mammal Mean body mass / kg Mean lung volume / cm

Bat 0.096 12.48

Shrew 0.024 0.72

Calculate how many times the lung volume per unit of body mass of the bat is greater

than that of the shrew.

Give your answer to an appropriate number of significant figures.

Give one suggestion to explain this difference.

[3 marks]

Answer

Explanation

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

02.1 1. Large(r) organisms have a small(er) surface 2.Accept short

area:volume (ratio); diffusion pathway

Small(er) organisms have a large(r) surface 2. Accept for ‘faster’,

area:volume (ratio); 2

2. Overcomes long diffusion pathway

Faster diffusion;

Mark in pairs, 1, and 2 OR 3. and 4.

02.2

1. Water has low(er) oxygen partial

pressure/concentration (than air);

2. So (system on outside) gives large surface area

(in contact with water)

So (system on outside) reduces diffusion

distance (between water and blood);

3. Water is dense(r) (than air);

– A-LEVEL BIOLOGY – –

4. (So) water supports the systems/gills;

1. In fish, blood leaving (V) has more oxygen than

02.3

water leaving (E);

2. (But) in humans, blood leaving (V) has less

oxygen than air leaving (E);

2 max

3. Difference in oxygen (concentration) between

artery and vein is greater in fish than in humans;

4. (So) fish remove a greater proportion from the

oxygen they take in;

02.4 1. Blood and water flow in opposite directions; Accept for 2 marks,

2 suitably labelled

2. Diffusion/concentration gradient (maintained)

diagram

along (length of) lamella/filament;

02.5 1. and 2. Correct answer for 2 marks, 4.3 (times

greater);;

Accept for 1 mark,

4.333333333 (correct answer not given to 2

significant figures)

OR 3

Evidence of 130 (cm3 kg-1) and 30 (cm3 kg-1)

Correct explanation for 1 mark,

3. Provides more oxygen for respiration;

How to answer it

Gas Exchange Systems & Surface Area to Volume Ratios

AQA A-Level Biology Exam Practice

What this question tests

This question assesses your understanding of mass transport systems, surface area to volume ratios in organisms of different sizes, environmental adaptations for gas exchange, data interpretation of oxygen concentration curves, the counter-current principle in fish, and stoichiometric/proportionality calculations involving biological variables with appropriate significant figures.

Question 02.1

Explain the advantage for larger animals of having a specialised system that facilitates oxygen uptake. [2 marks]

✅ Correct Answer

Large organisms have a smaller surface area to volume ratio.
Overcomes a long diffusion pathway / allows faster diffusion.

💡 Key Knowledge

As organisms increase in size, their volume increases much faster than their surface area.
Simple diffusion across the outer membrane is insufficient to supply deep-lying cells.

❌ Common Errors

Saying large animals have a "smaller surface area" instead of specifying a smaller surface area to volume ratio.
Vague phrasing like "they need more oxygen" without linking to diffusion distance or geometry.

Mark breakdown: 1 mark for surface area to volume ratio point, 1 mark for diffusion pathway/speed point.

Question 02.2

Suggest how the environmental conditions have resulted in adaptations of systems using Model A rather than Model B. [2 marks]

✅ Correct Answer

Any valid pair from: Water has a lower oxygen concentration/partial pressure than air (1), so an external system (Model A) provides a large surface area in direct contact with water (2); OR water is denser and supports the gills/external system (3 & 4).

🧠 Exam Technique

This question requires a paired marking point. You must link an environmental property (water vs. air) to a structural adaptation of the exchange surface.

Mark breakdown: 2 marks awarded in pairs (Point 1 + Point 2, OR Point 3 + Point 4).

Question 02.3

Use Figure 3 to justify the conclusion that the fish gas exchange system is more efficient than the human gas exchange system. [2 marks max]

✅ Correct Answer

In fish, blood leaving (V) has a higher oxygen concentration than water leaving (E).
In humans, blood leaving (V) has less oxygen than air leaving (E).
The difference in oxygen concentration between artery and vein is greater in fish than in humans.
Fish remove a greater proportion from the oxygen they take in.

❌ Common Errors

Misreading the axis labels (I, E, A, V) on the graphs provided in Figure 3.
Using general textbook definitions of counter-current flow rather than explicitly quoting data/trends visible in Figure 3.

Mark breakdown: Up to 2 marks for comparative statements directly derived from Figure 3.

Question 02.4

Explain how the counter-current principle allows efficient oxygen uptake in the fish gas exchange system. [2 marks]

✅ Correct Answer

Blood and water flow in opposite directions.
Maintains a diffusion/concentration gradient along the entire length of the lamella/filament.

💡 Key Knowledge

Counter-current flow prevents equilibrium from being reached, ensuring that a concentration gradient is maintained across the whole exchange surface, enabling maximum possible oxygen diffusion into the blood.

Mark breakdown: 1 mark for opposite directions, 1 mark for maintaining concentration/diffusion gradient.

Question 02.5

Calculate how many times the lung volume per unit of body mass of the bat is greater than that of the shrew. Give your answer to an appropriate number of significant figures. Give one suggestion to explain this difference. [3 marks]

📐 Step-by-Step Calculation

Bat lung volume per unit mass: 12.48 ÷ 0.096 = 130 cm³ kg⁻¹
Shrew lung volume per unit mass: 0.72 ÷ 0.024 = 30 cm³ kg⁻¹
Ratio: 130 ÷ 30 = 4.3333...
Apply significant figures: 4.3 (to 2 significant figures, matching data in the table).

✅ Correct Explanation

Bats fly, which requires significantly more energy / higher rate of respiration, therefore demanding a greater oxygen supply.

❌ Common Calculation Traps

Failing to round to 2 significant figures (leaving 4.33 loses a mark).
Dividing body mass by lung volume instead of lung volume per unit body mass.

Mark breakdown: 2 marks for the correct calculation to 2 sig fig (or 1 mark for correct intermediate values 130 and 30 if final rounding fails), plus 1 mark for the biological explanation linking flight to respiration.

Topics

Biology · 3.3 Organisms exchange substances with their environment

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.