AQA A-Level Biology Paper 1, June 2023: Question 7

6 marks · Medium difficulty · Extended Answer

Analyze how carbon monoxide exposure affects the loading and unloading of oxygen by haemoglobin using a dissociation curve, and evaluate scientists' conclusions based on WHO recommendations and mathematical models.

Practise this question

Question

Question text

07.1 Carbon monoxide (CO) is released during incomplete combustion of fossil fuels.

Figure 7 shows the dissociation curve for oxyhaemoglobin when:

• not exposed to CO

• exposed to CO such that 50% of the oxygen binding sites are occupied by CO

(50% COHb).

Figure 7

Using Figure 7, what can you conclude about how exposure to CO affects the loading

and unloading of oxygen by haemoglobin?

Explain your answer.

[3 marks]

07.2 The World Health Organisation (WHO) suggests that to avoid long-term health effects,

COHb concentrations should be kept below 2.5%.

WHO recommends that people should not be exposed to:

• air with > 10 mg m–3 CO for more than 8 hours

• air with > 30 mg m–3 CO for more than 1 hour.

*18* Scientists have used a mathematical model to calculate the exposure to carbon

monoxide that would result in 2.5% COHb in both adults and children.

Table 2 shows the scientists’ results.

Table 2

CO concentration in the air / mg m–3

Exposure duration at rest / resulting in 2.5% COHb

hours

Child Adult

1 31.2 40.2

89.6 10.6

The scientists suggest that the WHO recommendations for carbon monoxide

concentrations resulting in 2.5% COHb should be reduced.

Evaluate the scientists’ conclusion.

[3 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. Less (oxygen) loaded at high pO2 (compared Ignore references to

with no CO) ‘binding sites

occupied by CO’

1. Accept ‘in lungs’ for

Maximum Hb saturation is 50% (oxygen);

‘high pO2’

2. (At low pO2) Hb has higher affinity (for oxygen);

1. Accept ‘levels (off)’

3. Hb has more oxygen at low pO2 OR ‘plateau at 50%’

for maximum

31 Accept ‘only 50%’

Hb unloads less oxygen at low pO2; for ‘maximum (of)

07.1 (1 x

AO2, 2 50%’

x AO3) 3. Accept ‘in

(respiring)

cells/tissues’ for ‘low

pO2/cells’

3. Ignore ‘more

loading of oxygen at

low pO2’

3. Accept ‘less readily’

for less

(For scientists’ suggestion) Award 3 max only if

answer contains MP1

1. Children exposed (to CO) for 8 hours;

OR MP2

(Against scientists’ suggestion)

2. Children exposed (to CO) for 1 hour

3. Accept ‘(based on)

OR invalid assumptions’

Adults exposed (to CO) for 1 hour and 8 hours; for ‘may not be

accurate’

(But)

3. Maths model may not be accurate

3. Accept test OR

OR experiment OR

investigation for

Maths model did not use people; ‘maths model’

3 max

4. (Recommendation for people) only at rest

07.2 (3 x

AO3)

Unknown effects of exercise (on CO uptake);

5. (People vary in) size/age/ethnicity

OR 5. Accept gender

Different sex; 6. Accept no SDs to

show if differences

6. No statistical test to show if differences are are significant

18 significant;

7. Unknown effects on CO exposure for people with

respiratory disease 7. Accept any named

respiratory disorder;

OR eg asthma OR

Unknown effects on CO exposure for smokers; coronary heart

disease

8. Might not be able to reduce CO (concentration)

in air below 10(mg m-3);

How to answer it

Carbon Monoxide and Haemoglobin Dissociation Study Guide

📌 What this question tests

This assessment evaluates your ability to interpret oxyhaemoglobin dissociation curves in unfamiliar contexts (AO1/AO3), understand how competitive/irreversible inhibitors like carbon monoxide affect oxygen loading and unloading (AO2), and critically evaluate scientific data and mathematical models regarding public health recommendations (AO3).

Question 07.1: Interpreting the Dissociation Curve

Using Figure 7, explain how exposure to CO affects the loading and unloading of oxygen by haemoglobin. (3 marks)

✅ Correct Answer Breakdown

Mark 1: Less oxygen is loaded at high partial pressure of oxygen ( pO₂ ) / maximum haemoglobin saturation is capped at 50% (compared to no CO).
Mark 2: At low pO₂ , haemoglobin has a higher affinity for oxygen.
Mark 3: Haemoglobin unloads less oxygen at low pO₂ (in respiring tissues).

💡 Key Biological Knowledge

Carbon monoxide (CO) binds permanently and tightly to the iron/haem groups of haemoglobin, forming carboxyhaemoglobin (COHb).
Because 50% of the binding sites are occupied by CO, only the remaining 50% of sites are available to bind O₂. This explains why the curve plateaus at 50% saturation.
The curve shifts to the left, indicating an increased affinity for the remaining oxygen binding sites, which makes unloading oxygen to respiring tissues much harder.

🧠 Exam Technique & Guidance

Split your answer: Address both loading (lungs / high pO₂ ) and unloading (tissues / low pO₂ ) explicitly.
Use data reference points from the graph (e.g., noting the maximum saturation is 50%) to anchor your points securely.

❌ Common Student Errors

Vague phrasing such as "haemoglobin holds onto oxygen" without specifying whether this happens at high or low pO₂ .
Stating that CO "destroys" haemoglobin rather than occupying binding sites and altering affinity.

Mark Scheme Allocation: 3 marks total (1 × AO2, 2 × AO3). Accept alternative phrasing like "levels off at 50%" for max saturation.

Question 07.2: Evaluating a Scientific Conclusion

Evaluate the scientists' conclusion that the WHO recommendations for carbon monoxide concentrations resulting in 2.5% COHb should be reduced. (3 marks)

✅ Correct Answer / Marking Points (Any 3)

For the suggestion: Table 2 shows that for an 8-hour exposure, children reach 2.5% COHb at a lower concentration ( 9.6 mg m⁻³ ) than the WHO recommendation ( 10 mg m⁻³ ).
Against the suggestion (Children 1 hr): For a 1-hour exposure, children require 31.2 mg m⁻³ , which is above the WHO recommendation of 30 mg m⁻³ .
Against the suggestion (Adults): Adults require higher concentrations for both 1 hour ( 40.2 mg m⁻³ ) and 8 hours ( 10.6 mg m⁻³ ) to reach 2.5% COHb.
Validity issues: The mathematical model may not be accurate, may not have used human subjects, or fails to account for variables like exercise, age, gender, underlying health conditions (e.g., smokers, respiratory disease), or lack statistical testing.

🧠 Exam Technique & Guidance

Evaluation Questions Rule: To score full marks on an "Evaluate" question, you must give a balanced argument. You need at least one point supporting the scientists' view and at least one point challenging it.
Quote data directly from Table 2 to support your points (units are essential: mg m⁻³ ).

❌ Common Student Errors

Only looking at one row or column in Table 2 and missing the broader comparison between adults and children or 1-hour vs 8-hour exposures.
Making generic statements like "the data is wrong" instead of pointing out limitations of mathematical models or unmeasured confounding variables (exercise, pre-existing conditions).

Mark Scheme Allocation: Max 3 marks (3 × AO3). Note: Award 3 max only if the response contains at least one supporting point and one counter-point.

Topics

Biology · Practical skills · 3.3 Organisms exchange substances with their environment · Data analysis

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.