Gas exchange questions

Gas exchange syllabus

Questions on gas exchange can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 2.46 describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes
• 2.47 understand the role of the intercostal muscles and the diaphragm in ventilation
• 2.48 explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries
• 2.49 understand the biological consequences of smoking in relation to the lungs and the circulatory system, including coronary heart disease
• 2.50 practical: investigate breathing in humans, including the release of carbon dioxide and the effect of exercise

How to answer gas exchange questions

1. Trace the named substance through the relevant organs, tissues and cells in the correct order.
2. Link structural adaptations to diffusion distance, surface area, concentration gradient or transport rate.
3. Use balanced biological equations where required and distinguish respiration from breathing.
4. When explaining a data trend, quote evidence and connect it to the underlying biological process.

Gas exchange example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Plotting fish breathing rates

Adapted from January 2014, Paper 1B · Question 7(b)(i)

Question 1

Fish breathe by opening their mouths to allow water containing oxygen to pass over their gills. This is shown in the diagram.

Students investigated the effect of temperature on the breathing rate of fish. They put fish of the same size in tanks of water at different temperatures. They measured the breathing rate by counting the number of times the fish opened their mouths in a minute.

The results are shown in the table.

Water temp. (°C)	Breathing rate in breaths per minute
	Trial 1	Trial 2	Trial 3	Trial 4	Mean
2	2	3	3	4	3
8	30	33	27	30	30
14	54	52	53	53	53
20	80	75	81	84	80
26	101	98	102	101	101

Plot a line graph to show the effect of water temperature on the average breathing rate of the fish. Join the points with straight lines.

Final answer

Use linear scales: water temperature from 0 to 30 °C on the horizontal axis and average breathing rate from 0 to 120 breaths per minute on the vertical axis. Label the axes Water temperature (°C) and Average breathing rate (breaths per minute). Plot (2, 3), (8, 30), (14, 53), (20, 80) and (26, 101), then join consecutive points with straight lines.

Mark scheme points

1. M1 Use a linear scale.
2. M2 Join the plotted points with straight lines.
3. M3 Put water temperature on the horizontal axis and breathing rate on the vertical axis.
4. M4 Label the axes with breathing rate per minute and temperature in °C.
5. M5 Plot the five average values accurately: (2, 3), (8, 30), (14, 53), (20, 80) and (26, 101).

Explanation

Use the average breathing rates in the table, not the separate trial results. Put temperature on the horizontal axis and breathing rate on the vertical axis. A linear scale means equal steps on an axis represent equal changes in value. Plot each temperature with its matching average, then connect the points in order using straight line segments.

Common mistakes

• Plotting individual trial results instead of the average for each temperature.
• Using uneven scale intervals or drawing a smooth curve instead of straight lines through the points.

Example 2: Explaining the effects of lung disease

Adapted from January 2022 (R), Paper 1B · Question 10(b)

Question 2

The graph shows how the volume of air exhaled varies with time during one breath.

This is shown for a person with a lung disease and a person with healthy lungs.

Explain why the person with lung disease is often breathless and unable to exercise.

Final answer

The person with lung disease exhales a smaller maximum volume of air and cannot exhale as rapidly. Less oxygen is taken into the blood, so the diffusion gradient into the blood is less steep. Less oxygen reaches the muscles for respiration, so exercise is limited; carbon dioxide is also removed less rapidly.

Mark scheme points

The mark scheme asks for four references from the listed points for this 3-mark answer. The points below show one full-credit route; alternatives are included where relevant.

1. M1 State that the maximum volume of air exhaled is lower.
2. M2 State that the person cannot exhale as rapidly.
3. M3 Add relevant consequences to make four references overall: less oxygen is taken in, the diffusion gradient into the blood is less steep, less carbon dioxide is removed, or respiration in cells is reduced. More anaerobic respiration or lactic acid accumulation is also accepted.

Explanation

Use the graph to identify both differences: the person with lung disease exhales a smaller maximum volume of air and does so less rapidly. This means less oxygen can enter the blood. A less steep diffusion gradient means oxygen diffuses into the blood less quickly.

With less oxygen available to the muscles, aerobic respiration is reduced during exercise. The muscles may rely more on anaerobic respiration, which can lead to lactic acid accumulation. Less rapid removal of carbon dioxide is another relevant consequence.

Common mistakes

• The examiner report noted that strong answers identified both the lower volume and slower rate of exhalation, then linked these to reduced oxygen delivery to muscles or slower carbon dioxide removal. State the graph differences and explain their consequences rather than just describing types of lung disease.
• Do not give only “the person is breathless”; include the evidence from the graph and relevant effects on gas exchange or respiration.

Practise Gas exchange questions

Build a focused practice set from this part of the Edexcel IGCSE Biology syllabus.