Cambridge International AS & A Level Biology 9700: Gas exchange – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover Topic 9, Gas exchange, in the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: section 9.1 The gas exchange system, outcomes 1 to 7. It is AS Level content, examined on Papers 1 and 2, and Paper 4 can draw on it. Calculators are allowed in all 9700 papers.

Learn it first: Gas exchange study guide · Recall: Gas exchange revision notes · Course hub: Cambridge A Level Biology · Checklist: 9700 checklist

Questions

1. Name:

(a) the tissue that forms C-shaped rings in the wall of the trachea [1] (b) the cells in the airway epithelium that secrete mucus [1] (c) the type of epithelium that lines the alveoli. [1]

2. Describe the pathway taken by a molecule of oxygen from the trachea into the blood. [4]

3. For each of these tissues, state whether it is present in the wall of a bronchiole, an alveolus, both or neither: cartilage; goblet cells; smooth muscle; squamous epithelium. [4]

4. Describe how goblet cells, mucous glands and ciliated epithelial cells maintain the health of the gas exchange system. [4]

5. A photomicrograph shows a transverse section of an airway. The wall has a complete ring of smooth muscle and a folded lining of ciliated cells. No cartilage is visible.

(a) Identify the airway. Give two reasons for your answer. [3] (b) The airway is 36 mm across in the photomicrograph and 0.45 mm across in real life. Calculate the magnification. [2]

6. A photomicrograph of a transverse section of the trachea has a scale bar 25 mm long labelled 500 µm. The cartilage layer measures 40 mm thick on the same image. Calculate the actual thickness of the cartilage layer in µm. Show your working. [3]

7. A student is asked to make a plan diagram of the wall of a bronchus from a slide. State four features the diagram should have. [4]

8. Explain the function of:

(a) cartilage in the trachea [2] (b) elastic fibres in the walls of the alveoli. [2]

9. Air in an alveolus has a partial pressure of oxygen of 12.8 kPa. Blood entering the surrounding capillary has a pO₂ of 5.2 kPa and a pCO₂ of 6.3 kPa. The pCO₂ of alveolar air is 5.5 kPa.

(a) Calculate the oxygen gradient between the alveolar air and the incoming blood. [1] (b) Describe gas exchange between the air in this alveolus and the blood. [4]

10. A model treats each alveolus as a sphere of diameter 0.25 mm. Surface area of a sphere = 4πr².

(a) Calculate the surface area of one alveolus in mm². Give your answer to 3 significant figures. [2] (b) An adult’s lungs contain 3.5 × 10⁸ alveoli. Calculate the total alveolar surface area in m². [2] (c) The same adult’s skin has an area of 1.8 m². Calculate how many times larger the alveolar surface is. [1]

11. In a lung disease, the walls between neighbouring alveoli are destroyed and the remaining alveolar walls thicken with extra tissue. In electron micrographs at ×20 000, the barrier between alveolar air and capillary blood measures 10 mm in a healthy lung and 16 mm in a diseased lung.

(a) Calculate the actual thickness of the barrier in each lung, in µm. [2] (b) The alveolar surface area of the patient fell from 70 m² to 42 m². Calculate the percentage decrease. [2] (c) Using the data, explain why this patient becomes breathless during exercise. [4]

12. When a person breathes in an irritant, the smooth muscle in a bronchiole contracts and its internal diameter falls from 2.0 mm to 1.2 mm.

(a) Calculate the percentage decrease in diameter. [1] (b) Calculate the cross-sectional area of the lumen before and after, and the percentage decrease in area. Use area = πr². [3] (c) Explain why a bronchiole can narrow in this way but the trachea cannot. [3]

Answers

1. (a) Cartilage [1] (b) Goblet cells [1] (c) Squamous epithelium [1]

Examiner insight: One-word naming questions need the exact term: “flat cells” is not credited for squamous epithelium, and “mucus cells” is not accepted for goblet cells.

2. Trachea to a bronchus [1]; bronchus to bronchioles [1]; bronchioles to an alveolus [1]; oxygen diffuses across the alveolar wall and the capillary wall into the blood [1].

Examiner insight: The last mark needs both walls named; “diffuses into the blood” alone describes the destination, not the pathway.

3. Cartilage: neither [1]. Goblet cells: bronchiole only (larger bronchioles) [1]. Smooth muscle: bronchiole only [1]. Squamous epithelium: alveolus only [1].

Examiner insight: Each tissue is marked separately, so one error costs only one mark; do not leave a row blank.

4. Goblet cells and mucous glands secrete mucus [1]; mucus traps dust, pollen and pathogens [1]; cilia beat in a coordinated way [1]; to move mucus up towards the throat and away from the alveoli, where it is swallowed [1].

Examiner insight: “Cilia trap bacteria” contradicts the mechanism and is not credited; the trapping mark belongs to mucus.

5. (a) Bronchiole [1]; no cartilage present, unlike the trachea and bronchi [1]; smooth muscle forms a ring / large proportion of the wall, or the lining is folded [1]. (b) 0.45 mm = 450 µm and 36 mm = 36 000 µm, or keep both in mm [1]; 36 ÷ 0.45 = ×80 [1].

Examiner insight: Magnification has no units other than “×”; writing “80 mm” loses the accuracy mark even when the division is right.

6. Magnification = 25 000 µm ÷ 500 µm = ×50 [1]; 40 mm = 40 000 µm [1]; actual thickness = 40 000 ÷ 50 = 800 µm [1].

Examiner insight: Allow ECF from a wrong magnification for the final mark, but the answer must be in the unit asked for (µm), so 0.8 mm alone would lose that mark.

7. No individual cells drawn; only tissue layers outlined [1]. Clear, continuous lines with no shading [1]. Layers drawn in the correct proportions to each other [1]. All layers shown, e.g. epithelium, glands, cartilage plates, outer layer, with ruled label lines [1].

Examiner insight: Each feature must be specific to plan diagrams; “neat” or “large” earns nothing unless it is stated as a checkable rule such as no shading.

8. (a) Cartilage supports the trachea / keeps it open [1]; preventing collapse when air pressure inside falls during breathing in [1]. (b) Elastic fibres stretch as the alveoli fill with air [1]; then recoil to help push air out of the alveoli [1].

Examiner insight: Elastic fibres “contract” is a common wrong verb and is not credited; use “recoil”.

9. (a) 12.8 − 5.2 = 7.6 kPa [1] (b) Oxygen diffuses from the alveolar air into the blood, down its partial pressure / concentration gradient [1]. Carbon dioxide diffuses from the blood into the alveolar air, because 6.3 kPa is higher than 5.5 kPa [1]. The distance is short: one layer of squamous epithelium and one layer of capillary endothelium [1]. Oxygen then combines with haemoglobin in red blood cells, while blood flow keeps the gradient steep [1].

Examiner insight: When data are given, quote them: the carbon dioxide mark needs a direction supported by the two values, not just “carbon dioxide moves out”.

10. (a) r = 0.125 mm; area = 4 × π × 0.125² [1] = 0.196 mm² [1] (b) 0.196 mm² = 1.96 × 10⁻⁷ m² (1 m² = 10⁶ mm²) [1]; 1.963 × 10⁻⁷ (unrounded) × 3.5 × 10⁸ = 68.7 m² [1] (c) 68.7 ÷ 1.8 = 38.2 times [1]

Examiner insight: Using the diameter instead of the radius gives four times the area and loses the method mark; allow ECF in (b) and (c) from a wrong (a).

11. (a) Healthy: 10 000 µm ÷ 20 000 = 0.5 µm [1]; diseased: 16 000 ÷ 20 000 = 0.8 µm [1]. (b) (70 − 42) ÷ 70 × 100 [1] = 40% [1] (c) The diffusion distance increases from 0.5 µm to 0.8 µm [1]. The surface area for diffusion is 40% smaller [1]. So the rate of oxygen diffusion into the blood is lower (and carbon dioxide removal slower) [1]. During exercise muscles need more oxygen for respiration, which the lungs cannot supply, so the person breathes faster / becomes breathless [1].

Examiner insight: “Using the data” means the numbers must appear in the answer; a correct explanation that never quotes 0.5 µm, 0.8 µm or 40% can lose the first two marks.

12. (a) (2.0 − 1.2) ÷ 2.0 × 100 = 40% [1] (b) Before: π × 1.0² = 3.14 mm² [1]; after: π × 0.6² = 1.13 mm² [1]; decrease = (3.14 − 1.13) ÷ 3.14 × 100 = 64% [1] (c) Bronchioles have no cartilage [1], and a large proportion of smooth muscle that can contract and narrow the lumen [1]. The trachea has C-shaped cartilage rings that hold it open, so it cannot be narrowed in this way [1].

Examiner insight: In (b), halve the diameter before squaring; using 2.0 and 1.2 as radii gives the same percentage but wrong areas, so the two area marks are lost.

Where marks are usually lost

• Saying cilia trap dust or pathogens, instead of moving the mucus that traps them.
• Writing that mucus is swept down into the lungs.
• Giving bronchioles cartilage, or saying the trachea has smooth muscle “only”.
• Using “contract” for elastic fibres.
• Leaving out the capillary wall when describing the diffusion distance.
• Using diameter as radius in area calculations.
• Forgetting to convert mm to µm before dividing by magnification.
• Adding units to magnification.
• Ignoring the data when a question says “using the data”.

Next steps

• Gas exchange revision notes
• Gas exchange study guide
• Transport in mammals study guide
• Cambridge A Level Biology hub
• 9700 printable checklist
• Free 9700 AS diagnostic
• Book a free trial class

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, Cambridge University Press & Assessment – Topic 9, Gas exchange (section 9.1).