This free quiz has 11 real AQA A-level Biology exam questions on gas exchange (27 marks, about 35 minutes). It marks itself: for each answer you check your work against the real AQA mark scheme and tick the points you made.
What the quiz covers (AQA 3.3.2)
naming structures of the human gas exchange system, including the external intercostal muscles
how we breathe in
the pathway an oxygen molecule takes from an alveolus to the blood
how a single alveolus is adapted for gas exchange
why FEV₁ falls with age, and evaluating data on smoking and COPD
the gill lamellae and filaments, and the counter-current principle in fish
how the insect tracheal system supplies oxygen and limits water loss (5 marks)
how leaves are adapted for gas exchange (4 marks), and why plants short of water grow slowly
How to use it
Write each answer in full before you open the mark scheme, then tick only the points you really made. Say what each muscle does and how that moves the ribcage or diaphragm, and write diffusion rather than just “gas exchange” when you explain an adaptation. For the COPD graph, describe the pattern for men and for women separately.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
Alveolar epithelium, not walls. “Alveoli have thin walls” was a common answer, and “thin membrane” was rejected; the mark needs the epithelium or its cells, because it is the cells that are thin. Many also wrote about the large surface area of all the alveoli when asked about one alveolus.
✔ Gains credit: the alveolar epithelium is one cell thick, made of thin, flat (squamous) cells, so the diffusion pathway is short.
Link muscles to movement. Many students said the diaphragm contracts without saying it flattens, or didn't link the external intercostal muscles to the ribcage moving. Writing just “intercostal muscles”, “chest” instead of thoracic cavity, or only the route air takes cost marks.
✔ Gains credit: the diaphragm contracts and flattens, and the external intercostal muscles contract, pulling the ribcage up and out, so volume increases and pressure decreases in the thoracic cavity.
Breathing out. Many students treated the two sets of intercostal muscles as one, or described the diaphragm as “flattened” or “domed” without saying whether it had contracted or relaxed. Some mixed up the movements of the diaphragm altogether.
✔ Gains credit: the diaphragm relaxes and moves up, and the internal intercostal muscles contract, moving the ribcage down and in, so volume decreases and pressure in the thoracic cavity rises above atmospheric pressure.
Slower diffusion, not “less”. When gill or alveolar tissue is damaged and thickened, answers about “less” diffusion didn't score, because examiners wanted a change in rate. Vague differences such as “the gills are thicker” also missed the mark.
✔ Gains credit: thicker gill filaments or lamellae (more cells between the water and the blood) give a longer diffusion pathway, so diffusion is slower.
Tracheoles and diffusion. Some answers described only the spiracles, or wrote about abdominal pumping and water at the ends of the tracheoles, which are not structural features of the gas exchange surface. Many lost marks by leaving out the word diffusion or writing “gas exchange” instead.
✔ Gains credit: oxygen diffuses through the tracheae and tracheoles, and the tracheoles are highly branched, giving a large surface area and a short diffusion distance to the cells.
Leaf cells, not leaves. Students usually gave a large surface area for the whole leaf rather than for the cells, and many wrote “for gas exchange” without mentioning diffusion. Some wrote “stroma” or “spiracles” instead of stomata.
✔ Gains credit: many stomata allow gases to diffuse in and out, and air spaces in the mesophyll give a large surface area of cell membranes for rapid diffusion.
Water vapour, not water. A common error with xerophytes was saying hairs trap water or moisture rather than water vapour, with no link to the water potential gradient. Some said a waxy cuticle “prevents water escaping” or simply “reduces water loss”, which wasn't detailed enough.
✔ Gains credit: hairs or sunken stomata trap water vapour, so the water potential gradient is reduced, and a thick waxy cuticle reduces evaporation (transpiration).
Keep practising
The other AQA 3.3 self-marking quizzes: Surface area to volume · Digestion and absorption · Mass transport in animals · Mass transport in plants
Topic 3.2 quizzes start with cell structure
The AQA topic index
weekly AQA group classes: Year 12 on Tuesdays at 5.15pm, Year 13 on Wednesdays or Thursdays at 6.30pm
Questions © AQA, from past A-level and AS Biology papers, with their mark schemes and examiners’ reports.