CAIE A Level Biology 9700 · Gas exchange
A student examines a prepared slide of mammalian lung tissue under a light microscope. They observe numerous hollow sacs (alveoli) with extremely thin walls, each surrounded by a dense network of capillaries. The inner surface of the alveoli appears moist. Explain how these structural features, together with others you know, adapt the alveoli for efficient gas exchange between the alveolar air and the blood in the capillaries.
5 marks, so the mark scheme is looking for about 5 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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