CAIE A Level Biology 9700 · Gas exchange
A photomicrograph of lung tissue shows alveoli with very thin walls, each surrounded by a dense network of capillaries. The alveolar walls consist of a single layer of flattened cells, and the inner surface is lined with a thin film of fluid. Explain how the features of alveoli, as shown in the photomicrograph, are adapted for efficient gas exchange between alveolar air and 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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