CAIE A Level Biology 9700 · Gas exchange
The alveoli in the human lungs are the main sites of gas exchange. Explain how the structure of the alveoli and their associated blood capillaries are adapted to ensure rapid diffusion of oxygen and carbon dioxide between alveolar air and blood.
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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