CAIE A Level Biology 9700 · Gas exchange
An oxygen molecule enters the trachea during inhalation. Describe the pathway it takes to reach the blood in the pulmonary capillaries. For each structure it passes through, explain how its structure is adapted to facilitate efficient gas exchange.
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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