CAIE A Level Biology 9700 · Gas exchange
Describe the path that an oxygen molecule takes from the trachea to the blood in the pulmonary capillaries. In your answer, name the structures it passes through and explain how the tissues and cells of the gas exchange system (including cartilage, ciliated epithelium, goblet cells, smooth muscle, elastic fibres, and squamous epithelium) contribute to maintaining the health of the system and facilitating 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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