CAIE A Level Biology 9700 · Gas exchange

Gas exchange: explain question

Explain5 marksStandard

An oxygen molecule enters the trachea during inhalation. Describe the complete path of this oxygen molecule from the trachea to the blood in a pulmonary capillary. Explain how the structures and tissues of the gas exchange system are adapted to facilitate this journey and to maintain the health of the airways.

How this question is marked

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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