CAIE A Level Biology 9700 · Gas exchange
Explain the pathway of an oxygen molecule as it moves from the trachea into the blood of a pulmonary capillary. In your answer, name the structures it passes through and describe how each structure is adapted to facilitate 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.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.