CAIE A Level Biology 9700 · Gas exchange
A student is studying the human gas exchange system. She traces the path of an oxygen molecule from the trachea to the blood. Describe the structures the oxygen molecule passes through and explain 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.