CAIE A Level Biology 9700 · Gas exchange
A student examines a stained section of human lung under a light microscope. The walls of the alveoli are seen to consist of a single layer of flattened cells. Numerous capillaries are observed in close contact with these walls. Using this information and your knowledge of lung structure, explain how the alveoli are adapted for 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.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.