CAIE A Level Biology 9700 · Gas exchange
A histologist is examining a photomicrograph of a section of human lung tissue. She notes that the alveoli are surrounded by a dense network of capillaries and have walls that appear very thin. Describe and explain how the structure of the alveoli is adapted to maximise the rate of gas exchange between alveolar air and blood.
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.