CAIE A Level Biology 9700 · Gas exchange
A lung biopsy sample examined under an electron microscope reveals that the alveolar walls consist of a single layer of extremely flattened cells, with a dense network of capillaries in close contact. Describe how the structure of the alveolar epithelium and the associated capillary network is adapted for efficient gas exchange between alveolar air and blood.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.