CAIE A Level Biology 9700 · Gas exchange
A micrograph of a mammalian lung shows that the wall of an alveolus is lined with a single layer of very thin cells. Branches of the pulmonary capillary network are seen immediately adjacent to this layer. Describe how the structure of the alveolar epithelium is adapted for efficient gas exchange between alveolar air and blood in the capillaries.
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.