CAIE A Level Biology 9700 · Gas exchange
A student inhales a breath of air. Describe the path taken by an oxygen molecule from the trachea to the blood in the lungs, and explain how the structures encountered along this path are adapted to facilitate 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.