The herbicide atrazine works by disabling plastoquinone, one of the proton pumps in photosystem II. Explain how atrazine would kill a susceptible plant (5 marks) (OCR A specimen paper 3)

Atrazine prevents photophosphorylation from occurring because when plastoquinone is disabled H+ ions cannot be pumped into the thylakoid space therefore their will be a reduced proton motive force. ATP and reduced NADP will therefore not be produced. Without ATP and reduced NADP the Calvin cycle cannot occur therefore no triose phosphate and no glucose will be produced. An inability to produce glucose by photosynthesis means that the plant lacks a respiratory substrate and is therefore unable to respire leading to the death of the plant.