Spinach chloroplasts are illuminated in the absence of ADP and P_i, then the light is turned off and ADP and P_i are added. ATP is then synthesized for a short time in the dark. Which of the following best explains this?
I.
ATP is generated from the proton gradient accumulated in the light reactions.
II.
ATP synthesis stops after the proton gradient dissipates.
III.
ATP can be generated in the dark reactions if it becomes necessary for the plant to survive.

Question

Quizplus · Accepted Answer

In the absence of ADP and P_i, there is no ATP synthesis occurring in the light reactions. However, the chloroplasts are still being illuminated, so a proton gradient is being generated across the thylakoid membrane. When ADP and P_i are added, ATP synthesis can occur, as the proton gradient from the light reactions can be used to drive ATP synthesis. This explains option I, that ATP is generated from the proton gradient accumulated in the light reactions. Option II, that ATP synthesis stops after the proton gradient dissipates, is also correct, as once the proton gradient is dissipated, there is no more energy to drive ATP synthesis. Option III, that ATP can be generated in the dark reactions if it becomes necessary for the plant to survive, is not correct, as the dark reactions do not directly generate ATP – they use ATP generated in the light reactions to power their reactions.

Spinach Chloroplasts Are Illuminated in the Absence of ADP and Pi

Spinach Chloroplasts Are Illuminated in the Absence of ADP and P_i