Questions: Of the following events from the light reactions of photosynthesis, which would be expected to occur first? A Light-induced reduction of the primary electron acceptor in the reaction center of PS II takes place. (B) ATP synthase makes ATP (C) NADP+ gets reduced (D) Protons are pumped into the thylakoid space

The answer is the first one (A): Light-induced reduction of the primary electron acceptor in the reaction center of PS II takes place.

Explanation for each option:

A. Light-induced reduction of the primary electron acceptor in the reaction center of PS II takes place.

• This is the initial step in the light reactions. When light is absorbed by photosystem II (PS II), it excites electrons, which are then transferred to the primary electron acceptor.

B. ATP synthase makes ATP.

• This occurs later in the process. ATP is synthesized as protons flow back across the thylakoid membrane through ATP synthase, driven by the proton gradient established by earlier steps.

C. NADP+ gets reduced.

• This happens after the electrons have passed through the electron transport chain and reached photosystem I, where they are used to reduce NADP+ to NADPH.

D. Protons are pumped into the thylakoid space.

• This occurs after the initial electron transfer from PS II, as electrons move through the electron transport chain, contributing to the proton gradient used for ATP synthesis.

Summary: The first event in the light reactions of photosynthesis is the light-induced reduction of the primary electron acceptor in PS II, making option A the correct answer.