Questions: At low concentrations of NADPH + H^+, excited electrons in photosystem I Select one: a. are passed to photosystem II. b. power ATP synthase. c. reduce coenzymes. d. create a pH gradient in the lumen.

The answer is c: reduce coenzymes.

Explanation for each option:

a. are passed to photosystem II: This is incorrect. In the photosynthetic electron transport chain, electrons flow from photosystem II to photosystem I, not the other way around. Photosystem I receives electrons from photosystem II via the electron transport chain.

b. power ATP synthase: This is incorrect. ATP synthase is powered by the proton gradient across the thylakoid membrane, not directly by electrons. The movement of protons through ATP synthase drives the synthesis of ATP.

c. reduce coenzymes: This is correct. In photosystem I, excited electrons are used to reduce NADP+ to NADPH. This process involves the transfer of electrons to coenzymes, specifically NADP+, which is reduced to NADPH.

d. create a pH gradient in the lumen: This is incorrect. The pH gradient is created by the movement of protons (H+) into the thylakoid lumen during the electron transport chain, primarily associated with photosystem II and the cytochrome b6f complex, not directly by the electrons in photosystem I.

In summary, at low concentrations of NADPH, excited electrons in photosystem I are primarily used to reduce coenzymes, specifically converting NADP+ to NADPH.