What is the role of NADP+ reductase in photosynthesis?

• A. Transfers protons across the thylakoid membrane.
• B. Synthesizes ATP directly using light.
• C. Splits water to release O2.
• D. Transfers electrons to NADP+, forming NADPH.

Answer: (D)

NADP+ reductase sits at the end of the light reactions, taking electrons from ferredoxin and handing them to NADP+ to form NADPH, with a proton from the stroma contributed in the process. This NADPH provides the reducing power needed for carbon fixation in the Calvin cycle, supplying electrons to convert CO2 into carbohydrate. The enzyme itself doesn’t move protons across the membrane or synthesize ATP; those roles belong to other components of the thylakoid electron transport chain and ATP synthase. Water splitting to release O2 happens at photosystem II, not at NADP+ reductase. So the transfer of electrons to NADP+ to produce NADPH is the essential function of this enzyme.