How is NADPH formed in the light-dependent reactions of photosynthesis?

• A. NADPH forms during the photolysis of water at PSII.
• B. NADPH is produced by ATP synthase using the proton gradient.
• C. NADPH is produced when CO2 is fixed in the Calvin cycle.
• D. Electrons pass via PSI to ferredoxin and are transferred to NADP+ by ferredoxin-NADP+ reductase, forming NADPH.

Answer: (D)

NADPH is formed when electrons that have been energized by light in photosystem I are transferred to NADP+ through ferredoxin-NADP+ reductase. After PSI captures light, its excited electrons move through ferredoxin, and this protein donates them to NADP+ via the enzyme ferredoxin-NADP+ reductase, producing NADPH in the stroma. This step couples light energy to the reduction of NADP+, providing the reducing power the Calvin cycle needs to convert CO2 into sugars.

Photolysis of water at PSII feeds electrons into the chain but does not produce NADPH directly; it mainly helps sustain the flow of electrons and the proton gradient that drives ATP synthesis. ATP synthase uses that gradient to make ATP, not NADPH. Likewise, NADPH isn’t formed during carbon fixation in the Calvin cycle; NADPH is produced earlier in the light reactions and then used in the Calvin cycle.