How do chloroplasts generate reducing power (NADPH) during photosynthesis?

• A. NADPH is produced in the mitochondria during cellular respiration.
• B. NADPH is produced by substrate-level phosphorylation in glycolysis.
• C. NADPH is produced during the night by non-photosynthetic processes.
• D. In the light reactions, electrons are transferred to NADP+, forming NADPH.

Answer: (D)

During the light reactions, chloroplasts generate NADPH by transferring electrons to NADP+. Light energy excites electrons in the photosynthetic electron transport chain, starting with photosystem II as water is split to release electrons. The electrons travel through to photosystem I, where they are boosted again by light and handed to ferredoxin. Ferredoxin-NADP+ reductase then uses these electrons to reduce NADP+ to NADPH. This NADPH provides the reducing power for the Calvin cycle to drive carbon fixation. NADPH isn’t produced in mitochondria during respiration, glycolysis doesn’t generate NADPH (it makes ATP and NADH), and NADPH production at night by non-photosynthetic processes isn’t how chloroplasts provide reducing power.