Which sequence correctly describes the flow of electrons in the light-dependent reactions?

• A. Water → Photosystem II → Photosystem I → NADPH.
• B. NADPH → Photosystem I → Water → ATP.
• C. Oxygen evolves directly to NADP+.
• D. Carbon dioxide fixation precedes electron transport.

Answer: (A)

In the light-dependent reactions, electrons start from water and move through the two photosystems to NADP+. Light excites Photosystem II, pushing electrons into the electron transport chain and driving water splitting to replace the lost electrons. The electrons then pass from Photosystem II to Photosystem I via the mobile carriers, getting re-energized at Photosystem I, and finally reducing NADP+ to NADPH through NADP+ reductase. This flow also creates a proton gradient that powers ATP synthase, producing ATP for use in the Calvin cycle. So the sequence is water → Photosystem II → Photosystem I → NADPH. The other statements don’t match the actual electron flow: NADPH is a product, not a donor; oxygen is released during water splitting and isn’t transported to NADP+; and carbon fixation happens later, using the ATP and NADPH from the light reactions.