Describe non-cyclic electron flow and its outputs.

Describe non-cyclic electron flow and its outputs.

• A. Electrons travel from water through PSII, through the chain to PSI and ultimately reduce NADP+ to NADPH; O2 is evolved; ATP is formed
• B. Electron flow is cyclic only in PSI; only ATP produced
• C. Electrons go from NADPH to NADP+; no ATP produced
• D. Photosystems operate independently; no electron transport chain involvement

Answer: (A)

Non-cyclic electron flow uses both photosystems to move electrons from water all the way to NADP+. Light energy excites PSII, which splits water and provides electrons to the chain. The electrons travel through the chain (plastoquinone, cytochrome b6f, plastocyanin) to PSI, where another round of excitation lifts them to ferredoxin and then to NADP+ reductase to form NADPH. As electrons are drawn from water at PSII, oxygen is released. The movement of electrons powers proton pumping across the thylakoid membrane, and the resulting proton gradient drives ATP synthase to make ATP. So the outputs are NADPH, ATP, and O2. This is distinct from cyclic flow, which involves only ATP production around PSI without NADPH formation or oxygen evolution, and from scenarios where photosystems would act independently without the connected electron transport chain.

Non-cyclic electron flow uses both photosystems to move electrons from water all the way to NADP+. Light energy excites PSII, which splits water and provides electrons to the chain. The electrons travel through the chain (plastoquinone, cytochrome b6f, plastocyanin) to PSI, where another round of excitation lifts them to ferredoxin and then to NADP+ reductase to form NADPH. As electrons are drawn from water at PSII, oxygen is released. The movement of electrons powers proton pumping across the thylakoid membrane, and the resulting proton gradient drives ATP synthase to make ATP. So the outputs are NADPH, ATP, and O2.

This is distinct from cyclic flow, which involves only ATP production around PSI without NADPH formation or oxygen evolution, and from scenarios where photosystems would act independently without the connected electron transport chain.