What is the function of photosystem II in noncyclic photosynthesis?

• A. It reduces NADP+.
• B. It oxidizes water to produce oxygen.
• C. It makes ATP directly.
• D. It stores energy as glucose.

Answer: (B)

In noncyclic photosynthesis, the function of photosystem II is to start the electron flow by splitting water. Light excites the P680 chlorophyll in PSII, which donates electrons to the electron transport chain. To keep the chain going, water is split by the oxygen-evolving complex in PSII, providing fresh electrons to replace those lost, and releasing oxygen gas as a byproduct. As electrons move through the chain, a proton gradient builds up across the thylakoid membrane, powering ATP synthesis, while the electrons eventually reach photosystem I to help form NADPH. Glucose is produced later in the Calvin cycle, using ATP and NADPH from the light reactions, not directly by PSII. So PSII’s primary role here is oxidizing water to produce oxygen and supply electrons to drive the chain.