In photosynthesis, which component's primary role is to capture light energy to oxidize water, releasing oxygen and providing electrons to the electron transport chain?

• A. Photosystem I
• B. Rubisco
• C. Photosystem II
• D. ATP synthase

Answer: (C)

Light-dependent reactions rely on two photosystems working in order. The first photosystem is the one that captures light energy and uses it to drive the oxidation of water. When light excites the reaction-center chlorophyll (P680), the energy is transferred to an oxygen-evolving complex that splits water into oxygen gas, protons, and electrons. Those electrons are then funneled into the electron transport chain to replace the ones lost by the photosystem, helping establish the proton gradient used to make ATP. Photosystem I, in contrast, mainly serves to re-energize electrons so they can reduce NADP+ to NADPH, not to oxidize water. Rubisco fixes CO2 in the Calvin cycle, which happens later, and ATP synthase uses the proton gradient but does not capture light or oxidize water.