What is the role of water in photosynthesis according to the material?

• A. It acts as the primary carbon source
• B. It is a waste product
• C. It acts as an energy carrier
• D. It donates electrons and hydrogen during the light reactions

Answer: (D)

Water serves as the electron donor and hydrogen source that powers the light-dependent reactions. In the oxygen-evolving complex of Photosystem II, water is split through photolysis, delivering electrons to replace those that the chlorophyll loses after absorbing light. The reaction also releases protons into the thylakoid space and produces oxygen gas as a byproduct. Those electrons travel through the electron transport chain and ultimately help reduce NADP+ to NADPH, while the pumped protons create a gradient that drives ATP synthase to make ATP. This combination—ATP and NADPH—provides the energy and reducing power for the subsequent carbon-fixing steps in the Calvin cycle. Water, therefore, is the source of electrons and hydrogen for the light reactions, not the carbon source and not an energy carrier itself.