In the light reactions of photosynthesis, what role does water play?

• A. It donates carbon atoms that are fixed into sugars during the Calvin cycle.
• B. It directly absorbs light energy as the primary pigment to excite electrons in Photosystem II.
• C. It is split (photolyzed) to provide electrons to replace those lost from Photosystem II, releasing $O_2$ and contributing protons to the thylakoid lumen.
• D. It serves as the final electron acceptor in the photosynthetic electron transport chain, forming water from oxygen.

Answer: (C)

Verified step by step guidance

1. Understand the context of the light reactions in photosynthesis, which occur in the thylakoid membranes of chloroplasts and involve the absorption of light energy to drive electron transport.
2. Recognize that Photosystem II absorbs light and loses electrons that need to be replaced to continue the electron transport chain.
3. Identify the source of these replacement electrons: water molecules are split in a process called photolysis, which occurs in Photosystem II.
4. Note that the splitting of water releases electrons to replace those lost by Photosystem II, protons (H+) that contribute to the proton gradient in the thylakoid lumen, and oxygen (O2) as a byproduct.
5. Conclude that water's role is not to donate carbon atoms, absorb light as the primary pigment, or serve as the final electron acceptor, but specifically to provide electrons through photolysis, releasing oxygen and protons.