In the light reactions of photosynthesis, why is water (H2O) necessary?

• A. It provides carbon atoms that are incorporated into sugars during the light reactions.
• B. It is the final electron acceptor in the photosynthetic electron transport chain, forming water as a product.
• C. It directly absorbs light energy and transfers that energy to chlorophyll in the photosystems.
• D. It is split (photolyzed) to replace electrons lost from photosystem II and to provide protons for building a proton gradient, releasing O2 as a byproduct.

Answer: (D)

Verified step by step guidance

1. Understand the role of water in the light reactions of photosynthesis, which occur in the thylakoid membranes of chloroplasts.
2. Recognize that during the light reactions, chlorophyll molecules in photosystem II lose electrons when they absorb light energy and become excited.
3. Know that these lost electrons must be replaced to keep the electron transport chain functioning; water molecules are split (photolyzed) to provide these replacement electrons.
4. Realize that the splitting of water also releases protons (H+) into the thylakoid lumen, contributing to the proton gradient used to produce ATP, and oxygen (O2) is released as a byproduct.
5. Conclude that water is essential because it supplies electrons to photosystem II, helps generate a proton gradient for ATP synthesis, and produces oxygen, but it does not provide carbon atoms or act as the final electron acceptor.