In the light reactions of photosynthesis, what is the primary role of water (H2O)?

• A. It serves as the final electron acceptor at the end of the light reactions to form water.
• B. It donates carbon atoms that are incorporated into glucose during the Calvin cycle.
• C. It directly phosphorylates ADP to ATP by providing a phosphate group.
• D. It is split at photosystem II to provide electrons and protons and to release oxygen gas as a byproduct.

Answer: (D)

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 conversion of light energy into chemical energy (ATP and NADPH).
2. Recognize that water (H2O) plays a crucial role in these light reactions by undergoing a process called photolysis, where it is split into electrons, protons (H+), and oxygen gas (O2).
3. Identify that the electrons released from water replace the electrons lost by chlorophyll in photosystem II when it absorbs light energy, thus maintaining the flow of electrons through the electron transport chain.
4. Note that the protons (H+) contribute to the proton gradient across the thylakoid membrane, which drives ATP synthesis via ATP synthase.
5. Conclude that oxygen gas (O2) is released as a byproduct of water splitting, which is essential for maintaining atmospheric oxygen levels.