In the light reactions of photosynthesis, what role does water (H2O) play?

• A. It donates carbon atoms that are directly incorporated into glucose during the Calvin cycle.
• B. It acts as the final electron acceptor, being reduced to form water at the end of the light reactions.
• C. It absorbs light energy and directly converts it into ATP without involvement of chlorophyll.
• D. It is split (photolysis) to provide electrons to replace those lost by Photosystem II, releasing O2 and contributing H+ to the thylakoid lumen.

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.
2. Recognize that water (H2O) is involved in a process called photolysis during the light reactions, where it is split by the enzyme complex associated with Photosystem II.
3. Identify that the splitting of water provides electrons that replace those lost by Photosystem II when it absorbs light and excites electrons to a higher energy state.
4. Note that the splitting of water also releases oxygen (O2) as a byproduct, which diffuses out of the chloroplast and eventually the plant cell.
5. Understand that the hydrogen ions (H+) released from water contribute to the proton gradient across the thylakoid membrane, which is used to drive ATP synthesis.