In the light reactions of photosynthesis, how is $O_2$ produced?

• A. By reduction of carbon dioxide during the Calvin cycle
• B. By the terminal electron acceptor reaction in the mitochondrial electron transport chain
• C. By photolysis of water at photosystem II (the oxygen-evolving complex splits $2H_{2}O$ to form $O_2$, electrons, and protons)
• D. By ATP synthase combining oxygen atoms from ADP and phosphate during photophosphorylation

Answer: (C)

Verified step by step guidance

1. Understand that the light reactions of photosynthesis occur in the thylakoid membranes of chloroplasts and involve the absorption of light energy to drive electron transport.
2. Recognize that oxygen (O\_2) production is linked to the splitting of water molecules, a process known as photolysis, which occurs specifically at Photosystem II.
3. Recall that the oxygen-evolving complex (OEC) associated with Photosystem II catalyzes the splitting of 2 H\_2O molecules into 4 electrons, 4 protons (H\^+), and 1 molecule of O\_2.
4. Note that the electrons released from water replace those lost by chlorophyll in Photosystem II during light absorption, while the protons contribute to the proton gradient used for ATP synthesis.
5. Conclude that oxygen is produced as a byproduct of water photolysis during the light reactions, not from carbon dioxide reduction, mitochondrial processes, or ATP synthase activity.