In the light reactions of photosynthesis, how is oxygen gas ($O_2$) produced?

• A. By reduction of ${\mathrm{NADP}}_{+}$ to $\mathrm{NADPH}$ during electron transport
• B. By fixation of ${\mathrm{CO}}_2$ into carbohydrates during the Calvin cycle
• C. By photolysis of water at photosystem II, splitting $H_{2}O$ to release $O_2$, electrons, and protons
• D. By ATP synthase converting ADP and inorganic phosphate into ATP, releasing $O_2$ as a byproduct

Answer: (C)

Verified step by step guidance

1. Understand that oxygen gas (O\_2) production in photosynthesis occurs during the light reactions, which take place in the thylakoid membranes of chloroplasts.
2. Recognize that the light reactions involve two main photosystems: Photosystem II (PSII) and Photosystem I (PSI), which work together to convert light energy into chemical energy.
3. Focus on Photosystem II, where light energy is used to excite electrons. These high-energy electrons are passed down the electron transport chain, but to replace the lost electrons, water molecules (H\_2O) are split in a process called photolysis.
4. Photolysis of water involves breaking down H\_2O into oxygen gas (O\_2), protons (H\^+), and electrons (e\^-). The electrons replenish those lost by PSII, the protons contribute to the proton gradient used for ATP synthesis, and oxygen is released as a byproduct.
5. Note that the other options are incorrect because NADP\^+ reduction to NADPH and ATP synthesis do not produce oxygen, and CO\_2 fixation occurs in the Calvin cycle, which is separate from the light reactions.