During the light reactions of photosynthesis, how is molecular oxygen (O2) produced?

• A. By photolysis of water at photosystem II, splitting $H_{2}O$ to release $O_2$, electrons, and protons
• B. By splitting $N_2$ in the thylakoid lumen to form $O_2$
• C. By fixation of $C_{\mathrm{O}}2$ during the Calvin cycle, releasing $O_2$ as a byproduct
• D. By reduction of $N_{\mathrm{A}}{D}_{\mathrm{P}}$ to $N_{\mathrm{A}}{D}_{\mathrm{P}}H$, which generates $O_2$

Answer: (A)

Verified step by step guidance

1. Understand that molecular oxygen (O\_2) is produced during the light reactions of photosynthesis, which occur in the thylakoid membranes of chloroplasts.
2. Recall that the source of O\_2 is water (H\_2O), which undergoes a process called photolysis (light-driven splitting) at Photosystem II.
3. Photolysis of water splits H\_2O molecules into oxygen (O\_2), protons (H\^+), and electrons (e\^-). The overall reaction can be represented as: 2H\_2O \rightarrow 4H\^+ + 4e\^- + O\_2.
4. Recognize that the electrons released from water replace those lost by chlorophyll in Photosystem II when it absorbs light energy, while the protons contribute to the proton gradient used to produce ATP.
5. Conclude that the production of O\_2 during the light reactions is directly linked to the photolysis of water at Photosystem II, not from nitrogen fixation, Calvin cycle, or NADP reduction.