During the light reactions of photosynthesis, where is molecular oxygen ($O_2$) produced?

• A. In the mitochondrial matrix during oxidative phosphorylation
• B. At Photosystem II on the lumenal side of the thylakoid membrane, where water is split by the oxygen-evolving complex
• C. At Photosystem I during reduction of $N_A+$ to $NADPH$
• D. In the chloroplast stroma during carbon fixation by Rubisco in the Calvin cycle

Answer: (B)

Verified step by step guidance

1. Identify the stage of photosynthesis where molecular oxygen (O\_2) is produced. Oxygen is generated during the light-dependent reactions, not during the Calvin cycle or mitochondrial processes.
2. Recall that the light reactions occur in the thylakoid membranes of chloroplasts, involving Photosystem II and Photosystem I.
3. Understand that Photosystem II contains the oxygen-evolving complex (OEC), which catalyzes the splitting of water molecules (H\_2O) into oxygen, protons, and electrons.
4. Recognize that this water-splitting reaction occurs on the lumenal side (inside) of the thylakoid membrane, releasing O\_2 as a byproduct.
5. Conclude that molecular oxygen is produced at Photosystem II on the lumenal side of the thylakoid membrane where water is split by the oxygen-evolving complex.