During which stage of photosynthesis is molecular oxygen ($O_2$) produced as a byproduct?

• A. Citric acid cycle in the mitochondrial matrix
• B. Calvin cycle (light-independent reactions) in the stroma
• C. Light reactions (photophosphorylation) in the thylakoid membrane
• D. Glycolysis in the cytosol

Answer: (C)

Verified step by step guidance

1. Identify the main stages of photosynthesis: the light reactions (also called photophosphorylation) and the Calvin cycle (light-independent reactions).
2. Understand that the light reactions occur in the thylakoid membranes of the chloroplast and involve the absorption of light energy to produce ATP and NADPH.
3. Recognize that during the light reactions, water molecules (H\_2O) are split in a process called photolysis, which releases electrons, protons, and molecular oxygen (O\_2) as a byproduct.
4. Note that the Calvin cycle takes place in the stroma and uses ATP and NADPH to fix carbon dioxide into glucose, but it does not produce oxygen.
5. Conclude that molecular oxygen (O\_2) is produced specifically during the light reactions (photophosphorylation) in the thylakoid membrane, not during the citric acid cycle or glycolysis.