In which stage of photosynthesis is oxygen produced?

• A. Calvin cycle
• B. Light-dependent stage
• C. Krebs cycle
• D. Dark reaction

Answer: (B)

Oxygen is produced during the light-dependent stage because that part of photosynthesis uses light to drive the splitting of water. In the thylakoid membranes, light energy powers the flow of electrons through photosystems II and I and the electron transport chain. To replace the electrons lost by chlorophyll in photosystem II, water is split in a process called photolysis, releasing oxygen gas as a byproduct. The captured energy is then used to make ATP and NADPH, which feed the later carbon-fixing steps.

The other stages don’t release oxygen. The Calvin cycle (often called the dark reactions) uses ATP and NADPH to convert carbon dioxide into sugars and occurs without producing O2. It takes place in the stroma and consumes energy to build carbohydrates. The Krebs cycle is part of cellular respiration in mitochondria, not photosynthesis, and it does not generate oxygen in the chloroplast context.