What is the source of the oxygen released during photosynthesis?

• A. The splitting of water by the oxygen-evolving complex in PSII.
• B. Release of oxygen during NADPH formation at PSI.
• C. Breakdown of O2 by the Calvin cycle.
• D. Oxygen is produced by the chloroplast envelope.

Answer: (C)

Oxygen released in photosynthesis comes from water that is split during the light-dependent reactions, specifically by the oxygen-evolving complex of photosystem II. Light energy powers the splitting of two water molecules into oxygen, protons, and electrons: 2 H2O → O2 + 4 H+ + 4 e−. The O2 gas exits the plant, while the electrons flow through the electron transport chain to photosystem I, helping form NADPH, and the proton motive force drives ATP synthesis. The Calvin cycle then uses ATP and NADPH to build sugars from CO2, but it does not produce oxygen. The chloroplast envelope is simply the outer layer and does not generate O2.