Plants release oxygen during the light-dependent reactions of photosynthesis. This oxygen forms when water molecules are split at photosystem II, a process called photolysis.
The light-dependent reactions occur in the thylakoid membranes of chloroplasts. Light energizes electrons in photosystem II, and the resulting electron shortage is replaced by electrons extracted from water. Splitting water produces electrons, hydrogen ions, and molecular oxygen. The electrons continue through the electron transport chain, while the hydrogen ions help generate ATP.
The oxygen is a by-product rather than the fuel of photosynthesis. ATP and NADPH made during these reactions power the Calvin cycle, where carbon dioxide is incorporated into organic molecules. A common mix-up is to assume that the released oxygen comes from carbon dioxide; isotope-tracing experiments showed that it comes from water. Oxygenic photosynthesis is characteristic of plants, algae, and cyanobacteria, although some bacteria perform anoxygenic photosynthesis and release no oxygen.