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Where does the oxygen released during photosynthesis come from?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

The oxygen released during photosynthesis is derived from water molecules.5

During the light-dependent reactions, chlorophyll pigments absorb light energy, which excites electrons and initiates an electron transport chain. To replace these lost electrons, water molecules are split in a process known as photolysis. This reaction splits two molecules of water into molecular oxygen ( $O_{2}$ ), four protons ( $H^{+}$ ), and four electrons. The oxygen is then released into the environment as a by-product of the process.1234

This conclusion was definitively supported by experiments in the 1940s using heavy isotopes of oxygen, which demonstrated that the oxygen gas produced by plants originates from the water they consume rather than from carbon dioxide.5