Why does photosynthesis release oxygen gas?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

Photosynthesis releases oxygen gas as a necessary by-product of the light-dependent reactions, which are photochemical reactions driven by light energy.1

During these light reactions, chlorophyll and other pigments absorb sunlight, causing their electrons to become excited and enter an electron transport chain. To replace these lost electrons, water molecules are split in a process known as photolysis.2

This oxidation of water produces oxygen gas ( $O_{2}$ ), hydrogen ions, and electrons. While the hydrogen ions and electrons are utilized to generate energy-carrying molecules like ATP and NADPH for later use in carbon fixation, the oxygen is released into the environment as a by-product of the reaction.132