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How does photosynthesis add oxygen to Earth’s atmosphere?

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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.

Photosynthesis adds oxygen to Earth’s atmosphere by utilizing water ( $H_{2} O$ ) as a reducing agent to convert carbon dioxide into carbohydrates.12

During the light-dependent reactions of this process, specifically within photosystem II, light energy is captured by chlorophyll to initiate electron transfer. To replace electrons lost during this reaction, the plant enzymatically splits water molecules into molecular oxygen, protons, and electrons. While the protons and electrons are used to fuel the synthesis of energy-carrying molecules like ATP and NADPH, the oxygen is released as a by-product.34

The chemical process is summarized as:
$6 C O_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}$

Once released by the chloroplast, this oxygen exits the plant and enters the atmosphere. While most plants release oxygen primarily during the day, this continuous cycle of production by terrestrial plants and aquatic algae is essential for maintaining the oxygen levels in our biosphere.315