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How does photosynthesis release oxygen into the atmosphere?

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abundance of oxygen — evolution of the atmosphere A “best guess” reconstruction of the abundance of O₂ in Earth's atmosphere as a function of time. The O₂ abundance axis is logarithmic.

Photosynthesis is the biological process by which organisms, such as cyanobacteria and plants, convert carbon dioxide and water into carbohydrates using light energy captured by chlorophyll.123

During this reaction, molecular oxygen is produced as a by-product. The chemical process is commonly represented by the following formula:423

$6 C O_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}$

In this reaction, six molecules of carbon dioxide and six molecules of water are converted into one molecule of sugar and six molecules of oxygen. While the organism utilizes the sugar for growth and vital functions, the oxygen is released into the environment.231

Historically, this process was responsible for the Great Oxidation Event, which began approximately 2.4 billion years ago. Initially, much of the released oxygen was consumed by the oxidation of dissolved iron in the oceans, forming banded-iron formations. Once these iron sinks were saturated, oxygen began to accumulate in the atmosphere, fundamentally transforming Earth's biosphere.45