The oxygen evolved during photosynthesis comes from splitting of

The oxygen evolved during photosynthesis is derived from the splitting of water molecules, a process known as photolysis [3]. This occurs during the light-dependent reactions within the chloroplasts [1]. When chlorophyll absorbs light energy, it facilitates the conversion of this energy into chemical energy, leading to the oxidation of water [1]. Specifically, water molecules are split into oxygen gas (O2), protons (H+), and electrons (e-). The electrons are used to replenish the electron transport chain, while the oxygen is released into the atmosphere as a byproduct [3]. Historically, the Hill reaction provided critical evidence for this mechanism, demonstrating that isolated chloroplasts could release oxygen from water even in the absence of carbon dioxide fixation. While carbon dioxide is essential for photosynthesis, it is reduced to form carbohydrates (glucose) and does not contribute to the evolved oxygen [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > QUESTIONS > p. 82
2. [3] https://www.sciencedirect.com/topics/chemistry/hill-reaction