Where Does The Oxygen Liberated During Photosynthesis Come From

The splitting or hydrolysis of water in green plants is the source of oxygen liberated during photosynthesis.

Cornelius van Niel was the first to experimentally prove that the oxygen produced during photosynthesis comes from water and not from carbon dioxide. He conducted experiments on green and purple sulphur bacteria and found that when H2S was used as the hydrogen donor instead of H2O, the oxidation product of the reaction was sulphur or sulphates, and no O2 was released.

Bacterial Photosynthesis – Not only plants, but blue-green algae and certain bacteria such as purple and green-sulphur bacteria can also perform photosynthesis. This type of photosynthesis is mostly an anoxygenic process, meaning that oxygen is not produced.

For more information: Difference between Bacterial Photosynthesis and Plant Photosynthesis

H2O in green plants acts as a hydrogen donor and reduces CO2 to carbohydrates, which in turn gets oxidised to O2. This reaction takes place on the inner side of the thylakoid membrane.

6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2

Water’s role in the release of oxygen is further confirmed by radioisotope techniques.

Splitting of Water in Photosynthesis

The water splitting complex is located on the inner side of the thylakoid membrane and is associated with Photosystem II.

2H₂O → 4H⁺ + O₂ + 4e⁻

Electrons released during the splitting of water compensate for the electrons moved from PS II and enable PS II to continuously supply electrons to PS I through the Electron Transport Chain (ETS). The protons produced during the process accumulate in the lumen and account for the proton gradient present across the membrane, which facilitates the ATP production.

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