Where does the oxygen liberated during photosynthesis come from?

Oxygen liberated during photosynthesis comes from the splitting or hydrolysis of water in the green plants.

Cornelius van Niel experimentally proved for the first time that the oxygen liberated during photosynthesis comes from water and not from carbon dioxide. He based his experiments on green and purple sulphur bacteria and showed that when H₂S is utilised as hydrogen donor during photosynthesis instead of H₂O, then the oxidation product of the reaction is sulphur or sulphates and O₂ is not released.

Bacterial Photosynthesis – Not only plants but blue-green algae, as well as some bacteria like purple and green-sulphur bacteria, perform photosynthesis. Bacterial photosynthesis is mostly an anoxygenic process. Thus oxygen is not evolved.

Also See: Difference between Bacterial Photosynthesis and Plant Photosynthesis

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

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂

Water accounts for the release of oxygen is also proved later by radioisotope techniques.

Splitting of Water in Photosynthesis

Water splitting complex is present at the inner side of the thylakoid membrane, associated with PS II.

The reaction is as follows: 2H₂O → 4H⁺ + O₂ + 4e^–

Electrons released during the splitting of water compensate for the electrons moved from PS II and enable PS II to supply electrons continuously 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.

This was in brief about the splitting of water during photosynthesis. Get access to all the NEET Questions with explanations, only at BYJU’S.

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