The source of oxygen which is evolved during the photosynthesis is water. In 1937, an English bio-chemist named, Robin Hill proved this idea by a reaction. This reaction is named as Hill reaction.
The Hill reaction is properly defined as the photoreduction of an electron acceptor by the hydrogen of water, with the advancement of oxygen. In the organism the last electron acceptor is NADP+. We can measure the rate of the Hill reaction in isolated chloroplasts.
The reaction is as follows:
Robin hill put chloroplast water and some inorganic oxidizers; e.i., hydrogen acceptors together in absence of Carbon dioxide (CO2) and in presence of light is seen that no carbohydrate is produced in absence of carbon dioxide oxygen is released in this process. In fact, hydrogen of water releases oxygen by the reduction of hydrogen acceptor. It is proved from Hill’s experiment that the oxygen evolved in photosynthesis comes out of water.
Inorganic oxidizers or Hydrogen acceptors + 2H2O → (light and Chlorophyll) → 2AH2 + O2
Light is absorbed and the energy is used to drive electrons from water to generate NADPH and to drive protons across a membrane. These protons return through ATP synthase to make ATP.