When a molecule of chlorophyll-a (acting as the reaction center) receives light energy in the form of photons (quanta), it becomes energy-rich and activated. This is called the excited state of chlorophyll. In this excited state, the chlorophyll-a expels one electron. With the loss of an electron, the chlorophyll-a develops a positive charge. This is called ionized chlorophyll-a. The expelled electron contains an extra amount of energy received from light. In other words, the energy-rich expelled electron represents light energy. This electron energy is used for the formation of ATP during photosynthesis.

In the photosynthesis of green plants, water is used as a source of hydrogen required for the reduction of carbon dioxide to form carbohydrates.

In this process, water is oxidized in presence of light and chlorophyll. Hydrogen is removed from water, and oxygen is released as follows :

This is described as photolysis of water, or photochemical oxidation of water. This was first indicated by Van Niel (1931). It was demonstrated with the help of an experiment by R. Hill (1937). Hill suggested that during the photolysis of water, hydrogen combines with some hydrogen acceptor in the plant, while oxygen is released.

(where A = unknown H-acceptor in plants).

Later on, Arnon (1951) discovered this hydrogen acceptor to be a coenzyme called NADP (nicotinamide adenine dinucleotide phosphate).

Substance A = NADP

Similarly, Ruben, Kamen, et al. (1941) confirmed that the oxygen evolved during photosynthesis comes from the splitting of water only. They used heavy isotope of oxygen (O18) to demonstrate the fact as follows :

Chapter 5