Light reaction (or hill reaction)

                         It takes place in the grana of chloroplast in the presence of light. The light reaction was studied by R. Hill in 1937 AD. So, it is also called the hill reaction. In light reactions, chlorophyll absorbs light to break H₂O into H⁺ and OH^– ions. The H⁺ ion is utilized to form NADPH₂ which is necessary to reduce CO₂ in dark reactions and O₂ is evolved. The light reaction is completed in the following steps:-

1. Photoexcitation of chlorophyll A.

                               A fraction of light falling in leaves is absorbed by photosynthetic pigment. The photosynthetic pigment absorbs different wavelengths of light and transfers it to the reaction center(chlorophyll A). Such pigments are called accessory pigments (or supporting pigments). The reaction center absorbs light energy to start a chemical reaction to convert light energy to chemical energy. Each chloroplast has two groups of co-operate in photosynthesis called photosystem I and Photosystem II. Each photosystem has a reaction center and many accessory pigments. In the photosystem, the reaction center absorbs light of wavelength 700 nm called P₇₀₀ and photosystem II absorbs light wavelength 680 nm. So, it is called P₆₈₀.

2. .Photolysis of water:

                        The process of breakdown of water and formation of O₂ in the presence of light is called the photolysis of water.

4H₂O    ———–> 4H⁺ +4OH^–

4OH^{–        ——————->}  2H₂O + O₂ +4e^–

3. Photo phosphorylation :

                It is the process of synthesis of ATP using light energy. It is of two types:

a) Cyclic photophosphorylation:

                         The electrons released by P₇₀₀ of photosystem I in the presence of light are taken up by the primary acceptor and are passed on to Fd, PQ, and PC and finally come back to the same molecules (p₇₀₀). So, it is called cyclic phosphorylation. In this phase, ATP molecules are formed.

Fig: Cyclic photophosphorylation

b) Non-cyclic photo phosphorylation:

                             The electrons lost by P₆₈₀ (photosystem II) are taken up by P₇₀₀ (photosystem I) and don’t come back to P₆₈₀. So, it is called non-cyclic photophosphorylation. The electron passes through the primary acceptor, PQ, cytochrome complex, PC, and finally to P₇₀₀. The electron is given out by P₇₀₀ is taken up by the primary acceptorcombinese with hydrogen ions and reduced NADP to form NADPH₂.

                                                Non-cyclic photo phosphorylation needs a constant supply of water and forms O₂ as a bi-product, ATP molecules, and NADPH₂ which are utilized in dark reactions.

Fig: Non-cyclic photophosphorylation