Light reaction (or hill reaction)

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

1. Photo excitation of chlorophyll A.

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

2. .Photolysis of water:

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

4H₂O  ——>  4H⁺ +4OH^–

4OH^{–   ———–>}   2H₂O + O₂ +4e^–

3. Photo phosphorylation :

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

a) Cyclic photo phosphorylation:

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

b) Non-cyclic photo phosphorylation:

                             The electron lost by P₆₈₀ (photosystem-II) are taken up by P₇₀₀ (photosystem I) and doesn’t come back to P₆₈₀. So, it is called non-cyclic photo phosphorylation. The electron pass through primary acceptor, PQ, cytochrome complex, PC and finally to P₇₀₀. The electron give out by P₇₀₀ taken up by the primary acceptor combine with hydrogen ions and reduced NADP to form NADPH₂.

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