LIGHT REACTION/HILL'S REACTION/PHOTOCHEMICAL REACTION
Hill reaction or photolysis of water was proposed by Robin (Robert Hill). Splitting up of water and liberation of O2by chlogophyll in presence of light and hydrogen acceptor is called photolysis of water. Water splits in presence of Mn2+ and Cl-
and results in the formation of 2H+ , 2OH-, 2e-, O2 is formed from OH.
Two pigment systems of photo systems are included in the light phase of photosynthesis i.e., PS I and PS II. PS I is located primarily in stromathylakoid and PS II mainly in grana of the chloroplast. PS I consists of more of Chl a, Chl b, comparatively less carotenoids and photo center, a special Chl a molecule called P700. PS II consists of Chla andChl b, carotenoids (its contents is higher as compared to that of PS I) and photo center (P680). Photo system I has reducing agent X, FeS center B are ferredoxin, plastoquinone, cytochrome complex and plastocyanin. PS I takes part in both cyclic and noncyclic photophosphorylation. PS II also contains Mn2+, Cl-, quencher molecule Q, plastoquinone (PQ), cytochrome complex and plastocyanin. It picks up electrons released duringsphotolysis of water. The same is extruded on absorption of light energy. PS II can operate only in conjunction with PS I .