Photosynthetic Units

A photosynthetic unit is a small group of pigment molecules which takes part in photochemical reaction and changing light energy into chemical energy. In one unit there will be one photocentre or reaction centre which will be feed by about 200 harvesting pigment molecules. The photocentre consists of special chlorophyll a molecules, P700 and P680. Reaction centre absorbs light energy at longer wavelength. Harvesting molecules are also called light harvesting complex ( LHC). There are different LHCS for two photosystems I and II. Two types of molecules are presesnt in LHC these are called antenna molecules and core molecules. The antenna molecules absorbs variou wavelength of light shorter than photocentre. After absorbtion antenna molecules get excited and in excited form these molecules hand over energy to core molecules by resonance and come to ground state. The energy from core molecules will supply to the photocentre. On absorbtion of light energy photocentre get excited and by changing the state of electrons from excited to again grond state helps the photocentre to change the light energy to chemical energy. The frequency of excitation of photocentre is very high so it cannot meet by direct absorption of energy.

                                                   Harvesting of light by photosynthetic unit