In light reaction of photosynthesis two types of photosystems are involved. PS-I consists of plenty of chlorophyll-a and very less quantity of chlorophyll-b. These pigments absorb light energy and transfer it to the reaction centre - P₇₀₀. After absorbing adequate amount of light energy electron gets excited from P₇₀₀ molecule and moves to iron-sulphur protein complex, designated as A (Fe-S). It gets reduced after accepting electrons. It later gives these electron to ferredoxin and gets oxidised again.