In photosystem I, the primary electron acceptor is probably a Fe-S protein. The reduced primary acceptor transfers the electrons to the secondary electron acceptor P430. The reduced P430 passes its electrons to ferredoxin (Fd) present at the outer surface of the thylakoid membrane. Related Theory 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 this electron to ferredoxin and gets oxidized again. 