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 $\mathrm{P}_{430}$. The reduced $\mathrm{P}_{430}$ passes its electrons to ferredoxin $(\mathrm{Fd})$ 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 - $P_{700}$ After absorbing adequate amount of light energy electron gets excited from $P_{700}$ 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.