The electrons lost from the reaction center of photosystem i are replaced by electrons from

The electron lost from the reaction center of photo system l are replaced by electron form water. 
There are two photosystems in the photosynthesis process and they occur during the light reaction. Photosystem l refers to the second system in the photosynthetic light reaction of plant. When photosystem ll absorbs light, electrons in the reaction-center chlorophyll become excited and they move to a higher energy level where they are trapped by the primary electron acceptors. These electrons that are trapped are then replaced by electrons from water.