Explanation

The electrons that were moved from photosystem II must be replaced. This is achieved by electrons available due to splitting of water. The splitting of water is associated with the PS-II. Water is split into $H^{+}$, O and electrons. This creates oxygen, one of the net products of photosynthesis. The electrons that are needed to replace those removed from photosystem-I are provided by photosystem-II $2H_{2}O \rightarrow 4H^{+} + O_{2} + 4e^{-}$