When two photosystems work in a series then electron flow from  

In photosystem II the reaction centre chlorophyll ‘a’ absorbs 680nm wavelength of red light causing
electrons to become excited and jump into the orbit farther from the atomic nucleus. These electrons
are picked up by an electron acceptor (Pheophytin) which passes them to an electrons transport
system consisting of cytochromes (b_⁶ and f) which donate electrons to plastocyanin. The electrons are not used up as they pass through the electron transport chain, but are passed on to the pigments
of photosystem PSI.