In photosystem II the reaction centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit farther from the atomic nucleus.
These electrons are picked up by an electron acceptor which passes them to an electrons transportsystem consisting of cytochromes (Figure 13.5). This movement of electrons is downhill, in terms of an oxidation-reduction or redox potential scale.
The electrons are not used up as they pass through the electron transport chain, but are passed on to the pigments of photosystem PS I. Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm and are transferred to another accepter molecule that has a greater redox potential.
These electrons then are moved downhill again, this time to a molecule of energy-rich NADP+. The addition of these electrons reduces NADP+ to NADPH + H+.
This whole scheme of transfer of electrons, starting from the PS II, uphill to the accepter, down the electron transport chain to PS I, excitation of electrons, transfer to another accepter, and finally down hill to NADP+ causing it to be reduced to NADPH + H+ is called the Z scheme, due to its characterstic shape.
This shape is formed when all the carriers are placed in a sequence on a redox potential scale.