T HE E LECTRON T RANSPORT In photosystem II the reaction centre chlorophyll a absorbs 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 transport Photon Reaction centre Pigment molecules Primary acceptor Figure . The light harvesting complex system consisting of cytochromes (Figure . ).
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 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 acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally down hill to NADP + reducing it to NADPH + H + is called the Z scheme , due to its characterstic shape (Figure . ). This shape is formed when all the carriers are placed in a sequence on a redox potential scale.
. . Splitting of Water You would then ask, How does PS II supply electrons continuously? 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 2H + , [O] and electrons. This creates oxygen, one of the net products of photosynthesis. The electrons needed to replace those removed from photosystem I are provided by photosystem II.
H O H O e → − We need to emphasise