In biology, only photosystem II is able to withdraw electrons from water and to generate molecular oxygen as a waste product. This remarkable specialization of photosystem II is conferred by the unique properties of one of the two chlorophyll molecules of its special pair and by a manganese cluster linked to the protein. These chlorophyll molecules and the manganese cluster form the catalytic core of the photosystem II reaction center, whose mechanism is outlined in Figure 14–48.
Water is an inexhaustible source of electrons, but it is also extremely stable; therefore a large amount of energy is required to make it part with its electrons. The only compound in living organisms that is able to achieve this feat after its ionization by light, is the chlorophyll special pair called P680 (P680/P680+ redox potential = +1270 mV). The reaction 2H2O + 4 photons →4H+ + 4e – + O2 is catalyzed by its adjacent manganese cluster. The intermediates remain firmly attached to the manganese cluster until two water molecules have been fully oxidized to O2, thus
Figure 14–47 Changes in redox potential during photosynthesis. the redox potential for each molecule is indicated by its position along the vertical axis. photosystem II passes electrons derived from water to photosystem I, which in turn passes them to NaDp+ through ferredoxin-NaDp+ reductase. the net electron flow through the two photosystems is from water to NaDp+, and it produces NaDph as well as an electrochemical proton gradient. this proton gradient is used by the atp synthase to produce atp. Details in this figure will be explained in the subsequent text.
ensuring that no dangerous oxygen radicals are released as the reaction proceeds. The protons released by the two water molecules are discharged to the thylakoid space, contributing to the proton gradient across the thylakoid membrane (pH lower in the thylakoid space than in the stroma). The unique protein environment that endows life with this all-important ability to oxidize water has remained essentially unchanged throughout billions of years of evolution (Figure 14–49).
All of the oxygen in the Earth’s atmosphere has been generated in this way. Although the exact details of the water-oxidation reaction in photosystem II are still not fully understood, scientists are trying to construct an artificial system that mimics the process. If successful, this might provide a virtually endless supply of clean energy, helping to solve the world’s energy crisis.
the Cytochrome b6-f Complex Connects photosystem II  to photosystem I
Following the path shown previously in Figure 14–48, the electrons extracted from water by photosystem II are transferred to plastoquinol, a strong electron donor similar to ubiquinol in mitochondria. This quinol, which can diffuse rapidly in the lipid bilayer of the thylakoid membrane, transfers its electrons to the cytochrome b6-f complex, whose structure is homologous to the cytochrome c reductase in mitochondria. The cytochrome b6-f complex pumps H+ into the thylakoid space using the same Q cycle that is utilized in mitochondria (see Figure 14–21), thereby adding to the proton gradient across the thylakoid membrane.
The cytochrome b6-f complex forms the connecting link between photosystems II and I in the chloroplast electron-transport chain. It passes its electrons