Photosystems I and Ii Summary

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Within the thylakoid membranes of the chloroplast, are two photosystems. Photosystem I optimally absorbs photons of a wavelength of 700 nm. Photosystem II optimally absorbs photons of a wavelength of 680 nm. The numbers indicate the order in which the photosystems were discovered, not the order of electron transfer. Under normal conditions electrons flow from PSII through cytochrome bf (a membrane bound protein analogous to Complex III of the mitochondrial electron transport chain) to PSI. Photosystem II uses light energy to oxidize two molecules of water into one molecule of molecular oxygen. The 4 electrons removed from the water molecules are transferred by an electron transport chain to ultimately reduce 2NADP+ to 2NADPH. During the…show more content…
Manganese is the core of this redox center because it has four stable oxidation states (Mn2+, Mn3+,Mn4+ and Mn5+) and coordinates tightly to oxygen containing species. Each time the P680 is excited and an electron is kicked out, the positively charged special pair extracts an electron from the manganese center. 2H2O O2 + 4e4 electrons must be transferred to 2 molecules of plastoquinone in order to oxidize H2O to molecular oxygen. This requires 4 photochemical steps. The Manganese center is oxidized one electron at a time, until two molecules of H2O are linked to form O2 which is then released from the center. A tyrosine residue not shown participates in the proton electron transfers. The structures are designated S0 through S4 to indicated the number of electrons removed. When isolated chloroplasts that have been held in the dark are illuminated with very brief flashes of light, O2 evolution reaches a peak on the third flash and every fourth flash there after as shown to the left. The oscillation in O2 evolution dampens over repeated flashes and converges to an average value. We know the manganese center exists in five different oxidation states numbered for S0 to S4 as shown above. One electron and a proton are removed during each photochemical step. When S4 is attained, an O2 molecule is released and two new molecules of water bind. The reason the third pulse

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