Photosystems I and Ii Summary

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 electron transport process a proton gradient is generated across the thylakoid membrane. This proton motive force is then used to drive the synthesis of ATP. This process requires PSI, PSII, cytochrome bf, ferredoxin-NADP+ reductase and chloroplast ATP synthase. I. Photosystem II Photosystem II transfers electrons from water to plastoquinone and in the process generates a pH gradient.
O H3 C H3 C CH3 C H2 O C H CH3 C C H2 H

n = 6-10

2e- + 2H+ Plastoquinone

Plastoquinone (PQ) carries the electrons from PSII to the cytochrome bf complex. Plastoquinone is an analog of Coenzyme Q. The only differences are the methyl groups replacing the methoxy groups of Q and a variable isoprenoid tail. Plastoquinone can functions as a one or two electron acceptor and donor. When it is fully reduced to PQH2 it is called plastoquinol. Like CoQ, PQ is a lipophilic mobile electron carrier carrying electrons from PSII to cytochrome bf.

Photosystem II is homologous to the purple bacterial photoreaction center we talked about previously. PSII is an integral membrane CH CH H C protein. The core of this membrane protein is formed by two subunits H C C C C C H D1 and D2. These two subunits span the membrane and are H H H n homologous to subunits L and M of