PSI Photosystem

Both PSI and PSII are necessary for photosynthesis, but the systems do not operate in the implied temporal sequence. There is also considerable pooling of electrons in intermediates between the two photosystems, and the indicated photoacts seldom occur in unison. The terms PSI and PSII have come to represent two distinct, but interacting reaction centers in photosynthetic membranes (36,37) the two centers are considered in combination with the proteins and electron-transfer processes specific to the separate centers. 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

All photosynthetic cells contain some form of photosystem. Photosynthetic bacteria, unlike cyanobacteria and eukaryotic phototrophs, have only one photosystem. Interestingly, bacterial photosystems resemble eukaryotic PSII more than PSI, even though photosynthetic bacteria lack Og-evolving capacity. 

Photosynthetic electron flow, 13 287 Photosystem I (PSI), 13 286. See also PSI transport processes Photosystem I inhibitors, 13 286-288 Photosystem II (PSII), 13 286. See also PSII entries... 

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

These two reaction centers in plants act in tandem to catalyze the light-driven movement of electrons from HaO to NADP+ (Fig. 19-49). Electrons are carried between the two photosystems by the soluble protein plastocyanin, a one-electron carrier functionally similar to cytochrome c of mitochondria. To replace the electrons that move from PSII through PSI to NADP+, cyanobacteria and plants oxidize H20 (as green sulfur... 

Kargul, J., Nield, J., Barber, J. (2003) Three -dimensional reconstruction of a light-harvesting Complex I-Photosystem I (LHCI-PSI) supercomplex from the green alga Chlamydomonas rein-hardtii., J. Biol. Chem. 278, 16,135-16,141. 

The structure of the core antenna of Photosystem I (PSI) stands in striking contrast to the bacterial system. The 96 non-equivalent chlorophyll (Chi) molecules are very densely... 

Fig. 1. Space filling in Photosystem I illustrated from three points of view above file membrane plane (top), side view (bottom) and in between (middle). The left hand column shows the Chls in PSI with the special pair highlighted in black. The middle column contains all of the atoms in the structure with the Chls in black. The right hand column depicts the Chls in the licorice representation.

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