Photosystems PSI and PSII

Eukaryotic photosynthesizing cells possess two photosystems, PSI and PSII, which are connected in series in a mechanism referred to as the Z scheme. The water-oxidizing clock component of PSII generates 02. The protons are used in the synthesis of ATP in a chemiosmotic mechanism. PSI is responsible for the synthesis of NADPH. 

Plants contain two photosystems, PSI and PSII, which have different functions and are physically separated in the thylakoid membrane. PSII splits H2O into O2. PSI reduces NADP to NADPH. Cyanobacteria have two analogous photosystems. 

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,... 

The capacity of photosystem particles to bind to Ti02 particles and retain their photosynthetic activities was studied before immobilizing the particles on to the electrodes. Both PSI and PSII were able to bind Ti02 powder with retention of 80% PSI activity and 50% PSII activity. 

Fig. 2. Energy levels of the charge-separted states of Photosystems I and II (PSI and PSII).

Figure 1. The major transmembrane photosynthetic reaction centers (RC) (top) and respiratory complexes (bottom) are composed of light (zigzag) activated chains (dark gray) of redox centers (open polygons) that create a transmembrane electric field and move protons (double arrows) to create a transmembrane proton gradient, fulfilling the requirements of Mitchell s chemiosmotic hypothesis. Diffusing substrates include ubiquinone (hexagon) and other sources of oxidants and reductants. PSI and PSII, photosystems I and II, respectively.

As in the reaction center of green and purple bacteria, each chloroplast photosystem contains a pair of specialized reaction-center chlorophyll a molecules, which are capable of Initiating photoelectron transport. The reaction-center chlorophylls in PSI and PSII differ in their light-absorption maxima because of differences in their protein environment. For this reason, these chlorophylls are often denoted Peso... 

FIGURE 9.1 Plant-type ferredoxin (Fd) isoforms with diverse redox potential could donate or receive electrons involved in many metabolic processes (modified from the model of chlo-roplast described in Tognetti et al. 2006 and Hanke et al. 2004). PSI and PSII, photosystem I and II, respectively FNR, Fd NADP reductase FTR, Fd thioredoxin reductase Trx, thiore-doxin PGR5, proton gradient regulation protein 2-Cys Prx, 2-Cys peroxiredoxin PQ, plas-toquinone PC, plastocyanin. 

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