Photosystem I, reduced

Photosystem I Reduces NADP+ by Way of Iron-Sulfur Proteins... 

Photooxidation of P700 in photosystem I reduces a chlorophyll, which transfers electrons to a series of membrane-bound iron-sulfur centers, probably by way of a quinone. From the iron-sulfur centers, electrons move to the soluble iron-sulfur protein, ferre-... 

Photosystem I reduces NADP to NADPH. Its reaction centre absorbs maximally at 700 run, and it is found mostly on intergrana. 

How does photosystem I reduce NADP The electrons generated by photosystem II reduce NADP+ to NADPH in the reactions of photosystem II. 

The electrons undergo the equivalent of a partial oxidation process ia a dark reaction to a positive potential of +0.4 V, and Photosystem I then raises the potential of the electrons to as high as —0.7 V. Under normal photosynthesis conditions, these electrons reduce tryphosphopyridine-nucleotide (TPN) to TPNH, which reduces carbon dioxide to organic plant material. In the biophotolysis of water, these electrons are diverted from carbon dioxide to a microbial hydrogenase for reduction of protons to hydrogen ... 

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,... 

In photosystem I, absorption of a photon leads to an excited state that functions as a reducing agent. The electrons are passed from one species to another with several intermediate species that include ferrodoxin (a protein containing iron and sulfur) before finally reducing C02. In photosystem II, electrons are transferred to a series of intermediates, of which a cytochrome bf complex is one entity. Ultimately, the transfer of electrons leads to the reaction... 

Cyclic photophosphorylation is also a highly energetic reaction. The bipyridyliums, paraquat and diquat (Figure 2.2), divert the electron flow of cyclic photophosphorylation (photosystem I). The capture of an electron from the chlorophyll reduces the herbicide and the reduced herbicide reacts with oxygen to form superoxide. Superoxide produces hydrogen peroxide within the chloroplast and these two compounds interact to form hydroxyl radicals in the presence of an iron catalyst. Hydroxyl radicals are very damaging and lead to the destruction of the cellular components leading to rapid plant death. 

Plant photosystem I passes electrons from its excited reaction center, P700, through a series of carriers to ferredoxin, which then reduces NADP+ to NADPH. 

Boosting the Reducing Power of Photosystem I by Light Absorption When photosystem I absorbs red light at 700 nm, the standard reduction potential of P700 changes... 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

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