Photosystem photosynthesis

Tetranuclear iron-sulfur clusters are key relay stations in the electron flow in photosynthesis. Photosystem I comprises three subunits, PsaA, PsaB and PsaC. The latter contains two [Fe4S4] centres FA and FB. The core subunits PsaA and B, respectively, house a [Fe4S4] centre denoted FX in addition to other, organic cofactors. The role of this latter cluster was probed in preparations partially devoid of PsaC. It was concluded from the results that FX has a major role in controlling the electron transport through PS I.236 Since the final acceptor of the electrons in PS I is a ferredoxin with a [Fe2S2] cluster it was of interest to study a... 

Melis A. (1989). Spectroscopic methods in photosynthesis photosystem stoichiometry and chlorophyll antenna size. Phil. Trans. R. Soc. Lortd. B 323, 397-409. 

Photosynthesis is a chemical reaction that converts sunlight into energy for living organisms. It s one of the most remarkable reduction-oxidation reactions scientists have observed (see Chapter 3 for details on this type of reaction). There are two currently known systems of photosynthesis Photosystem I (PSI) and Photosystem 11 (PSll). They each use the light in a series of reactions that pass on electrons. The essential difference is that PSI creates hydrogen sulfide (H2S) by reduction of CO2 and is only conducted by anaerobic bacteria and was more common in prehistoric times. 

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 ... 

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. 

A method of detecting herbicides is proposed the photosynthetic herbicides act by binding to Photosystem II (PS II), a multiunit chlorophyll-protein complex which plays a vital role in photosynthesis. The inhibition of PS II causes a reduced photoinduced production of hydrogen peroxide, which can be measured by a chemiluminescence reaction with luminol and the enzyme horseradish peroxidase (HRP). The sensing device proposed combines the production and detection of hydrogen peroxide in a single flow assay by combining all the individual steps in a compact, portable device that utilises micro-fluidic components. 

The existence of two separate but interacting photosystems in photosynthetic eukaryotes was demonstrated through analysis of the photochemical action spectrum of photosynthesis, in which the oxygen-evolving capacity as a function of light wavelength was determined (Figure 22.10). 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

Whereas the 2[4Fe-4S] ferredoxin may have been replaced by the [2Fe-2S] ferredoxins in oxygenic photosynthesis, another 2[4Fe-4S] protein, the so-called FA/FB-binding subunit (see Fig. 1), appears to be common to all RCI-type photosystems. 

Davis, M.S., Forman, A., and Fajer, J., Ligated chlorophyll cation radicals their function in photosystem II of plant photosynthesis, PNAS, 76, 4170, 1979. 

Unlike the photosynthetic apparatus of photosynthetic bacteria, that of cyanobacteria consits of two photosystems, PS I and II, connected by an electron transport chain. The only chlorophyll present is chlorophyll a, and, therefore, chlorophylls b—d are not of interest in this article. Chlorophyll a is the principal constituent of PS I. Twenty per cent of isolated pigment-protein complexes contain one P700 per 20—30 chlorophyll a molecules the other 80% contain only chlorophyll a20). The physical and chemical properties of chlorophyll a and its role in photosynthesis have recently been described by Meeks77), Mauzerall75), Hoch60), Butler10), and other authors of the Encyclopedia of Plant Physiology NS Vol. 5. 

Principle Chlorophyll fluorescence is a sensitive and early indicator of damage to photosynthesis and to the physiology of the plant resulting from the effect of allelochemicals, which directly or indirectly affects the function of photosystem II (Bolhar-Nordenkemf et ah, 1989, Krause and Weiss 1991). This approach is convenient for a photosynthesis analysis in situ and in vivo and quick detection of otherwise invisible leaf damage. The photosynthetic plant efficiency was measured using the method of induced chlorophyll fluorescence kinetics of photosystem II [Fo, non-variable fluorescence Fm, maximum fluorescence Fv=Fm-Fo, variable fluorescence t /2, half the time required to reach maximum fluorescence from Fo to Fm and photosynthetic efficiency Fv/Fm]. 

Atrazine enters plants primarily by way of the roots and secondarily by way of the foliage, passively translocated in the xylem with the transpiration stream, and accumulates in the apical meristems and leaves (Hull 1967 Forney 1980 Reed 1982 Wolf and Jackson 1982). The main phytotoxic effect is the inhibition of photosynthesis by blocking the electron transport during Hill reaction of photosystem II. This blockage leads to inhibitory effects on the synthesis of carbohydrate, a reduction in the carbon pool, and a buildup of carbon dioxide within the leaf, which subsequently causes closure of the stomates, thus inhibiting transpiration (Stevenson et al. 1982 Jachetta et al. 1986 Shabana 1987). 

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