Photosynthetic electron flow

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

Figure 12.13 Photosynthetic pigments are used hy plants and photosynthetic bacteria to capture photons of light and for electron flow from one side of a membrane to the other side. The diagram shows two such pigments that are present in bacterial reaction centers, bacteriochlorophyll (a) and ubiquinone (b). The light-absorbing parts of the molecules are shown in yellow, attached to hydrocarbon "tails" shown in green.

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... 

The transfer of phosphoryl groups is a central feature of metabolism. Equally important is another kind of transfer, electron transfer in oxidation-reduction reactions. These reactions involve the loss of electrons by one chemical species, which is thereby oxidized, and the gain of electrons by another, which is reduced. The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all work done by living organisms. In nonphotosynthetic organisms, the sources of electrons are reduced compounds (foods) in photosynthetic organisms, the initial electron donor is a chemical species excited by the absorption of light. The path of electron flow in metabolism is complex. Electrons move from various metabolic intermediates to specialized electron carriers in enzyme-catalyzed reactions. 

Compare the oxidative photosynthetic carbon cycle (C2 cycle), also called photorespiration, with the mitochondrial respiration that drives ATP synthesis. Why are both processes referred to as respiration Where in the cell do they occur, and under what circumstances What is the path of electron flow in each ... 

The photosynthetic process, showing coupling of electron transport and ADP phosphorylation. The dashed line shows electron flow in cyclic photophosphorylation. See text for details. 

Trebst, A. (1979). Inhibition of photosynthetic electron flow by phenol and diphenyl ether herbicides in control and trypsin-treated chloroplasts. Z. Naturforsch. Sect. C Biosci., 34 986-991. 

Although photophosphorylation was reported to be very sensitive to metals in vitro, the reduction of photosynthetic ATP production was shown to be related to an inhibition of the electron flow rate in Euglena gracilis treated with toxic concentrations of zinc, cadmium and mercury (De Filippis et al., 1981 b), in cadmium-treated Lycoper-sicon esculentum (Bazinsky et al., 1980) and in zinc-treated Phaseolus vulgaris (Van Assche and Clijsters, 1986a). 

Fig. 2. Diagrammatic representation of the energy —transducing steps involved in the photosynthetic noncyclic electron flow.

Effect of Monochromatic Light on Electron Flow The extent to which an electron carrier is oxidized or reduced during photosynthetic electron transfer can sometimes be observed directly with a spectrophotometer. When chloroplasts are illuminated with 700 run light, cytochrome plastocyanin, and plastoquinone are oxidized. When chloroplasts are illuminated with 680 nm light, however, these electron carriers are reduced. Explain. 

Dyson speculated on the properties of a highly efficient, self-repairing, self-replicating, photosynthetic. machine for space colonization - and could not get past plants]. The photosynthetic ETC involves two ETC-linked light absorbing photosystems (photosystems I and II) and is described by the so-called Z scheme. Electron flow in downhill noncyclic and cyclic sections of the chain is coupled to ATP synthesis by noncyclic and cyclic photophosphorylation, respectively. 

Herbicides that inhibit photosynthetic electron flow prevent reduction of plastoquinone by the photosystem II acceptor complex. The properties of the photosystem II herbicide receptor proteins have been investigated by binding and displacement studies with radiolabeled herbicides. The herbicide receptor proteins have been identified with herbicide-derived photoaffinity labels. Herbicides, similar in their mode of action to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) bind to a 34 kDa protein, whereas phenolic herbicides bind to the 43-51 kDa photosystem II reaction center proteins. At these receptor proteins, plastoquinone/herbicide interactions and plastoquinone binding sites have been studied, the latter by means of a plastoquinone-deriv-ed photoaffinity label. For the 34 kDa herbicide binding protein, whose amino acid sequence is known, herbicide and plastoquinone binding are discussed at the molecular level. 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

D. An uncoupler is a compound that decreases the ATP formation coupled to photosynthetic electron flow. When such a compound is added to chloroplasts incubated at a high photon flux density, the O2 evolution rate eventually becomes less than a control without the uncoupler. Explain. 

Figure 6-4. Energy aspects of photosynthetic electron flow. The lengths of the arrows emanating from the trap chi s of Photosystems I and II represent the increases in chemical potential of the electrons that occur upon absorption of red light near the Xmax s of the trap Chip s. The diagram shows the various midpoint redox potentials of the couples involved (data from Table 5-3) and the three types of election flow mediated by ferredoxin. Spontaneous electron flow occurs toward couples with higher (more positive) redox potentials, which is downward in the figure.

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