Photophosphorylation cyclic

Proton translocations accompany these cyclic electron transfer events, so ATP synthesis can be achieved. In cyclic photophosphorylation, ATP is the sole product of energy conversion. No NADPFI is generated, and, because PSII is not involved, no oxygen is evolved. The maximal rate of cyclic photophosphorylation is less than 5% of the rate of noncyclic photophosphorylation. Cyclic photophosphorylation depends only on PSI. 

FIGURE 22.22 The pathway of cyclic photophosphorylation by PSI. (Adapted from Arnon, D. 1984. Trends in Biochemical Sciences 9 258.)... 

If noncyclic photosynthetic electron transport leads to the translocation of 3 H /e and cyclic photosynthetic electron transport leads to the translocation of 2 H /A, what is the relative photosynthetic efficiency of ATP synthesis (expressed as the number of photons absorbed per ATP synthesized) for noncyclic versus cyclic photophosphorylation (Assume that the CFiCEq ATP synthase yields 1 ATP/3 H. )... 

Bendall, D. S., and Manasse, R. S., 1995. Cyclic photophosphorylation and electron transport. Biochimica et Biophysica Acta 1229 23-38. 

Fig. 5.2. The photosynthetic membrane of a green sulfur bacterium. The light-activated bacte-riochlorophyll molecule sends an electron through the electron-transport chain (as in respiration) creating a proton gradient and ATP synthesis. The electron eventually returns to the bacteri-ochlorophyll (cyclic photophosphorylation). If electrons are needed for C02 reduction (via reduction of NADP+), an external electron donor is required (sulfide that is oxidised to elemental sulfur). Note the use of Mg and Fe.

It can be seen from the normal potentials E° (see p. 18) of the most important redox systems involved in the light reactions why two excitation processes are needed in order to transfer electrons from H2O to NADP"". After excitation in PS II, E° rises from around -IV back to positive values in plastocyanin (PC)—i. e., the energy of the electrons has to be increased again in PS I. If there is no NADP" available, photosynthetic electron transport can still be used for ATP synthesis. During cyclic photophosphorylation, electrons return from ferredoxin (Fd) via the plastoquinone pool to the b/f complex. This type of electron transport does not produce any NADPH, but does lead to the formation of an gradient and thus to ATP synthesis. 

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. 

Table 2.1 Herbicides that interupt electron flow in -cyclic photophosphorylation (photosystem II)...

By regulating the partitioning of electrons between NADP+ reduction and cyclic photophosphorylation, a plant adjusts the ratio of ATP to NADPH produced in the light-dependent reactions to match its needs for these products in the carbon-assimilation reactions and other biosynthetic processes. As we shall see in Chapter 20, the carbon-assimilation reactions require ATP and NADPH in the ratio 3 2. 

Function of Cyclic Photophosphorylation When the [NADPH]/[NADP+] ratio in chloroplasts is high, photophosphorylation is predominantly cyclic (see Fig. 19 49). Is 02 evolved during cyclic photophosphorylation Is NADPH produced Explain. What is the main function of cyclic photophosphorylation ... 

In contrast, the reaction centers of green sulfur bacteria resemble PSI of chloroplasts. Their reaction centers also receive electrons from a reduced quinone via a cytochrome be complex.245 However, the reduced form of the reaction center bacteriochlorophyll donates electrons to iron-sulfur proteins as in PSI (Fig. 23-17). The latter can reduce a quinone to provide cyclic photophosphorylation. Cyanobacteria have a photosynthetic apparatus very similar to that of green algae and higher plants. 

Cyclic photophosphorylation in purple bacteria. QH2 is eventually dehydrogenated in the cytochrome bc1 complex, and the electrons can be returned to the reaction center by the small soluble cytochrome c2, where it reduces the bound tetraheme cytochrome or reacts directly with the special pair in Rhodobacter spheroides. The overall reaction provides for a cyclic photophosphorylation (Fig. 23-32) that pumps 3-4 H+ across the membrane into the periplasmic space utilizing the energy of the two photoexcited electrons. 

The pathways involved in cyclic photophosphorylation in chloroplasts are not yet established. Electrons probably flow from the Fe-S centers Fdx, Fda, or Fdb back to cytochrome b563 or to the PQ pool as is indicated by the dashed line in Fig. 23-18. Cyclic flow around PSII is also possible. The photophosphorylation of inorganic phosphate to pyrophosphate (PP ) occurs in the chromatophores (vesicles derived from fragments of infolded photosynthetic membranes) from Rho-dospirillum rubrum. The PP formed in this way may be used in a variety of energy-requiring reactions in these bacteria.399 An example is formation of NADH by reverse electron transport. 

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

Figure E9.1 illustrates the photosynthetic process as it occurs in higher plants. This is called noncyclic photophosphorylation to distinguish it from cyclic photophosphorylation in photosynthetic bacteria. Cyclic photophosphorylation requires only photosystem I and a second series of electron carriers to return electrons to the electron-deficient chlorophyll. The dashed line in Figure E9.1 indicates the flow of electrons in cyclic photophosphorylation. ATP is produced during the cyclic process just as in the noncyclic process, but NADPH is not.

In a study designed to determine the mode of action of atrazine in higher plants, Shimabukuro and Swanson (1969) concluded that atrazine inhibits the Hill reaction and its noncyclic phosphorylation, while being ineffective against cyclic photophosphorylation. Atrazine readily penetrated the chloroplast of resistant as well as susceptible plants. In tolerant plants such as sorghum, the metabolism of atrazine was postulated to occur outside the chloroplasts to form water-soluble and insoluble residues that reduced the concentration of photosynthetic inhibitors in the chloroplasts. 

Van Rensen, J.J.S. (1971). Action of some herbicides in photosynthesis of Scenedesmus, as studied by their effects on oxygen evolution and cyclic photophosphorylation. Wageningen H. Veenman. 80 p. 

When little NADP+ is available to accept electrons, an alternative electron transport pathway is used. The high-energy electron donated by photosystem I passes to ferredoxin, then the cytochrome bf complex, then plastocyanin and back to the P700 of photosystem I. The resulting proton gradient generated by the cytochrome bf complex drives ATP synthesis (cyclic photophosphorylation) but no NADPH is made and no 02 is produced. 

In-vitro approach Data are available in abundance concerning metal effects on isolated chloroplasts (for a review, see Clijsters and Van Assche, 1985). All the metals studied were found to be potential inhibitors of photosystem 2 (PS 2) photosystem 1 (PS 1) was reported to be less sensitive. From the in-vitro experiments, at least two potential metal-sensitive sites can be derived in the photosynthetic electron transport chain the water-splitting enzyme at the oxidising side of PS 2, and the NADPH-oxido-reductase (an enzyme with functional SH-groups) at the reducing side of PS 1 (Clijsters and Van Assche, 1985). Moreover, in vitro, non cyclic photophosphorylation was very sensitive to lead (Hampp et al., 1973 b) and mercury (Honeycutt and Korgmann, 1972). Both cyclic and non-cyclic photophosphorylation were proven to be inhibited by excess of copper (Uribe and Stark, 1982) and cadmium (Lucero et al, 1976). 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

The bipyridyliums support both noncyclic and cyclic photophosphorylation, are photoreduced by illuminated chloroplasts under anaerobic conditions, and inhibit the photoreduction of NADP. 

Gimenez-Gallego, G., del Valle-Tascon, S. and Ramifrez, J.M. 1978. Photooxidase system of Rhodospirillum rubrum II. Its role in the regulation of cyclic photophosphorylation. Z.Pflanzenphysiol., 87, 25-36. 

Answer Plants have two photosystems. Photosystem I absorbs light maximally at 700 nm and catalyzes cyclic photophosphorylation and NADP+ reduction (see Fig. 19-56). Photosystem II absorbs light maximally at 680 nm, splits H20 to 02 and H+, and donates electrons and H+ to PSI. Therefore, light of 680 nm is better in promoting 02 production, but maximum photosynthetic rates are observed only when plants are illuminated with light of both wavelengths. 

Thus, H20 is not split and 02 is not produced. In addition, NADPH is not formed because the electrons return to P700. The function of cyclic photophosphorylation is to produce ATP. 

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