Photophosphorylation and

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

Sharkey, T.D. Badger, M.R. (1982). Effects of water stress on photosynthetic electron transport, photophosphorylation and metabolite levels of Xanthium strumarium mesophyll cells. Planta, 156, 199-206. 

Besides these external processes, formation of ROS may also take place intrac-ellularly. Photooxidative stress, including UVB, stimulates various cellular processes leading to the production of superoxide radicals and hydrogen peroxide, as well as singlet-oxygen and hydroxyl radicals. The sources and production sites of ROS are mainly related to photosynthetic activities such as the pseudocyclic photophosphorylation and the Mehler reaction, which stimulate the accumulation of hydrogen peroxide (Asada 1994 Elstner 1990). 

In the dark, the production of ATP and NADPH by photophosphorylation, and the incorporation of C02 into triose phosphate (by the so-called dark reactions), cease. The dark reactions of photosynthesis were so named to distinguish them from the primary light-driven reactions of electron transfer to NADP+ and synthesis of ATP, described in Chapter 19. They do not, in... 

Formation of ATP by photosynthetic systems is often called photophosphorylation. Although photophosphorylation and oxidative phosphorylation are very similar, they do differ in a few details. The respiratory chain pumps protons... 

Focusing on the mechanisms of action of BOA into the plant cell, Barnes et al.7 suggested that the chlorotic seedlings observed in the presence of BOA and DIBOA could be the consequence of a benzoxazinone effect on the photophosphorylation and electron transport into the plant metabolism. In this way, Niemeyer et al.28 studied the effects of BOA on energy-linked reactions in mitochondria and reported an inhibition of the electron transfer between flavin and ubiquinone in Complex I, with complete inhibition of electron transport from NADH to oxygen in SMP. They could also detect an inhibition of BOA on ATP synthesis by acting directly on the ATPase complex at the F1 moiety. 

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. 

The electrons are activated by a light energy-capturing photosystem. These cells produce their ATP by photophosphorylation and their NADPH from H2S oxidation. Because H20 is not split, 02 is not evolved (photosystem II is absent). 

ETC, which conveys electrons from reduced coenzymes to the terminal electron acceptor 02. The exergonic downhill flow of electrons to 02 is coupled to the endergonic synthesis of ATP from ADP and P , a process analogous to photophosphorylation and called oxidative phosphorylation . The overall yield of ATP from this process of glycolysis, the mitochondrial TGA cycle and mitochondrial oxidative phosphorylation is about 38 ATP per glucose oxidized. 

CFi was originally isolated as a coupling factor, that is, a protein which when removed from the thylakoid membrane leaves a membrane unable to catalyse photophosphorylation, and which when reconstituted into it restores this ability. The... 

JVeed killer 1. Dichlorophenyldimethylurea (DCMU), a herbicide, interferes with photophosphorylation and O2 evolution. However, it does not block O2 evolution in the presence of an artificial electron acceptor such as ferricyanide. Propose a site for the inhibitory action of DCMU. 

The halftime for the PPj-induced changes is around 0.4 s which is one-tenth of the half-time when ATP is used as the energy donor [4]. The extent of the changes obtained with PPj is usually about two to three times that obtained with ATP. The effects of PPj and ATP are additive [2], The reoxidation of h-type cytochrome starts when the concentration of PP falls below 25 juM [4], Uncouplers of photophosphorylation and electron transport inhibitors abolish both the PPj and the ATP-in-duced reactions. In line with the results with PP synthesis and hydrolysis, the PPj-induced reactions are not inhibited by oligomycin. 

PS I appears to promote cyclic photophosphorylation and proceeds best in light of wavelengths greater than 700 nm, whereas PS II promotes noncyclic photophosphorylation and proceeds best in light of wavelengths shorter than... 

The extent of coupling between photophosphorylation and electron transport in chloroplasts is usually expressed by the ratio of ATP formed per pair of electrons transferred, written as ATP/Cj or P/Cj. This parameter expresses the amount of ATP formed divided by the number of pairs of electrons transferred through the electron-transfer chain. The P/ej ratio for phosphorylation coupled to the transfer of electrons from water to photosystem I can be computed by taking the HVe ratio of 2 (4 protons per electron-pair transferred) and the HVATP ratio of 3 (three protons required to flow through CFo F to produce one ATP), to obtain the P/c2 value of 1.33. 

Weed killer 1. Dichlorophenyldimethylurea (T)C2VIU), a ner-bicide, inlerteres with photophosphorylation and 0. evolution. 

Cespedes, C. L. Achnine, L. Lotina-Heimsen, B. Salazar, J. R. Gomez-Garibay, F. Calderon, J. S., Inhibition of photophosphorylation and electron transport by flavonoids and biflavonoids from endemic Tephrosia spp. of Mexico, Pestic. Biochem. Physiol, 2001, 69, Gi-16. 

The electron transport inhibitors do not directly affect photophosphorylation or interfere with mitochondrial electron transport and phosphorylation ( 1), However, the inhibitory uncouplers, in addition to interfering with electron transport in thylakoids, uncouple photophosphorylation and oxidative phosphorylation, and inhibit mitochondrial electron transport. 

Contrast the formation of ATP by cyclic photophosphorylation and by oxidative phosphorylation. 

TABLE 1. Photophosphorylation and light-induced pH changes in chromatophores from various purple bacteria as a function of chromatophore treatment. 

Ajrfj+ = A P + Z pH where Z = 2.3 RT/ F The proton electrochemical potential (A Xjj+) difference across the coupled (energetically linked) plasma membrane in phototrophic bacteria plays an essential role in photophosphorylation and solute transport into bacterial cells. Therefore, the exact measurements of these quantities are required for studies of the mechanism of energy transduction. It was, for example, shown that R.ruhrum chrom-atophores, associated with a phospholipid-impregnated filter,... 

The above results demonstrate that the fluorescence decrease upon addition of uncouplers or inhibitors of the mitochondrial oxidases can be attributed to a transition to state II it occurs with similar kinetics, it is accompanied by a decrease in the ratio of F685nro/F7i5nnj fluorescence emissions at 77K and an increased phosphorylation of the LHC subunits and it is not observed in cytbg/f mutants which are blocked in state I. The mechanism by which ATP controls the state transitions remains to be elucidated. We conclude that ATP deprivation causes a reorganization of the photosynthetic apparatus suitable for an increase in PSI-mediated cyclic photophosphorylation and favours ATP synthesis versus NADPH production... 

Losada et al. (1961) reported the separation of the two light reactions in noncyclic photophosphorylation and NADP+ reduction in green plants. They blocked System 2, which oxidizes water, by adding the inhibitor DCMU and then adding a dye as an electron donor. Under these conditions they were able to demonstrate the photochemical reduction of NADP+ and the simultaneous noncyclic photophosphorylation. 

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