Photosynthesis photophosphorylation

Cyclic photophosphorylation In photosynthesis, Photophosphorylation (light-dependent ATP synthesis) that is linked to a cyclic flow of electrons firom photosystem 11 down an electron transport chain and back to photosystem II it is not coupled to the oxidation of H2O or to the reduction of NADP. Compare non-cyclic photophosphorylation. 

Noncyclic Photophosphorylation In photosynthesis. Photophosphorylation (light-dependent ATP synthesis)... 

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

The photoreductive synthetic process that promotes the assimilation of carbon dioxide into carbohydrates, other reduced metabolites, as well as ATP (synthesis of the latter is termed photophosphorylation). Photosynthesis is the primary mechanism for transducing solar energy into biomass, and green plants utilize chlorophyll a to capture a broad spectrum of solar radiant energy reaching the Earth s surface. Photosynthetic bacteria typically produce NADPH, the reductive energy of which is converted to ATP. 

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

ATP synthesis is not the only energy-conserving reaction of photosynthesis in plants the NADPH formed in the final electron transfer is (like its close analog NADH) also energetically rich. The overall equation for noncyclic photophosphorylation (a term explained below) is... 

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... 

In the overall scheme of the photosynthesis of green plants the electron transport cycle starts with the excitation of chlorophyll a in photosystem 2. The excited electron then follows a downward electron acceptor chain which eventually reaches the chlorophyll a of photosystem 1 (P700) in which it can fill the positive hole left by electronic excitation. The energy released in the electron transport chain which links photosystems 2 and 1 is used for other biochemical processes which are thereby related to photosynthesis. One of these is the process of photophosphorylation which is the production of molecules with phosphate chains used as energy transfer agents in many biochemical reactions. 

Figure 20-7 Simplified representation of the photoreactions in photosynthesis. The oxidation of water is linked to the reduction of NADP by an electron-transport chain (dashed line) that is coupled to ATP formation (photophosphorylation).

The oxygen formed clearly comes from H20 and not from C02, because photosynthesis in the presence of water labeled with lgO produces oxygen labeled with 180, whereas carbon dioxide labeled with 180 does not give oxygen labeled with 180. Notice that the oxidation of the water produces two electrons, and that the formation of NADPH from NADP requires two electrons. These reactions occur at different locations within the chloroplasts and in the process of transferring electrons from the water oxidation site to the NADP reduction site, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP see Section 15-5F for discussion of the importance of such phosphorylations). Thus electron transport between the two photoprocesses is coupled to phosphorylation. This process is called photophosphorylation (Figure 20-7). 

Oxidative phosphorylation resembles photophosphorylation, discussed in Section 20-9, in that electron transport in photosynthesis also is coupled with ATP formation. 

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. 

Photosynthesis can be affected in many ways. Metals can influence biosynthesis of biomembranes and photosynthetic pigments, especially chlorophyll. They may inactivate enzymes by oxidising SH-groups necessary for catalytic activity or by substitution for other divalent cations in metalloenzymes. They finally can also interact with the photosynthetic electron transport and with the related photophosphorylation. 

In contrast to the previous results, Weigel (1985 a, b) reported that in mesophyll protoplasts of Valerianella locusta and in intact chloroplasts of Spinacea oleraceae cadmium affects photosynthesis by inhibition of several reaction steps of the Calvin cycle and not by interaction with the electron transport or photophosphorylation (cf. section on photosynthetic C02 fixation). 

The photochemical reaction of photosynthesis involves the removal of an electron from an excited state of the special chlorophyll that acts as an excitation trap. The movement of the electron from this trap chi to an acceptor begins a series of electron transfers that can ultimately lead to the reduction of NADP+. The oxidized trap chi, which has lost an electron, can accept another electron from some donor, as in the steps leading to O2 evolution. Coupled to the electron transfer reactions in chloroplasts is the formation of ATP, a process known as photophosphorylation. In this section we will consider some of the components of chloroplasts involved in accepting and donating electrons a discussion of the energetics of such processes will follow in Chapter 6 (Section 6.3). 

ATP formation coupled to electron flow in mitochondria is usually called oxidative phosphorylation. Because electron flow involves both reduction and oxidation, more appropriate names are respiratory phosphorylation and respiratory-chain phosphorylation, terminology that is also more consistent with photophosphorylation for ATP formation in photosynthesis. As with photophosphorylation, the mechanism of oxidative phosphorylation is not yet fully understood in molecular terms. Processes like phosphorylation accompanying electron flow are intimately connected with membrane structure, so they are much more difficult to study than are the biochemical reactions taking place in solution. A chemiosmotic coupling mechanism between electron flow and ATP formation in mitochondria is generally accepted, and we will discuss some of its characteristics next. 

The result of such a process would be that two electrons are cycled twice through the PQ, and the ratio of H /e between PS II and PS I would be higher than one. This, if definitively confirmed, would be of great importance from the point of view of understanding the coupling of electron transport to the synthesis of ATP, and of the quantum yield of photosynthesis (see discussion under photophosphorylation). 

Govindjee and Wydrzynski, T. (1981) in Photosynthesis II. Electron Transport and Photophosphorylation (Akoyunoglou, G., ed.) pp. 293-303, Balaban Inti. Science Service, Philadelphia, PA. 

The diphenyl ether herbicides are active only in the presence of light and canse chlorosis of leaf tissue. They inhibit the Hill reaction in photosynthesis and photophosphorylation. However, the primary mode of action probably involves the photosynthetic rednction to form radicals, which initiate destructive reactions in lipid membranes leading to cell leakage. 

The mode of action of the substituted ureas is relatively well known. It results in an inhibition of photosynthesis by blocking photosynthetic electron transport and photophosphorylation. 

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