Carbon dioxide photosynthetic reduction

Significant differences in net photosynthetic assimilation of carbon dioxide are apparent between C, C, and CAM biomass species. One of the principal reasons for the generally lower yields of C biomass is its higher rate of photorespiration if the photorespiration rate could be reduced, the net yield of biomass would increase. Considerable research is in progress (ca 1992) to achieve this rate reduction by chemical and genetic methods, but as yet, only limited yield improvements have been made. Such an achievement with C biomass would be expected to be very beneficial for foodstuff production and biomass energy appHcations. 

However, process (6.5.5) cannot be a universal photosynthetic process because H2S is unstable and is not available in sufficient quantities in nature. Water is the only substance that can be used in the reduction of carbon dioxide whose presence in nature is independent of biological processes. 

From a fundamental viewpoint, carbon dioxide reduction is a model reaction which can help us to understand better the mechanism of natural photosynthesis.11 Development of artificial photosynthetic systems, by mimicking functions of green plants, is one of... 

The photoassisted reduction of aqueous carbon dioxide in the presence of inorganic minerals has been examined as a model of prebiological photosynthesis,120 a potential precursor to the photosynthetic fixation of C02 by plants. 

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. 

Four Steps Involved in Oxidation of Water The photosynthetic process in green plants consists of splitting the elements of water, followed by reduction of carbon dioxide. 

An analogous use of ATP is found in photosynthetic reduction of carbon dioxide in which ATP phos-phorylates ribulose 5-P to ribulose bisphosphate and the phosphate groups are removed later by phosphatase action on fructose bisphosphate and sedoheptulose bisphosphate (Section J,2). Phosphatases involved in synthetic pathways usually have a high substrate specificity and are to be distinguished from nonspecific phosphatases which are essentially digestive enzymes (Chapter 12). 

These are involved in a wide range of electron-transfer processes and in certain oxidation-reduction enzymes, whose function is central to such important processes as the nitrogen cycle, photosynthesis, electron transfer in mitochondria and carbon dioxide fixation. The iron-sulfur proteins display a wide range of redox potentials, from +350 mV in photosynthetic bacteria to —600 mV in chloroplasts. 

Table 1 summarizes several redox transformations that can be accomplished in artificial photosynthetic assemblies including the photolysis of water, carbon dioxide reduction, and nitrogen fixation processes. The endoergicities of these transformations, and the number of electrons involved in the reduction processes, are also indicated in the table. It is evident that the energy per electron to drive the various transformations are met by visible light quanta. 

From the point of view of organic synthesis, the overall process consists of the formation of carbohydrates (CH20) by the reduction of carbon dioxide. The essence of the process is the use of photochemical energy to split water and concomitantly to reduce C02. Many proteins and small molecules are involved in photosynthetic machinery. Inorganic species are in the centre of photosynthesis as pigments in light harvesting, substrates, products, catalysts, and electron transfer mediators. 

Khan MMT, Rao NN, Chatterjee D. A novel photosynthetic mimic reaction catalysed by K[Ru(H-EDTA)C1]-2H20 reduction of carbon dioxide to formate and formaldehyde in the presence of an aqueous suspension of Pt-CdS-Ru02. / Photochem Photobiol A Chem 1991 60 311-18. 

Small carbon-containing molecules such as atmospheric CO2 are considered to be important renewable feedstocks (144,145). In the context of mankind s increasing demand for carbon-based materials, food, and liquid fuels, the photocatalytic reduction of carbon dioxide under solar light irradiation is an attractive option. Such types of artificial photosynthetic processes could greatly enlarge the possibilities of abiotic CO2 recycling. 

Transport of iron in carbonate waters, mainly in the form of Fe " bicarbonate, is more common. The decrease in COj due to the overall reduction in pressure when ground waters come to the surface, when carbon dioxide is consumed as a result of photosynthetic activity of plants or even, as Mokiyevskaya (1959) mentions, when the temperature rises, leads to deposition of FeCOj. In Strakhov s opinion such a process could lead to the formation of oolitic hydrogoethite-chamosite-siderite ores. The iron migrated in mobile form as Fe, which accumulated in solution in a reducing environment. Formation of the ores was related to the draining of high-iron waters formed in swampy regions. The near-shore parts of the sea with... 

In summary, current evidence [39-41] is thus consistent with the view that the ferredoxin/thioredoxin system functions in photosynthetically diverse types of plants as a master switch to restrict the activity of degradatory enzymes and activate biosynthetic enzymes in the light. It is significant that enzymes controlled by the ferredoxin/thioredoxin system (FBPase, SBPase, NADP-G3PDH, and PRK) function in the regenerative phase of the reductive pentose phosphate cycle that is needed to sustain its continued operation - i.e, to regenerate the carbon dioxide acceptor, Rbu-1,5-P2, from newly formed 3-PGA. It seems likely that one of these thioredoxin-linked enzymes limits the regeneration of Rbu-1,5-P2. 

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