Carbon metabolic processes

M. Gibbs and E. Lat2ko, eds.. Photosynthesis 11 Photosynthetic Carbon Metabolism and Kelated Processes, Tnyclopedia of Plant Physiology, N.S., Springer-Vedag, Berlin, 1979. 

Anaerobic respiration Metabolic process whereby electrons are transferred from an organic, or in some cases, inorganic compounds to an inorganic acceptor molecule other than oxygen. The most common acceptors are nitrate, sulfate, and carbonate. 

Carbon dioxide production Rate at which the pulmonary bloodstream transports carbon dioxide, produced by metabolic processes, to the pulmonary airstream. 

Carbon dioxide (CO2) The gas formed by complete combustion of carbon-containing substances. Also a product of the metabolic process. 

BOTH RIBOSE-5-P AND NADPH ARE NEEDED BY THE CELL In this case, the first four reactions of the pentose phosphate pathway predominate (Figure 23.37). N/VDPH is produced by the oxidative reactions of the pathway, and ribose-5-P is the principal product of carbon metabolism. As stated earlier, the net reaction for these processes is... 

Some aspects of the biochemistry of metabolic processes affecting nutrients appear to have significant consequences for the expected behavior of stable carbon isotopes as tracers of diet. Specifically, we have seen that the simple model of a total scrambling of carbon atoms during endogenous biosynthesis is inconsistent with the expected pathways of some nutrients, whereas other isotopic records in ancient human tissues can be adequately accounted for by this model. 

Complex metabolic processes carbon and amino acid starvation viral infection... 

The human body generates a steady flow of acidic by-products during its normal metabolic processes. Foremost among these is carbon dioxide, which is a major product of the reactions the body uses to produce energy (see Section 14-). An average person produces from 10 to 20 mol (440 to 880 g) of CO2 every day. Blood carries CO2 from the cells to the lungs to be exhaled. In aqueous solution, dissolved CO2 is in equilibrium with carbonic acid H2 O + CO2 H2 CO3... 

Once the product specifications have been fixed, some decisions need to be made regarding the reaction path. There are sometimes different paths to the same product. For example, suppose ethanol is to be manufactured. Ethylene could be used as a raw material and reacted with water to produce ethanol. An alternative would be to start with methanol as a raw material and react it with synthesis gas (a mixture of carbon monoxide and hydrogen) to produce the same product. These two paths employ chemical reactor technology. A third path could employ a biochemical reaction (or fermentation) that exploits the metabolic processes of microorganisms in a biochemical reactor. Ethanol could therefore also be manufactured by fermentation of a carbohydrate. 

The cells of the body require a continuous supply of oxygen to produce energy and carry out their metabolic functions. Furthermore, these aerobic metabolic processes produce carbon dioxide, which must be continuously eliminated. The primary functions of the respiratory system include ... 

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

For organic mass spectrometry the case of carbon is of much greater importance. Carbon is ubiquitous in metabolic processes and the most prominent example of variations in the isotopic ratio is presented by the different pathways... 

The carbon cycle, one of the most essential of all biological processes, involves the chemical conversion of carbon dioxide to carbohydrates in green plants by photosynthesis. Animals consume the carbohydrates and, through the metabolic process, reconvert the carbohydrates back into carbon dioxide, which is returned to the atmosphere to continue the cycle. 

The ultimate biodegradability of a substrate, such as is depicted in Figure 4, may, in addition to oxygen uptake, be measured by disappearance of organic carbon, CO2 evolution and the formation of water. A radiotracer approach provides a more accurate determination and is the only feasible way of measuring the formation of water in the aqueous medium required for all metabolic processes. 

Further investigation is needed to determine the fate of contaminants accumulated during phytoremediation. Evidence has shown that poplar trees degrade chlorinated solvents, such as trichloroethylene (TCE), through naturally occurring metabolic processes. The by-products, possibly carbon dioxide and chloride salts, may be stored in the tissues of the trees. It is not yet certain, however, the fate of many chemicals and metals. Contaminants that collect in the leaves may be released when the leaves drop, or may be eaten by animals and consequently bioaccumulated through the food chain. 

Oxidation Carbon-Sulfur Systems. The most common metabolic process that affects a C-S system is S-oxidation. The S atom is oxidized to a sulfoxide. In the case of thioketones, the C=S double bond is converted to a C=0 bond. For thioethers, oxidative S-dealkylation is a possibility. 

Carbon tetrachloride causes centrilobular liver necrosis and steatosis after acute exposure, and liver cirrhosis, liver tumors, and kidney damage after chronic administration. The mechanism underlying the acute toxicity to the liver involves metabolic activation by cytochrome P-450 to yield a free radical (trichloromethyl free radical). This reacts with unsaturated fatty acids in the membranes of organelles and leads to toxic products of lipid peroxidation including malondialdehyde and hydroxynonenal. This results in hepatocyte necrosis and inhibition of various metabolic processes including protein synthesis. The latter leads to steatosis as a result of inhibition of the synthesis of lipoproteins required for triglyceride export. 

In addition to deliberate enzyme-catalyzed processes, there are nonenzymatic processes that alter proteins. These include the degradative reactions described in Section 5 and also reversible reactions that may be physiologically important. For example, the N-terminal amino groups of peptides, and other amino groups of low p Ka can form carbamates with bicarbonate (Eq. 2-21 ).301-303 This provides an important mechanism of carbon dioxide transport in red blood cells (Chapter 7) and a way by which C02 pressure can control some metabolic processes. 

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