Aerobic glycolysis organisms

Aerobic glycolysis first involves a ten-step conversion of glucose to pyruvic acid or pyruvate, called the Embden-Meyerhoff-Pamas pathway, followed by its further conversion to carbon dioxide and water via what is variously called the tricarboxylic acid cycle, or citric acid cycle, or Krebs cycle after its discoverer. The net products discharged from the cycle are carbon dioxide and water, with recycle of a further product called oxaloacetic acid or oxaloacetate. Successive organic acids that contain three carboxyl groups (-COOH), are initially involved in the cycle starting with citric acid or a neutral salt of citric acid (citrate). Hence the designator tricarboxylic. 

The glycolytic pathway, or glycolysis, is a metabolic sequence in which glucose is broken down to pyruvic acid. The subsequent fate of pyruvate then depends upon whether or not the organism is aerobic or anaerobic Under aerobic conditions, pyruvate is oxidized via oxidative phosphorylation under anaerobic conditions, pyruvate is converted further into compounds such as lactate or ethanol, depending upon the organism. 

Glycolysis is a catabolic pathway in the cytoplasm that is found in almost all organisms— irrespective of whether they live aerobically or anaerobically. The balance of glycolysis is simple glucose is broken down into two molecules of pyruvate, and in addition two molecules of ATP and two of NADH+H"" are formed. 

Fig. 3.2 Biological abstraction. Yeast cells reflect anaerobic, reductive metabolism (intestine) as well as aerobic, oxidative metabolism (liver), if glycolysis is regarded as the most active pathway. Therefore, the yeast Saccharomyces cerevisiae is a good model organism for studies of xenobiotic metabolism.

The characteristic feature of carbohydrate breakdown in cestodes is the production of a range of complex end-products, usually organic acids, even under aerobic conditions (Table 5.4). This contrasts with predominantly aerobic organisms, such as most free-living metazoa, where the end-product of glycolysis is almost exclusively lactic acid formed from pyruvic acid. Lactic acid is produced as a result of rapid muscular contraction carried out essentially under anaerobiosis and its production ensures a rapid expenditure of energy without the limitation due to the rate of diffusion of oxygen. The anaerobic phase is followed by an aerobic phase, where pyruvic acid is metabolised to acetyl-coenzyme A which is in turn oxidised completely to... 

The first metabolic pathway that we encounter is glycolysis, an ancient pathway employed by a host of organisms. Glycolysis is the sequence of reactions that metabolizes one molecule of glucose to two molecules ofpyruvate with the concomitant net production of two molecules of ATP. This process is anaerobic (i.e., it does not require O2) inasmuch as it evolved before the accumulation of substantial amounts of oxygen in the atmosphere. Pyruvate can be further processed anaerobically (fermented) to lactate (lactic acidfermentation) or ethanol (alcoholic fermentation). Under aerobic conditions, pyruvate can be completely oxidized to CO2, generating much more ATP, as will be discussed in Chapters 17 and 18. 

The electron acceptor in the oxidation of glyceraldehyde 3-phosphate is NAD+, which must be regenerated for glycolysis to continue. In aerobic organisms, the NADH formed in glycolysis transfers its electrons to O2 through the electron-... 

The formation of acetyl CoA from carbohydrates is less direct than from fat. Recall that carbohydrates, most notably glucose, are processed by glycolysis into pyruvate (Chapter 16). Under anaerobic conditions, the pyruvate is converted into lactic acid or ethanol, depending on the organism. Under aerobic conditions, the pyruvate is transported into mitochondria in exchange for OH by the pyruvate carrier, an antiporter (Section 13.4). In the mitochondrial matrix, pyruvate is oxidatively decarboxylated by the pyruvate dehydrogenase complex to form acetyl CoA. 

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