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an·aer·o·bic res·pi·ra·tion(an'ār-ō'bik res'pir-ā'shŭn)
anaerobic respirationa type of cell respiration that takes place in ANAEROBES, and in which energy is released from glucose and other foods without the presence of oxygen. The reactions fall into two stages:
Stage 1: GLYCOLYSIS, in which glucose is converted to two molecules of pyruvic acid (pyruvate) in the general cell cytoplasm. The same reactions occur as in AEROBIC RESPIRATION, but in anaerobes the absence of oxygen prevents the two resulting molecules of reduced NAD from being oxidized via the ELECTRON TRANSPORT SYSTEM (ETS) in the MITOCHONDRIA. Instead, ATP is produced from ADP by SUBSTRATE-LEVEL PHOSPHORYLATION. Thus the net output of ATP in anaerobic respiration is 2 molecules (4 minus 2 used in the initial phosphorylation).
Stage 2: once pyruvate has been produced, two alternative pathways can occur. In plants and many microorganisms the pyruvate is broken down to ethanol via ethanal (acetaldehyde) in a process called ALCOHOLIC FERMENTATION, which requires hydrogen from NADH (see Fig. 34 ). In animals the pyruvate is changed via a single step into LACTIC ACID, a process called lactic-acid fermentation, which again requires hydrogen from NADH.
The role of NADH in both fermentations should be noted. Since the amount of NAD present in the cell is limited, glycolysis would quickly come to a halt if anaerobic respiration stopped at pyruvate. By going on to ethanol or lactic acid, NAD is freed in the fermentations to return to glycolysis and thus allow glucose CATABOLISM to continue. The yield of ATP in anaerobic respiration is poor because, firstly, the ETS cannot be used without oxygen, and, secondly, the end products still contain large amounts of energy. As a result, the free energy released and stored is only a fraction of the amount in the complete oxidation of glucose, as is shown in Fig. 35.