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No doubt, glycolysis can also function without O2, but further oxidation of pyruvic acid and NADH by mitochondria requires oxygen. Therefore, when O2 is limited, NADH and pyruvic acid begin to accumulate. Under this condition, plants carry out fermentation (anaerobic respiration), leading to the formation of CO2 and either ethanol or lactic acid (usually ethanol). During fermentation, the pyruvic acid releases one molecule of CO2 to produce acetaldehyde. The acetaldehyde, then reoxidises NADH, and is itself reduced to ethanol. These reactions are catalysed by the enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase. 

Aerobic Oxidation of Pyruvic acid

Pyruvic acid, generated in the cytosol, is transported to mitochondria and thus initiates the second phase of respiration. Before pyruvic acid enters Krebs cycle, operative in the mitochondria, one of the three carbon atoms of pyruvic acid is oxidized to carbon dioxide in a reaction called oxidative decarboxylation, that is pyruvate is first decarboxylated and then oxidized by the enzyme pyruvate dehydrogenase. The combination of the remaining 2-carbon acetate unit is readily accepted by a sulphur-containing compound, coenzyme A (CoA), to form acetyl-CoA. This is the connecting link between glycolysis and Kreb's cycle. During the process, NAD+ is reduced to NADH. The summary of the reaction is given below:


During this process, two molecules of NADH are produced (from the metabolism of two molecules of pyruvic acid produced during glycolysis) and thus, it results in a net gain of 6 ATP molecules (2NADH x 3 = 6 ATP).

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