Fate of Pyruvate
Two molecules of pyruvate (pyruvic acid) are formed in the 10-step glycolysis pathway. This glycolytic pathway is common to both aerobic and anaerobic respiration. The fate of pyruvate depends on the presence/absence of oxygen, and the type of cells present. For instance, in aerobic conditions and in eukaryotic cells, pyruvate would enter the mitochondrial matrix for further biochemical processing. On the other hand, in anaerobic conditions, pyruvate would form either lactic acid or ethanol in prokaryotic cells and eukaryotic muscle cells. Anaerobic reactions take place in the cytoplasm of a cell. A common term, fermentation, is used for the formation of lactic acid and ethanol.
FERMENTATION
It is an anaerobic respiration process. Glycolysis is common for both aerobic and anaerobic respiration. The end product of glycolysis is two molecules of pyruvic acid. Let’s see the fate of pyruvic acid in anaerobic respiration.
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Alcoholic fermentation:
The fate of pyruvate depends on the presence and absence of oxygen. In alcoholic fermentation, the 2 molecules of 3C pyruvic acid undergoes decarboxylation reaction. And the enzyme involved is pyruvate decarboxylase in the presence of TPP and Mg ions. After decarboxylation, 2 molecules of acetaldehyde will be formed. The acetaldehyde will undergo reduction (addition of H and e) by NADH2 in the presence of alcohol dehydrogenase enzyme. After reduction, 2 molecules of ethyl alcohol and 2 NAD+ will be formed. It is important to remember that 2 molecules of NADH2 formed in glycolysis have been used up in the fermentation process. Example: yeast. If the alcohol concentration in the yeast cell reaches to 13%, this would cause the yeast cell to die. Wine and beer can be formed through alcoholic fermentation, but if the drinks having higher alcohol concentration, this fermentation cannot be used rather an artificial method would be needed like distillation. Example, whisky, rum, vodka, etc.
2. Lactic acid fermentation:
The fate of pyruvate depends on the presence and absence of oxygen. Direct reduction of pyruvic acid will occur leading to the formation of two molecules of CH3-CHOH-COOH (lactic acid) in the presence of the enzyme lactate dehydrogenase (present in the cytoplasm) in the presence of FMN and Zn. Here, for reduction NADH2 will be utilized. Example: bacteria (LAB), muscles. Lactic acid fermentation occurs in the cytoplasm of a cell. It can occur in both prokaryotes and eukaryotes (muscle cells).
Note: in fermentation, the net ATP production is 2ATP. The net water released is zero. It is important to note that the total energy produced during aerobic respiration is 686 Kcal. In alcoholic fermentation, it is 58 Kcal and in lactic acid fermentation, it is 36 Kcal.
The efficiency of alcoholic fermentation = 58 / 686 x 100 = 8.4%
The efficiency of lactic acid fermentation = 36 / 686 x 100 = 5.2%