Glycolysis and Alcohol Fermentation

 

Glycolysis and Alcohol Fermentation

When there is oxygen deficiency or prolonged exercise, muscles obtain most of their energy from an anaerobic (without oxygen) process called glycolysis. Yeast cells obtain energy under anaerobic conditions using a very similar process called alcoholic fermentation. Glycolysis is the anaerobic breakdown of glucose to lactic acid in lactic acid bacteria and very active skeletal muscles. This process makes energy available for cell activity in the form of ATP. Alcoholic fermentation is identical to glycolysis except for the final step. In alcoholic fermentation, pyruvic acid is broken down into ethanol and carbon dioxide. Lactic acid from glycolysis produces a feeling of tiredness; the products of alcoholic fermentation have been used in baking and brewing.

Both alcoholic fermentation and Lactic acid fermentation are anaerobic fermentation processes that begin with the sugar glucose. Glycolysis requires 11 enzymes which degrade glucose to lactic acid. Alcoholic fermentation follows the same enzymatic pathway for the first 10 steps. The last enzyme lactate dehydrogenase, is replaced by two enzymes in alcoholic fermentation. These two enzymes, pyruvate decarboxylase and alcohol dehydrogenase, convert pyruvic acid into carbon dioxide and ethanol in alcoholic fermentation.


Fate of Pyruvate after glycolysis

Homolactic Fermentation

Instead of being immediately reoxidized the NADH molecule remains in its reduced form until pyruvate has been formed at the end of glycolysis. The pyruvate product of glycolysis gets further acted upon under anaerobic conditions by the enzyme lactate dehydrogenase (LDH). From the lactate product, lactic acid can be formed, which causes the muscle fatigue that accompanies strenuous workouts where oxygen becomes deficient.

Alcoholic Fermentation

There is another  way that the NADH molecule can be re-oxidized. Anaerobic conditions in yeast convert pyruvate to carbon dioxide and ethanol. This occurs with the help of the enzyme pyruvate decarboxylase which removes a carbon dioxide molecule from the pyruvate to yield an acetaldehyde. The acetaldehyde is then reduced by the enzyme alcohol dehydrogenase which transfers the hydrogen from NADH to the acetaldehyde to yield NAD and ethanol. This enzyme is not found in humans.

Alcohol fermentation

Assesment

What is the difference between lactic acid fermentation and alcohol fermentation?

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