Fermentation (Anaerobic Respiration)
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Fermentation is the incomplete oxidation of glucose under anaerobic conditions. It occurs in many prokaryotes and unicellular eukaryotes.
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It is of two types:
- Alcoholic fermentation: Pyruvic acid from glucose is converted to CO₂ and ethanol. The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyze these reactions. For example, yeast. Yeasts poison themselves when alcohol concentration reaches about 13%.
- Lactic acid fermentation: Pyruvic acid is converted to lactic acid. For example, some bacteria.
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The reducing agent NADH+H⁺ is reoxidized to NAD⁺ in both processes.
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In animals, when oxygen is inadequate during exercise, pyruvate in muscle cells is reduced to lactic acid by the enzyme lactate dehydrogenase.
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Net ATP production from fermentation of one glucose molecule is 2 ATP (4 ATP from glycolysis minus 2 ATP utilized).
- Limited energy production: Less than 7% of the energy in glucose is released, and not all is trapped as high-energy ATP bonds.
- Hazardous products: Acid or alcohol is formed.