Fermentation (Lactic Acid and Alcoholic)

When oxygen is not available quickly enough, the NADH+H⁺ produced during glycolysis must still be re-oxidised back to NAD⁺ so that glycolysis can keep running - and cells do this by handing that hydrogen on to the glycolytic end-product, pyruvic acid, instead of to oxygen. Depending on the organism, this gives one of two products.

Lactic acid fermentation (in muscle). In muscle cells, especially during vigorous exercise when the oxygen supply cannot keep pace with demand, the NADH+H⁺ produced during glycolysis is reoxidised to NAD⁺ by donating one proton and two electrons to pyruvic acid, which is thereby reduced to lactic acid:

Glucose → (Glycolysis) 2 CH₃COCOOH + 2NADH+H⁺ → 2 CH₃CHOHCOOH + 2NAD⁺

(Glucose → Pyruvic acid → Lactic acid.)

Skeletal muscle usually derives much of its immediate energy from this anaerobic route. After vigorous exercise, the accumulated lactic acid causes the familiar burning sensation and leads to muscle fatigue; during rest, this lactic acid is reconverted back to pyruvic acid and channelled into the aerobic respiration pathway.

Alcoholic fermentation (in yeast). In yeast, pyruvate is first decarboxylated to acetaldehyde, releasing CO₂, and the acetaldehyde is then reduced by NADH+H⁺ to ethanol:

Glucose → (Glycolysis) 2 CH₃COCOOH → CO₂↑ + 2 CH₃CHO + 2NADH+H⁺ → 2 C₂H₅OH

(Glucose → Pyruvic acid → Acetaldehyde → Ethanol.) …