Fermentation

Cells handle pyruvic acid in three main ways: lactic acid fermentation, alcoholic fermentation and aerobic respiration. Fermentation runs under anaerobic conditions in many prokaryotes and unicellular eukaryotes, and also in germinating seeds.

In yeast, glucose is incompletely oxidised without O₂: pyruvic acid is converted to CO₂ and ethanol by the enzymes pyruvic acid decarboxylase and alcohol dehydrogenase.

Some bacteria make lactic acid from pyruvic acid. In animal cells such as muscles during exercise, when O₂ falls short, lactate dehydrogenase reduces pyruvic acid to lactic acid.

Both kinds of fermentation use NADH + H⁺ as the reducing agent and turn it back into NAD⁺, which lets glycolysis keep going.

Fermentation frees under seven per cent of the energy stored in glucose, and only part of even that is trapped as ATP. The net gain is only 2 ATP per glucose, all from glycolysis.

Fermentation is hazardous to the cell because it produces acid or alcohol. Yeasts poison themselves to death when alcohol reaches about 13 per cent.

Fermentation versus aerobic respiration: glucose is only partly broken down rather than fully to CO₂ and H₂O; the net gain is 2 ATP rather than many more; and NADH goes back to NAD⁺ only slowly in fermentation but very vigorously in aerobic respiration.