Fermentation

Pyruvic acid, the end product of glycolysis, can go three ways: lactic acid fermentation, alcoholic fermentation, or aerobic respiration. Fermentation is named after its product: if alcohol is formed, it is alcoholic fermentation; if lactic acid, lactic acid fermentation.

• Alcoholic fermentation (in yeast): pyruvic acid is broken into CO₂ and ethanol, using the enzymes pyruvic acid decarboxylase and alcohol dehydrogenase. This is what makes bread dough rise.
• Lactic acid fermentation: some bacteria, and your muscles during hard exercise when oxygen runs short, turn pyruvic acid into lactic acid, using lactate dehydrogenase.

In both, NADH + H⁺ gives up its hydrogen and becomes NAD⁺ again. That is the real purpose of fermentation: without fresh NAD⁺, glycolysis would stop.

Fermentation is a poor deal. It releases less than 7% of the energy in glucose; the only ATP gained is the 2 ATP from glycolysis. It is also risky: acid or alcohol builds up. Yeasts are poisoned and die when the alcohol reaches about 13%.

Some organisms can live with or without oxygen (facultative anaerobes); others can live only without it (obligate anaerobes). The first cells on Earth probably lived without oxygen.