• It occurs in almost all cells.
• In eukaryotes, it occurs in the cytosol.
• C₆H₁₂O₆ + 2NAD⁺ -> 2C₃H₄O₃ + 2NADH + 2H⁺
• The free energy stored in 2 molecules of pyruvic acid is somewhat less than that in the original glucose molecule.
• Some of this difference is captured in 2 molecules of ATP.

                                 Pyruvic Acid 

In YEAST

• Pyruvic acid is decarboxylated and reduced by NADH to form a molecule of carbon dioxide and one of ethanol.
• C₃H₄O₃ + NADH + H⁺ -> CO₂ + C₂H₅OH + NAD⁺
• This accounts for the bubbles and alcohol in, for examples, beer and champagne.
• The process is called alcoholic fermentation.
• The process is energetically wasteful because so much of the free energy of glucose (some 95%) remains in the alcohol (a good fuel!).

In active MUSCLES

• Pyruvic acid is reduced by NADH forming a molecule of lactic acid.
• C₃H₄O₃ + NADH + H⁺ -> C₃H₆O₃ + NAD⁺
• The process is called lactic acid fermentation.
• The process is energetically wasteful because so much free energy remains in the lactic acid molecule. (It can also be debilitating because of the drop in pH of overworked muscles.)

In MITOCHONDRIA

• Pyruvic acid is oxidized completely to form carbon dioxide and water.
• The process is called cellular respiration.
• Approximately 40% of the energy in the original glucose molecule is trapped in molecules of ATP.