Fermentation

It is a form of anaerobic respiration occurring in certain microorganism. ATP (carrier of chemical energy) is generated in which organic compounds act as both donors and acceptors of electrons.

In alcohol fermentation, pyruvate, the end product of glycolysis, is converted to ethanol and carbon dioxide by eg. yeast

Pyruvate + NADH ® (pyruvate decarboxylase) ethanol + NAD⁺

                    Pyruvic acid                                             

                                 ¯

                      Ethanol                                                                           

In lactic acid fermentation, which occurs in many microorganisms (when sugar is in short supply) by Lactobacillus acidophilus, in animal cells the end product is lactic acid. 

Micoorganisms display a range of fermentations, producing not only ethanol or lactic acid but also other products, such as propionoic and butyric acids, acetate, and methane.

pyruvate + NADH + H^{+ ®} (lactate dehydrogenase) lactate + NAD⁺

                                                                    Pyruvic acid

                                                       Lactic acid                       Ethanol