Fermentation is the metabolism of organic compounds in the absence of air. It involves substrate-level phosphorylation in the absence of a respiratory electron transport chain. The equation for the reaction of glucose to form lactic acid is:
:  + 2 ADP + 2 P → 2  + 2 ATP + 2 
Anaerobic respiration is respiration in the absence of . Prokaryotes can utilize a variety of electron acceptors. These include nitrate, sulfate, and carbon dioxide.