Anaerobic Respiration does not require the presence of oxygen, but other electron acceptors. This process is commonly used by prokaryotes that have varying degrees of oxygen utilization: facultative anaerobes can metabolize aerobically or anaerobically, but obligate anaerobes anaerobically respire and require strict no-oxygen conditions. In aerobic respiration, oxygen is in the final electron acceptor in the electron transport chain (ETC) but in anaerobic respiration less-oxidized molecules like sulfate, nitrate sulphur and fumarate as used in the ETC. Due to their less-reducing properties, the electrochemical gradient is not as defined and therefore less efficient than aerobic respiration – produce less ATP. Anaerobic respiration is 15 times less efficient than aerobic respiration, as it generates a total of 2 ATP compared to 38 ATP respectively.
After glycolysis, 2 units of three-carbon pyruvate are produced and undergo fermentation in the cytosol of the cell. There are two types of fermentation: yeast and Homolactic fermentation. In yeast fermentation, ethanol (C2H6O) and carbon dioxide are produced, while in Homolactic fermentation lactic acid is produced. Fermentation uses substrate-level phosphorylation to produce ATP. This is where a phosphoryl group (PO3) is transferred to adenosine diphosphate (ADP) from an intermediate of oxidized compounds (sulfate, nitrate, sulphur, fumarate) to produce adenosine triphosphate (ATP).
Fermentation is important economically, as it is integral in ethanol production for fuel, beverages, cosmetics and more.