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Fermentation uses a range of substrates and forms a variety of metabolic end products. Of the 55 end products formed, the most common are acetate and lactate . [ 1 ] [ 2 ] Of the 46 chemically-defined substrates that have been reported, the most common are glucose and other sugars.
In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.
Anaerobic cellular respiration and fermentation generate ATP in very different ways, and the terms should not be treated as synonyms. Cellular respiration (both aerobic and anaerobic) uses highly reduced chemical compounds such as NADH and FADH 2 (for example produced during glycolysis and the citric acid cycle) to establish an electrochemical gradient (often a proton gradient) across a membrane.
Fermentative organisms are very important industrially and are used to make many different types of food products. The different metabolic end products produced by each specific bacterial species are responsible for the different tastes and properties of each food. Not all fermentative organisms use substrate-level phosphorylation.
Anaerobic respiration is used by microorganisms, either bacteria or archaea, in which neither oxygen (aerobic respiration) nor pyruvate derivatives (fermentation) is the final electron acceptor. Rather, an inorganic acceptor such as sulfate ( SO 2− 4 ), nitrate ( NO − 3 ), or sulfur (S) is used. [ 16 ]
Some obligate anaerobes use fermentation, while others use anaerobic respiration. [12] Aerotolerant organisms are strictly fermentative. [13] In the presence of oxygen, facultative anaerobes use aerobic respiration. [7] In the absence of oxygen, some facultative anaerobes use fermentation, while others may use anaerobic respiration. [7]
The most important initial issue when considering the application of anaerobic digestion systems is the feedstock to the process. [55] Almost any organic material can be processed with anaerobic digestion; [ 56 ] however, if biogas production is the aim, the level of putrescibility is the key factor in its successful application. [ 57 ]
Numerous fermentation pathways exist such as lactic acid fermentation, mixed acid fermentation, 2-3 butanediol fermentation where organic compounds are reduced to organic acids and alcohol. [8] [4] The energy yield of anaerobic respiration and fermentation (i.e. the number of ATP molecules generated) is less than in aerobic respiration. [8]