FERMENTATION Some organisms undergo the process of fermentation in order to generate ATP in the absence of oxygen, using other substrates to generate this type of energy. In fact, the scientific definition of fermentation is a metabolic or enzymatic process that creates energy out of an organic molecule in the absence of oxygen or an electron transport system. In such cases, the final electron receptor is an organic molecule rather than oxygen. Fermentation is believed to be the oldest metabolic pathway—something that prokaryotes and eukaryotes have in common. From an evolutionary standpoint, this was the pathway that developed before there was sufficient oxygen on earth. What it means is that, even though higher order animals get the vast majority of their ATP through aerobic mechanisms, the capacity to undergo fermentation still exists as part of an older evolutionary pathway. In fact, mammals undergo fermentation in muscle cells when intense exercise is done and there is insufficient oxygen. The end result is the creation of lactic acid, a common metabolite in muscle during exercise. Invertebrates have fermentation capabilities, producing things like alanine and succinate. Bacteria undergo fermentation in low oxygen environments, creating acetate and formate. These bacteria interact with methanogens, which are Archaea species; these convert the acetate to methane gas. Figure 23 describes lactate fermentation in mammals:
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