FERMENTATION Cells that are unable to undergo respiration may lack a final electron acceptor, may not have the genes to make the complexes necessary to undergo respiration, or may not have the genes to have a Krebs cycle. As you can see, some of these are geneticallyoriented, while others are related to the environment. Many prokaryotes just do not have the genetic makeup to participate in respiration, while others are considered facultative, meaning that they change their biochemistry depending on the environment. The cell’s only way to make ATP energy in some cases is glycolysis. The final electron acceptor is pyruvate, and the organism participates in fermentation. No other sources of ATP energy exist. Fermentation will be made use of in many human food processes that make fermented food, such as yogurt and beer. The fermentation that occurs in yogurt and related products as well as in human muscle tissue that is overextended is called lactic acid fermentation. Pyruvate plus NADH makes lactic acid and NAD+. There are bacteria that can do this. Homolactic fermentation involves only making lactic acid as an end product, while heterolactic fermentation involves the making of lactic acid, plus ethanol or acetic acid plus CO2. These pathways are used in cucumber and cabbage fermentation, which makes pickles or sauerkraut. Lactic acid fermentation is important in medical circles. It is the lactic acid bacteria in the vagina that make the pH of the vaginal milieu more acidic. This prevents things like yeast from proliferating. If these bacteria die off or are eliminated, then yeast forms can overgrow, leading to a yeast infection. This is why antibiotics can lead to yeast infections; they kill off healthy lactic acid-making bacteria. Alcohol fermentation makes ethanol as the end product. Pyruvate is the first substrate and both carbon dioxide gas and ethanol are end products. The yeast form called Saccharomyces cerevisiae is what makes bread rise and what makes alcoholic beverages. Other forms of fermentation include those that make acetone and butanol. Certain
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