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Fermentation
Fermentation is a metabolic process that converts sugar to acids, gases and/or alcohol. It occurs in yeast and bacteria, but also in oxygen-starved muscle cells (see "Lactic acid fermentation" below). Fermentation takes place in the absence of oxygen, when the electron transport chain is unusable. It is used by the cell not to generate energy directly, but to recycle NADH into NAD+ so that glycolysis can continue, as long as glucose is present.
The energy generated by the glycolysis-fermentation pathway, a form of substrate-level phosphorylation, is small compared with that of oxidative phosphorylation. Fermentation consumes NADH, which in aerobic conditions might have been used to generate energy in the electron transport chain. For that reason, cells generally benefit from avoiding fermentation when oxygen is available. Exceptions include obligate anaerobes, which cannot tolerate oxygen.
Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium. The science of fermentation is known as zymology. ...
Fermentation is a form of anaerobic digestion that generates adenosine triphosphate (ATP) by the process of substrate-level phosphorylation. The energy for generating ATP comes from the oxidation of organic compounds, such as carbohydrates. In contrast, during respiration the energy for ATP formation is derived from hydrogen protons gradient that is generated across the inner mitochondrial membrane via electron transport chain, in a process called oxidative phosphorylation.
Fermentation does not necessarily have to be carried out in an anaerobic environment . For example, even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to aerobic respiration, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect). The antibiotic activity of hops also inhibits aerobic metabolism in yeast.
Fermentation uses an endogenous, organic electron acceptor. A widely used endogenous electron acceptor is pyruvate. During fermentation, pyruvate is metabolized to various compounds through several processes:
ethanol fermentation, also referred to as alcoholic fermentation, is the conversion of pyruvate into ethanol and carbon dioxide
lactic fermentation (short for lactic acid fermentation) refers to two means of producing lactic acid:
1. heterolactic fermentation is the production of lactic acid as well as other acids and alcohols.
2. homolactic fermentation is the production of lactic acid from pyruvate Sugars are the most common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, lactose, and hydrogen gas (H2). However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone. Yeast carries out fermentation in the production of ethanol in beers, wines, and other alcoholic drinks, along with the production of large quantities of carbon dioxide. Fermentation occurs in mammalian muscle during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid. Chemistry :
Fermentation products contain chemical energy (they are not fully oxidized), but are considered waste products, since they cannot be metabolized further without the use of oxygen.
The chemical equation below shows the alcoholic fermentation of glucose, whose chemical formula is C6H12O6. One glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules:
C6H12O6 → 2 C2H5OH + 2 CO2
C2H5OH is the chemical formula for ethanol.
Before fermentation takes place, one glucose molecule is broken down into two pyruvate molecules. This is known as glycolysis
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