Ethanol fermentation is a form of anaerobic respiration used primarily by yeasts when oxygen is not present in sufficient quantity for normal cellular respiration, the cellular energy-producing system, to continue.
The need for Fermentation
In aerobic respiration (the branch of respiration ‘normal’ for most organisms), a molecule of Glucose (C6H12O6, shown left) is broken down through the process of glycolysis into pyruvate (C3H3O3, shown right). In the first half of glycolysis, two Adenosine triphosphate (ATP) molecules cause glucose to break into two molecules Glyceraldehyde-3-phosphate (G3P). In the next step, a Nicotinamide adenine dinucleotide (NAD+) molecule removes a hydrogen atom from a G3P, converting G3P to 3-Biphosphoglycerate and NAD+ to NADH.
When oxygen is present, NADH carries its hydrogen elsewhere. Eventually, oxygen reacts with the hydrogen carried by NADH to make water (H2O). When oxygen is not present in sufficient quantity, NADH cannot give up its hydrogen to oxygen, and so much of the cell’s supply of NAD+ is converted to NADH that G3P can no longer be converted to 3-Biphosphoglycerate, and the generation of ATP by the cell ceases, unless another substance can be used to remove the hydrogen from NADH.
The chemical process of fermentation
Fermentation is a chemical pathway that provides such a substance. In the ethanol fermentation used by yeasts and other organisms, the ionised carboxyl group (COO–) is removed from the pyruvate to generate a molecule of carbon dioxide, which is released by the yeast into its surroundings. The resulting molecule, acetaldehyde (C2H4O, see left), takes the place of oxygen as the chemical that accepts hydrogen from NADH. This hydrogen, together with an H+ ion released during an earlier stage of glycolysis, is added to the acetaldehyde, making ethanol. A hydrogen from NADH, and an H+ ion that is released during an earlier stage of glycolysis is added onto the acetaldehyde, making ethanol (C2H6O, see right).
Ethanol respiration can be summed up in this chemical equation: C6H12O6 → 2 C2H6O + 2 CO2 + 2ATP
Uses of Ethanol respiration
Ethanol respiration is the form of fermentation used to make alcohol and bread. Yeast cells in the dough of a bread will be cut off from their sources of oxygen, and will generate alcohol (which is boiled away due to the heat) and carbon dioxide (which form bubbles that cause bread to rise). It is also used to mass-produce alcoholic beverages. The yeasts, cut off from oxygen, will ferment a starchy grain or vegetable (such as wheat, corn, potatoes, rye). The ethanol is excreted by the yeasts, making an alcoholic beverage.
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