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The Anaerobic Option: Fermentation

Glycolysis occurs as the first step in glucose metabolism regardless of whether the cell is performing fermentation or aerobic respiration. It is important to note that glycolysis can never




A summary of glycolysis

occur alone. It must be coupled to other reactions to be useful. Since fermentation is a comparatively simple process, we will examine it first. If a cell performs fermentation, no oxygen is required, but the pyruvic acid generated in glycolysis must be further processed. While countless variations exist, two major types of fermentation are common (see Figure 4.2):
  • In ethanol fermentation, pyruvic acid is broken down into ethanol (a two-carbon compound) and carbon dioxide (CO2). This type of fermentation is especially prevalent in yeast, and is utilized in many commercial processes, including the baking of bread and the making of alcoholic beverages.
  • In lactic acid fermentation, the atoms of pyruvic acid are rearranged to form lactic acid (another three-carbon compound). This type of fermentation is carried out by vertebrates, usually in their muscle tissues. During heavy exertion, not enough oxygen may be available to supply ATP needs via the aerobic pathway.

A summary of fermentation reactions


It is important to note in examining Figure 4.2 that no additional ATP is produced by either of the fermentation reactions. Why, therefore, must they occur? The answer is simple: the two molecules of NADH formed during glycolysis are oxidized back to NAD+ during the fermentation reactions. If this did not occur, all of the NAD+ in a cell would quickly be used up, and glycolysis could no longer continue. This explains why glycolysis can never “stand alone”.

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