Fermentation
Although normal cellular respiration is an efficient process, it requires a constant supply of oxygen. So what if the cell's oxygen supply is cut off? In an anaerobic environment (one without oxygen), the cell cannot perform the citric acid cycle and oxidative phosphorylation since there's no oxygen at the end of the electron transport chain to accept electrons. Therefore, cells either have to use other electronegative substitutes instead of oxygen, or they have to rely on glycolysis exclusively for ATP production. The latter scenario is called fermentation.
There are two major problems with only using glycolysis. First, this process is much less efficient than respiration. In respiration we get 36-38 ATP's for every glucose molecule, but in fermentation we only net 2 ATP's. This is only about 5% the efficiency of respiration. Unfortunately, there's no way to compensate for that except to consume more food. The other problem is that since there's no electron transport chain, the NADH needs to find another way to become oxidized into NAD+. If this can be done, then glycolysis can continuously repeat itself, provided that sugar is present. There are two ways to solve the latter problem, depending on the type of fermentation involved - alcohol fermentation of lactic acid fermentation.
In alcohol fermentation, pyruvate gets converted to acetaldehyde, which in turn oxidizes NADH and becomes ethanol. This regenerates the supply of NAD+ for glycolysis. Many bacteria and yeast use this type of fermentation.
In lactic acid fermentation, pyruvate directly oxidizes NADH and becomes lactate. This process also replenishes the cell's NAD+ supply. Bacteria, fungi, and human muscle cells use this kind of fermentation when oxygen is not present.
Many biologists see glycolysis as evidence for evolution. First, it is the most widespread metabolic pathway on Earth. This implies that an ancient ancestor to all organisms developed this pathway and passed it on to its descendants. Second, glycolysis offers organisms a way to synthesize ATP without a significant source of oxygen. This supports the hypothesis because oxygen was not present in large amounts on Earth when early organisms appeared over 3.5 billion years ago. Therefore, it makes sense that these ancient organisms used glycolysis to make their ATP.
There are two major problems with only using glycolysis. First, this process is much less efficient than respiration. In respiration we get 36-38 ATP's for every glucose molecule, but in fermentation we only net 2 ATP's. This is only about 5% the efficiency of respiration. Unfortunately, there's no way to compensate for that except to consume more food. The other problem is that since there's no electron transport chain, the NADH needs to find another way to become oxidized into NAD+. If this can be done, then glycolysis can continuously repeat itself, provided that sugar is present. There are two ways to solve the latter problem, depending on the type of fermentation involved - alcohol fermentation of lactic acid fermentation.
In alcohol fermentation, pyruvate gets converted to acetaldehyde, which in turn oxidizes NADH and becomes ethanol. This regenerates the supply of NAD+ for glycolysis. Many bacteria and yeast use this type of fermentation.
In lactic acid fermentation, pyruvate directly oxidizes NADH and becomes lactate. This process also replenishes the cell's NAD+ supply. Bacteria, fungi, and human muscle cells use this kind of fermentation when oxygen is not present.
Many biologists see glycolysis as evidence for evolution. First, it is the most widespread metabolic pathway on Earth. This implies that an ancient ancestor to all organisms developed this pathway and passed it on to its descendants. Second, glycolysis offers organisms a way to synthesize ATP without a significant source of oxygen. This supports the hypothesis because oxygen was not present in large amounts on Earth when early organisms appeared over 3.5 billion years ago. Therefore, it makes sense that these ancient organisms used glycolysis to make their ATP.