All living things need a continuous supply of energy to keep their cells functioning normally and to stay healthy. Some organisms, called autotrophs, can produce their own energy using sunlight through the process of photosynthesis. Others, like humans, need to eat food in order to produce energy.
However, that is not the type of energy cells use to function. Instead they use a molecule called Adenosine triphosphate (ATP) to keep themselves going. The cells, therefore, must have a way to take the chemical energy stored in food and transform it into the ATP they need to function. The process cells undergo to make this change is called cellular respiration.
Two Types of Cellular Respiration
Cellular respiration can be aerobic (meaning "with oxygen") or anaerobic ("without oxygen"). Which route the cells take to create the ATP depends solely on whether or not there is enough oxygen present to undergo aerobic respiration. If there is not enough oxygen present for aerobic respiration, then the organism will resort to using anaerobic respiration.
In order to maximize the amount of ATP made in the process of cellular respiration, oxygen must be present. As eukaryotic species evolved over time, they became more complex with more organs and body parts. It became necessary for cells to be able to create as much ATP as possible to keep these new adaptations running properly.
Early Earth's atmosphere had very little oxygen. It wasn't until after autotrophs became abundant and released large amounts of oxygen as a byproduct of photosynthesis that aerobic respiration could evolve. The oxygen allowed each cell to produce many times more ATP than their ancient ancestors that relied on anaerobic respiration. This process happens in the cell organelle called the mitochondria.
More primitive is the process of anaerobic respiration (also known as fermentation). Anaerobic respiration starts out the same way as aerobic respiration, but it stops part way through the pathway because the oxygen is not available for it to finish the aerobic respiration process. This type of cellular respiration makes many fewer ATP and also releases byproducts of either lactic acid or alcohol. Like aerobic respiration, anaerobic respiration happens in the mitochondria.
Lactic acid fermentation is the type of anaerobic respiration humans undergo if there is a shortage of oxygen. For example, long distance runners experience a buildup of lactic acid in their muscles because they are not taking in enough oxygen to keep up with the demand of energy needed for the exercise. The lactic acid can even cause cramping and soreness in the muscles as time goes on.
Alcoholic fermentation does not happen in humans. Yeast is a good example of an organism that undergoes alcoholic fermentation. The same process that goes on in the mitochondria during lactic acid fermentation also happens in alcoholic fermentation. The only difference is that the byproduct of alcoholic fermentation is ethyl alcohol.
Alcoholic fermentation is important for the beer industry. Beer makers add yeast which will undergo alcoholic fermentation to add alcohol to the brew. Wine fermentation is also similar and provides the alcohol for the wine.
Which is Better?
Aerobic respiration is much more efficient at making ATP than anaerobic respiration. Without oxygen, the Krebs Cycle and the Electron Transport Chain in cellular respiration get backed up and will not work any longer. This forces the cell to undergo the much less efficient anaerobic respiration. While aerobic respiration can produce up to 36 ATP, anaerobic respiration can only have a net gain of 2 ATP.
Evolution and Respiration
The most ancient type of respiration is anaerobic. Since there was little to no oxygen present when the first eukaryotic cells evolved through endosymbiosis, they could only undergo anaerobic respiration. This was not a problem, however, since those first cells were unicellular. Producing only 2 ATP at a time was enough to keep the single cell running.
As multicellular eukaryotic organisms began to appear on Earth, the larger and more complex organisms needed to produce more energy. Through natural selection, organisms with more mitochondria that could undergo aerobic respiration survived and reproduced, passing on these favorable adaptations to their offspring. The more ancient versions could no longer keep up with the demand for ATP in the more complex organism and went extinct.