In biology terms, respiration is the process by which cells break down sugar. Within a cell, two types of respiration may occur: "aerobic" and "anaerobic." Aerobic respiration is the more productive of the two and requires the presence of oxygen. Without oxygen, anaerobic respiration, which is also known as "fermentation," occurs.
Adenosine Triphosphate, more commonly known as "ATP," is the usable product of both types of respiration, although anaerobic respiration yields far less -- two parts ATP to every part of sugar the cell processes, as composed to aerobic respiration's 38-to-1 ratio. While an organism can utilize ATP in the quantities aerobic respiration produces it, the two ATP produced during anaerobic respiration serve mainly to allow the cell another chance at going through the process.
After or during strenuous exercise, you may notice a dull burning sensation in your muscles. This occurs as a result of lactic acid buildup, lactic acid being the main by-product of anaerobic respiration. Not surprisingly, the burning you feel after exercise results from your body's inability to deliver adequate oxygen to the muscle paining you. The only way to stop this burning is for your muscles to receive more oxygen, usually by taking a break. At that point, aerobic respiration begins and the lactic acid build-up dissipates.
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Also known as ethanol, ethyl alcohol is the chief component of liquor, beer and wine. Beer producers place yeast in a no-oxygen environment to ferment it. What this means in biology terms is that they force the yeast cells to undergo anaerobic respiration, which also happens to be known as fermentation. Yeast cells do not produce lactic acid, nor do humans produce ethyl alcohol as by-products of their respective anaerobic respiration.