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Anaerobic metabolism refers to biological processes that produce energy for an organism without using oxygen. It is based on chemical reactions within the body in which carbohydrates are broken down to release chemical energy. This process occurs primarily when an organism needs a sudden, short-term burst of energy or during periods of intense exertion.
The primary source of energy for cellular functions is a molecule called adenosine triphosphate (C10H16N5O13P3), or ATP, which releases energy when it is broken down. Under most circumstances, the body produces most of its ATP from fats and carbohydrates through chemical reactions involving oxygen, called aerobic metabolism. The oxygen is carried from the respiratory system to cells through the bloodstream, and when an organism's physical activity increases, its breathing and heartbeat become more rapid to increase the supply of oxygen for these reactions. This process takes time, however, and so is inadequate if the organism needs more energy on short notice, to make a sudden, rapid movement, for instance. In addition, during intense activity, aerobic metabolism alone may not provide enough energy even once oxygen supplies have been increased.
This is where anaerobic metabolism is important. When an organism needs to rapidly increase its energy supply, anaerobic metabolism allows it to do so immediately instead of waiting for enough oxygen to fuel increased aerobic metabolism. It can also be used in combination with aerobic metabolism when high energy levels are needed. For example, a human athlete running a short high-speed sprint uses this metabolism to supply himself or herself with a short-term increase in energy, while someone going for an extended, leisurely jog will rely on anaerobic metabolism when starting but eventually shift to primarily aerobic metabolism once his or her body has had had time to adjust to the increased activity. If the athlete engages in high-intensity activity for an extended period, both forms of metabolism can be heavily exploited.
Anaerobic metabolism is based on a metabolic pathway, or series of chemical reactions in the body, called glycolysis. Glycolysis begins with the sugar glucose (C6H12O6) and, through a series of chemical reactions and intermediate compounds, uses them to produce ATP. This process is considerably less energy efficient than aerobic metabolism of glucose and produces fewer molecules of ATP per molecule of glucose, which is why the body will try to rely on aerobic metabolism to the greatest extent possible and uses anaerobic metabolism primarily when aerobic metabolism alone is inadequate. Anaerobic glycolysis produces byproducts that, when accumulated in sufficient quantities, enter the bloodstream and cause fatigue. Thus, extended bursts of this metabolism are not sustainable over time.