After two to three minutes, heart rate and breathing rate have increased, bringing more oxygen to the muscles. Now, aerobic metabolism can begin supplying ATP. To produce ATP, aerobic metabolism can use glucose, fatty acids, and sometimes amino acids from protein.
It produces ATP slower than anaerobic metabolism does, but it is much more efficient, producing about 18 times more ATP for each molecule of glucose.
The reactions of aerobic metabolism take place in cellular organelles called mitochondria. When oxygen is available, the pyruvate produced by anaerobic metabolism enters mitochondria. Here it loses a carbon atom as CO2 combines with a molecule called coenzyme A (CoA) to form a molecule called acetyl-CoA. When oxygen is available, pyruvate is used to make acetyl-CoA and no lactic acid is formed.
The electrons released by glucose breakdown are picked up by shuttling molecules and then used to generate ATP. When fatty acids are used to produce ATP, the fatty acid chain is first broken into 2-carbon units that form acetyl-CoA. This process is called beta-oxidation and releases electrons that can be used to produce ATP.
Acetyl-CoA, whether produced from beta-oxidation or glucose breakdown, enters the next stage of aerobic metabolism: the citric acid cycle. To begin the cycle, acetyl-CoA combines with a 4-carbon molecule that comes from carbohydrate metabolism. The result is a 6-carbon molecule called citric acid.
The citric acid cycle then removes one carbon at a time of CO2 until the 4-carbon molecule is formed again. These chemical reactions produce two ATP molecules per glucose molecule. They also release high-energy electrons. The electrons are passed to shuttling molecules for transport to the last stage of aerobic metabolism, the electron transport chain.
The electron transport chain involves a series of molecules associated with the inner membrane of the mitochondria. These molecules accept the electrons from the shuttling molecules and pass them down the chain until they are finally combined with oxygen to form water. As the electrons are passed along, their energy is trapped and used to make ATP.
Aerobic capacity is dependent on the amount of oxygen that can be delivered to and used by the muscles. A greater aerobic capacity allows a person to perform more intense exercise without relying on anaerobic metabolism.
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