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Skeletal Muscles and Energy Supply

As noted above, ATP supplies the energy for a muscle fiber to contract. Since muscular contraction often requires large amounts of energy, especially under extreme exertion, the supply of ATP that can be generated by aerobic cellular respiration is sometimes not enough to fuel the muscle’s needs. Under these circumstances, alternate means of generating ATP must be found, or muscle contraction will cease. Firstly, muscles store additional fuel in the form of glycogen, which can be quickly mobilized when energy is needed. Muscles also store another high energy compound called creatine phosphate, which reacts with ADP to regenerate ATP by donating a phosphate group. Thus as the ATP supply dwindles, it can be quickly “restocked” by creatine phosphate. If energy demands are still not being met, the body uses its last alternative: the anaerobic process of fermentation begins in the muscle cells. As noted earlier, glucose can be broken down by humans in the process of fermentation to produce two molecules of lactic acid (lactate). While extremely inefficient, the process can provide extra ATP for short periods of time. Since lactic acid is toxic, however, when the “emergency” need for energy is over it must be reconverted to glucose in a process known as gluconeogenesis. This process requires energy, which is usually supplied by aerobic respiration. The amount of energy required to dispose of the lactic acid that has been produced corresponds to the so-called “oxygen debt”, and explains why heavy breathing often continues even when activity has ceased. 

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