The Science of Exercise Physiology
There are three steps involved in cellular respiration; the conversion of food sources to energy:
1. Glycolysis: the breaking down of glucose molecules from carbohydrates into molecules of pyruvate, which can be used in the following stage the cytric acid cycle. This process occurs in the cytosol of the cell and can proceed regardless of the presence of oxygen.
2. The Cytric Acid Cycle:
As pyruvate is being shuttled from the cytosol to the interior of the mitochondrion, a microenzyme removes one carbon and two oxygens from each molecule, producing Aceytl CoA. The Cytric acid cycle is an aerobic process, meaning it needs oxygen to function.
3. The Electron Transport Chain:
Very little energy has been produced during glycolysis and the Cytric acid. Most of the energy locked in the original glucose molecule will be released by the electron transport chain and oxidative phosphorylation. The electron transport chain is a network of electron-carrying proteins. Therefore, the electron transport chain produces a gradient through which ATP can be made known as chemiosmosis.

ATP
ATP is produced in the mitochondria by an enzyme called ATP Synthase. ATP Synthase is powered by a transmembrane electrochemical potential gradient in a process called the electron transport chain. The function of the electron transport chain is to produce this gradient. In all living organisms, a series of reduction reactions (chemical reactions in which electrons are transferred from one molecule to another) are used to produce a transmembrane electrochemical potential gradient.
Adenosine triphosphate (ATP) is the source of energy for all muscle contractions. Energy is released when ATP is broken into ADP+Pi (adenosine diphosphate and phosphate group). Maintaining the availability of ATP for muscle contraction is the limiting factor for muscle performance since ATP is not stored in large amounts in skeletal muscle. Sources of ATP come from both anaerobic (does not require oxygen) and aerobic (requires oxygen). The primary energy source for a given level of activity will depend on the intensity of muscle contractions.
ATP energises the muscle contraction process by the ATPase activity of the exposed myosin head. When ATP is exposed to the myosin head, it is cleaved and energy is released. In addition to ATP, magnesium is very necessary in ATP energy releasing reactions. Before ATP can become active ATP magnesium must bind between the second and third phosphate. Clinically, magnesium deficiency may be related to such conditions as fibromyalgia and chronic fatigue syndrome.
The synthesis of almost any chemical compound requires energy. That energy is ATP, which is critically important to the biosynthesis of proteins, phospholipids, purines, pyrimidines and many other substances.
ATP is necessary for nerve transmission. Nerve transmission entails the release of nerve transmitter substance from the pre-synaptic terminal into the synaptic cleft, which is a space between one nerve ending and another. The neurotransmitter spans the cleft and attaches to the receptor of the other cell. The nerve transmitter substance must be constantly renewed in the presynaptic terminal for future release; the energy for this formation is supplied by ATP. There are many mitochondria in the pre-synaptic terminal to form and store the ATP for this process. At the post-synaptic terminal, the next nerve cell down the line, it is through active transport of sodium, potassium and calcium that concentration differences across the nerve cell membrane cause nerve firing and propagation of nerve signals to travel to the next presynaptic terminal. These concentration gradients could not be accomplished without ATPase active transport across nerve cell membranes.