Glycolysis
Glucose is a simple sugar that is used by much of the body to fuel individual cells. Glucose itself is not burned to fuel cells; instead, it is used to form chemical bonds that make high-energy molecules, such as adenosine triphosphate (ATP). According to the Medical Bio Chemistry Page, the first step in the breakdown and metabolism of glucose is called glycolysis, in which a glucose molecule is broken down into other forms. Glycolysis involves many different chemical reactions which produce a small amount of ATP but also produce the compound pyruvate, which is a smaller molecule that the body can then use to make more energy.
Aerobic Metabolism
Aerobic metabolism is the breaking down of glucose in the presence of oxygen and involves the formation of pyruvate as an intermediate. As the Muscle Physiology page at the University of California-San Diego explain, this way of breaking down glucose generates much more energy (in the form of ATP) than the reactions without oxygen. In the presence of oxygen, pyruvate can be entered into a complex series of chemical reaction, termed the Krebs cycle. In the Krebs cycle, all of the molecules of the pyruvate are broken down and used to form ATP and carbon dioxide (which is a waste product).
Anaerobic Metabolism
In some cases oxygen cannot be used during the breakdown of glucose. For example, when muscles in the body are being heavily exerted, they quickly use up all of the available oxygen. Glucose can still be broken down to form energy without oxygen via anerobic metabolism (or anerobic respiration). With anerobic respiration, the pyruvate does not enter into the Krebs cycle but is instead converted into lactate. This process produces some energy, but not as much as the Krebs cycle. The lactate then gets turned into lactic acid, which is the compound that causes cramping of muscles.
