Mitochondria are membrane-- bound organelles in the cytoplasm of eukaryotic cells. Their function is to supply cellular energy in the form of ATP to cells to use as chemical energy for life processes. Respiration is catabolic because large molecules are broken into smaller ones, accompanied by the release of energy. It is a combustion reaction, but, due to the presence of enzymes, it occurs in many small steps. In glycolysis, which occurs in the cytosol of cells, one molecule of glucose is converted into two molecules of pyruvate and 2 ATP's are produced. This step does not require oxygen. Next, pyruvate is oxidized to acetyl-CoA and CO2, by the enzyme pyruvate dehydrogenase complex or PDC. This is located in the mitochondria. This step is the link reaction as it links glycolysis to the next step--the Kreb's cycle. In the Krebs cycle or the citric acid cycle, oxygen is present. The mitochondria undergo aerobic respiration leading to this cycle. If no oxygen is present, the pyruvate undergoes fermentation. If oxygen is present, the acetyl COA that is produced from the pyruvate enters the Krebs cycle, in the mitochondrial matrix. Here, it is oxidized to produce the waste product CO2. NAD is reduced to NADH, which is used by the electron transport chain to create more ATP molecules. This is called oxidative phosphorylation. When a glucose is completely oxidized, two acetyl-Co A molecules are metabolized by the Kreb's cycle. The resulting waste products are H20 and C02. These are the wastes we exhale to remove them from the body. When a series of enzyme --controlled steps is finally completed, the end result of the oxidation of one glucose molecule is the production of a net gain of 36 ATP molecules to be used for cellular energy and the wastes carbon dioxide and water.