Enzymes are named for their substrates and have an active site specific for the substrate. Many enzymes have nonprotein molecules, called coenzymes, to assist them. Some coenzymes, like NAD, remove hydrogen from substrates. Glucose breaks down into carbon dioxide and water during aerobic cellular respiration. Glycolysis, the Krebs cycle, and the electron transport system are three subpathways of cellular respiration.
Glycolysis needs no oxygen and takes place in the cytoplasm of the cell.
A transition reaction converts pyruvate to active acetate that can enter the mitochondrion and the Krebs cycle. Two ATP and 4 CO2 molecules form as the cycle turns twice, once for each pyruvate molecule. Lactic acid is toxic to many cells and leads to muscle fatigue.