The gradual breakdown of food molecules in the presence of oxygen within cells, resulting in the formation of carbon dioxide and water and the release of energy in the forms of ATP and heat. In many intermediary reactions, substances other than oxygen act as oxidizing agents (i.e., hydrogen or electron acceptors). Reactions are catalyzed by respiratory enzymes, which include the flavoproteins, cytochromes, and other enzymes. Certain vitamins (nicotinamide, riboflavin, thiamine, pyridoxine, and pantothenic acid) are essential in the formation of components of various intracellular enzyme systems.