As fuels provided by our diet are broken down into smaller and smaller molecules they undergo a series of oxidation reactions. Energy to produce ATP is extracted from the fuels as they are oxidized in what is essentially a controlled burn. This gradual breakdown and oxidation of fuels allows us to capture some of the chemical energy in those fuels to do work, such as physical movement, anabolic reactions, and active transport.

Coenzymes synthesized from the vitamins niacin and riboflavin function as electron carriers and are involved in transferring electrons (and/or hydrogen atoms) from one substance to another in energy metabolism. As fuels are oxidized, high-energy electrons are transferred to these coenzymes. It is the reduction of coenzymes that conserves much of the chemical bond energy that is released during the oxidation of our metabolic fuels. The reduced coenzymes shuttle their high-energy electron cargo to the electron transport chain (ETC) in mitochondria where a series of electron-carrier molecules are embedded in the inner mitochondrial membrane. As electrons are passed from one molecule to another, energy is released. This energy can be used to synthesize ATP. See the Overview of Energy Metabolism illustration.