Food molecules contain chemical energy which is released when its chemical bonds are broken.
Food undergoes digestion in the digestive system and is broken down into its monomer units. Carbohydrates, which are the principal source of energy, are broken down into glucose. Glucose is the monomer unit of carbohydrates.
Glucose then is taken up by cells and is used in cellular respiration, which involves three main stages namely glycolysis, Krebs cycle and the electron transport chain, through which a glucose molecule is processed to form at least 36 molecules of ATP.
ATP is the form of energy that cells use.