The purpose of the digestive system is to break down food into simple molecules that the cells of the body can use.
Food is broken down into its simplest units called monomers. Proteins are broken down into amino acids, fats are broken down into fatty acids and glycerol while carbohydrates are broken down into glucose units.
Glucose is taken in by the cells of the body and through the process of cellular respiration is used to produce the ATP, the form of energy that cells use. Cellular respiration is the process by which cells use glucose to produce energy to power their functions.
~Not sure if this will help you, but I hope it did~