All living things require an outside source of materials and energy. The sum total of all chemical reactions that occur within a cell to provide these materials and energy is called metabolism.
Energy is the ability to perform work. In an organism's body energy is required to perform each and every processes inside the body. In eukaryotic organisms this energy is derived by the mitochondria. In humans and various other species, the major source of energy is ATP.
Metabolism is the sum total of all the reactions taking place inside the body. These reactions can be anabolic or catabolic. The anabolic reaction involve the formation of larger molecules from the smaller ones. The catabolic reactions involve the breakdown of larger molecules into smaller ones.
