Carbohydrates are composed of carbon, hydrogen and oxygen. They serve as the primary energy source for cells. There are three main types of carbohydrates: monosaccharides, disaccharides, and polysaccharides. Polysaccharides include starch and glycogen which are made of chains of glucose units with branch points. Carbohydrates are broken down into glucose through digestion and various enzyme-catalyzed pathways in the body like glycolysis and the citric acid cycle to generate energy in the form of ATP. The metabolism of carbohydrates is tightly regulated by hormones like insulin.