Organisms obtain most calories from fats, proteins, disaccharides and polysaccharides.
These complex molecules must be enzymatically hydrolyzed into simpler molecules or monomers that can enter an intermediate reaction of glycolysis or the Krebs cycle. Glycolysis can accept a wide range of carbohydrates for catabolism. Starch is hydrolyzed to glucose in the digestive tract of animals. In between meals, the liver hydrolyzes glycogen to glucose. Enzymes in the small intestine break down disaccharides to glucose or other monosaccharides.