A molecule of triglyceride is composed of a molecule of glycerol in which each of the three carbons is linked through an ester bond to a fatty acid. Triglycerides cannot be efficiently absorbed, and are enzymatically digested by pancreatic lipase into a 2-monoglyceride and two free fatty acids, all of which can be absorbed. Other lipases hydrolyse a triglyceride into glycerol and three fatty acids. The diversity of dietary carbohydrates necessitates discussion of several classes of these molecules, ranging from simple sugars to huge, branched polymers. These are the breakdown products of more complex carbohydrates and can be efficiently absorbed across the wall of the digestive tube and transported into blood.
Disaccharides are simply two monosaccharides linked together by a glycosidic bond.
Polysaccharides are the most abundant dietary carbohydrate for all except very young animals. Cellulose is the other major plant carbohydrate. It is the major constituent of plant cell walls, and more than half of the organic carbon on earth is found in cellulose. Herbivores subsist largely on cellulose, not because they can digest it themselves, but because their digestive tracts teem with microbes that produce cellulases that hydrolyze cellulose. Glycogen is the third large polymer of glucose and is the major animal storage carbohydrate.