These highly branched polysaccharides serve as a direct reserved source of energy in the muscles. In the liver they serve as a reserve source to maintain the blood sugar level. This ester is called a triglyceride and its structural formula is shown in Figure 14.24. All are polymers. Polymers are huge, chainlike molecules made of hundreds or thousands of smaller, repeating molecular units called monomers.
Polymers occur naturally in plants and animals.
Cellulose, for example, is a natural plant polymer made of glucose monomers. Wool and hair are natural animal polymers made of protein monomers. Synthetic polymers are manufactured from a wide variety of substances. Polyethylene is a familiar polymer used for vegetable bags, dry cleaning bags, grocery bags, and plastic squeeze bottles. Polyethylene is a polymer produced by a polymerization reaction of ethylene, which is derived from petroleum.