In nature, most carbohydrates exist as polymers, rather than monomers. Polysaccharides are one of the most abundant carbohydrates. They're polymers consisting of ten or more simple sugar units. They're composed of thousands or hundreds of thousands of simple sugars bonded together, and can have molecular masses as high as 100 million atomic mass units. The function of a polysaccharide is determined by the monomers it contains, and by the way they're linked together. When many monosaccharides combine in condensation reactions, the product is a polysaccharide. The other products are molecules of water that are removed in order to join one monosaccharide molecule to the next.

Let's look at some common polysaccharides. Cellulose is a polysaccharide found in all plants. Cellulose is the most abundant natural polymer in the world. Cellulose is made of beta-glucose monomers, where the O-H group on carbon one points up. The beta-glucose monomers in cellulose produce a nearly-linear molecule. The O-H groups on the glucose monomers can form hydrogen bonds to O-H groups on another cellulose molecule. These bonds enable cellulose strands to form fibers that give plants and trees a rigid structure.

Other polysaccharides can also be used for structural support. Invertebrate animals like snails, shellfish, and insects have exoskeletons made of a substance called chitin. The monomers in a chitin molecule are a beta-glucose derivative with a different functional group at carbon 2. The beta linkages between monomers mean that chitin is structurally similar to cellulose. As we can see here, both molecules consist of linear strands that can form hydrogen bonds to other strands.

Starch is another plant polysaccharide. Plants produce starch to store energy, but it's also an important source of energy for organisms that eat those plants. Starch is made of alpha-glucose monomers, where the OH group on carbon one points down.

Even though starch and cellulose are made from nearly-identical monomers, each has a unique structure that determines its properties and function. The alpha-glucose monomers in starch produce a helical, compact structure that's used for energy storage in plants. A similar molecule called glycogen is found in animal cells that need to store a lot of energy, like muscle cells. Glycogen is a polymer of alpha-D-glucose, with frequent branches off of carbon six. Since glycogen is even more dense than starch, it's a more efficient form of energy storage for organisms that move around.

The important thing to remember about polysaccharides is the relationship between their structure and function. Polysaccharides generally perform one of two functions: energy storage or structural support. Starch and glycogen are highly compact polymers that are used for energy storage. Cellulose and chitin are linear polymers that are used for structural support in plants and animals, respectively.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education