Carbohydrate biomolecules generally have a basic structural formula that is written as C(H2 O)n. For example, glucose has the structural formula C6 H12 O6 . The carbohydrates have many functions in cells. They can be used for energy storage (e.g., starch) and as structural molecules (e.g., cellulose). And as we will see, they also play a role in genetic information storage in nucleic acids (organic compounds that make up RNA and DNA).
The basic monomer of carbohydrates are the simple sugars (monosaccharides ). Monosaccharides include glucose (also known as dextrose), fructose, ribose, galactose, and ribulose. Note, the -ose suffix is used by chemists to describe all carbohydrates.
A disaccharide is a sugar formed by a condensation reaction between two monosaccharides. Condensation synthesis of maltose (glucose + glucose), a common disaccharide, is illustrated in the following animation. Other disaccharides include sucrose (glucose + fructose) and lactose (glucose + galactose); sucrose is common table sugar, and lactose is milk sugar. Many disaccharides are sweet tasting and are used as a source of energy for cell metabolism.
More complex forms of carbohydrates can be synthesized by cells via additional condensation reactions. Polysaccharide polymers can consist of up to several thousand monomers of simple sugars. The type of polysaccharide is determined by the number, type, and arrangement of its monomers. Starch is a polymer of glucose monomers. It is used by plants to store surplus sugar and when we and other animals eat a starchy plant (such as a potato) we use the starch for energy and nutrients. Animals store surplus sugar in their livers in the form of a molecule known as glycogen.
Cellulose, another polymer of glucose, is an important component of plant cell walls; it is also the most abundant polymer on the planet. Although humans cannot digest cellulose, it is a valuable part of our diet because this "fiber" helps to maintain a healthy digestive tract. Some animals, such as the crab shown below, and fungi use another polysaccharide, chitin, for structural purposes.
This video discusses the basics of carbohydrate structure and function:
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