Definition

What are carbohydrates? The formal definition is a class of substances having the formula C n(H2O)n, i.e., the molar ratio of C:H:O is 1:2:1. This definition, however, fails for oligosaccharides, polysaccharides, and sugar alcohols (viz., sorbitol, maltitol, mannitol, galactitol, and lactitol). Of the complex carbohydrate macromolecules known, the main member is plant starch (and the animal polymer glycogen), but the group includes pectins, cellulose, and gums. Simple carbohydrates include the hexose monosaccharides (glucose, galactose, and fructose) and the disaccharides maltose (glucose-glucose), sucrose (glucose-fructose), and lactose (glucose-galactose). Other carbohydrates include trioses (glycerose, C 3H6O3), tetroses (erythrose, C4H8O4), and pentoses (ribose, C5H1CO5). The latter are important constituents of nucleic acids. In general, oligosaccharides yield two to ten monosaccharides on hydrolysis, while polysaccharides yield more than ten. The polysaccharides serve storage and structural functions. Starch is the storage carbohydrate of plants; glycogen is that of animals (liver contains up to 6% and muscle about 1%) and is often called animal starch. There are many different types of starch, depending on the plant source. Inulin, for example, is a starch found in the tubers and roots of dahlias, artichokes, and dandelions and when hydrolyzed yields only fructose; hence it is a fructosan. The oligosaccharide cellulose consists of glucose units linked by b(1-4) bonds to form long straight chains strengthened by hydrogen bonding. It is the chief structural framework of plants and cannot be easily digested by humans because they do not secrete an intestinal carbohydrase that attacks the b(1-4) linkage. It is often thought of as a dietary fiber that gives bulk to food. Bacterial enzymes, however, can break down cellulose. A small amount is probably hydrolyzed by this process in the colons of humans, but it has little nutritive value.

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