Qualitative Tests for Carbohydrates

Carbohydrates are a class of organic molecules with the general chemical formula Cn(H2O)n. These compounds are literally carbon hydrates. Only the monomeric form of these compounds, the monosaccharides, fit this description precisely. Two monosaccharides can be polymerized together through a glycosidic linkage to form a disaccharide. When a few monosaccharide molecules are polymerized together, the result is an oligosaccharide. A polysaccharide is an extensive polymer of carbohydrate monomers.

The monosaccharide glucose is our primary energy source. The function of the polysaccharides starch (plants) and glycogen (animals) is to store glucose in a readily accessible form, as well as to lower the osmotic potential of internal fluids. Some polysaccharides serve a structural role in living organisms. The glucose polymer cellulose is a major component of plant cell walls. Chitin, a polymer of N-acetylglucosamine, is a major structural component of the exoskeleton of insects and crustaceans. Hyaluronic acid and chondroitin sulfate occurs in the connective tissues of animals, especially in cartilage. Oligosaccharide side chains of glycoproteins may also serve as signals for intracellular sorting of the protein (i.e. mannose-6-phosphate signal designating lysosomal enzymes).

Several qualitative tests have been devised to detect members of this biologically significant class of compounds. These tests will utilize a test reagent that will yield a color change after reacting with specific functional groups of the compounds being tested. The following exercises are reactions that can detect the presence or absence of carbohydrates in test solutions. They range in specificity to the very general (i.e. Molisch test for carbohydrates) to the very specific (i.e. mucic acid test for galactose).