Chemistry of Carbohydrates

Carbohydrates are compounds composed of carbon, hydrogen, and oxygen. These molecules get their name from the empirical formula of many monosaccharides that have two atoms of hydrogen and one atom of oxygen for every carbon.

Simple carbohydrates are monomers called monosaccharides and dimers called disaccharides. Complex carbohydrates are polymers called polysaccharides.

A monosaccharide can be classified as an aldose or a ketose by its functional group. Those having an aldehyde group are called aldoses and those having a ketone group are called ketoses.

Monosaccharides usually contain three to seven carbon atoms in their chains. Each of the carbons can be numbered starting with the number one from the end closest to the carbonyl carbon.

Sugars can form non-planar ring structures and in nature mostly exist in this cyclic form. Ring structures form when the carbonyl group reacts with a hydroxyl group at the opposite end of the molecule through a condensation reaction. The carbon attached to the reactive aldehyde or ketone is also known as the anomeric carbon.

These carbon-oxygen ring structures can exist as two anomers: alpha and beta. Anomers are diastereomers that differ in the configuration at the carbonyl carbon. In the alpha form, the hydroxyl group on the anomeric carbon and the CH₂OH group on the last carbon within the ring are located on opposite sides of the ring, and in the beta form, they are located on the same side.

A similar naming convention is used for bonds linking the monosaccharides to other molecules. Bonds formed below the ring structure are known as alpha glycosidic bonds, whereas those above the ring are known as beta glycosidic bonds. The numbers in the name of glycosidic linkages indicate the carbon numbers from the monosaccharides that are involved in the linkages.

When two monosaccharides link together through glycosidic linkages, they form disaccharides.

Glycosidic linkages are covalent bonds that are formed when the hydroxyl group of one monosaccharide reacts with the anomeric carbon of another monosaccharide with the elimination of a water molecule.

Polysaccharides or complex carbohydrates are a chain of monosaccharides held together by covalent bonds. Variation in the glycosidic linkages present in these molecules can result in different types of polysaccharides from the same monosaccharide.

Glucose molecules linked through β-1,4-glycosidic bonds form cellulose, a water insoluble polysaccharide. Glucose molecules linked through α-1,4-glycosidic bonds form amylose, a water-soluble polymer and a component of starch. Starch and cellulose are polysaccharides found in plants.

Glycogen is a water-insoluble molecule made of glucose linked through α-1,4-glycosidic bonds and intermittent branching through α-1,6-glycosidic bonds. Glycogen serves as a storage polysaccharide in animals.