Classification of Carbohydrates

Carbohydrates are also termed as saccharides and sugars. The word saccharide is derived from a Latin word “saccharum” meaning sugar. Carbohydrates are classified as either simple or complex sugars. Simple sugars are also termed as monosaccharides, and they cannot be further hydrolyzed into smaller sugars. Common examples of monosaccharides include glucose and fructose. On the other hand, complex sugars are made up of two or more monosaccharides linked to each other via acetal bonds. Common examples of complex sugars include sucrose (made up of glucose and fructose molecules) and cellulose (made up of several thousand glucose molecules).

Monosaccharides:

The monosaccharides are further classified as polyhydroxy ketones (ketoses) or polyhydroxy aldehydes (aldoses). Monosaccharides can have three carbon atoms (trioses), four carbon atoms (tetroses), five carbon atoms (pentoses), six carbon atoms (hexoses), seven carbon atoms (heptoses) etc.

Disaccharides:

Disaccharides upon hydrolysis produce two monosaccharides. Both monosaccharides and disaccharides are highly water soluble and have characteristic sweet taste. Most common example of disaccharide includes Sucrose which is also called table sugar. Sucrose on hydrolysis produces glucose and fructose molecules.

Other examples of disaccharides include lactose, maltose, trehalose, cellobiose, chitobiose etc.

Oligosaccharides:

Oligosaccharides are typically made up of three to ten monosaccharides. There are references which consider oligosaccharides comprising two to ten monosaccharides including disaccharide as oligosaccharide. Few examples of oligosaccharides are depicted in figure 3.

Polysaccharides:

Polysaccharides upon hydrolysis produce many (over thousand) of monosaccharides. They are naturally occurring biopolymers of carbohydrates. Typical examples include starch and cellulose made up of glucose monomers. Polysaccharides are further classified as homopolysaccharides and heteropolysaccharides.

Homopolysaccharides are made up of the same type of monosaccharides. Examples include glycogen (made up of α-glucose), cellulose (made up of β-glucose) and starch (α-glucose). Starch is further classified as amylose and amylopectin. Amylose is a straight chain polymer of α-glucose monomer units while amylopectin is a branched chain polymer of α-glucose monomer units (Figure 4).

Heteropolysaccharides are made up of different types of monosaccharides. Examples include hyaluronic acid (made up of D-glucuronic acid and N-acetyl-glucosamine), heparin (made up of D-glucuronic acid, L-iduronic acid, and N-sulfo-D-glucosamine).