Carbohydrates are generally composed of Carbon, Hydrogen and Oxygen in the ratio of 1:2:1. It is commonly represented as (CH2O)n. Carbohydrates are simply known as “Sugars” made of carbon compounds that has large amount of hydroxyl groups and has either aldehyde or ketone as functional group. They form the most fundamental class of biomolecules.1 These are found to be the basic component in almost all the biological systems. One of the familiar form of carbohydrates sugar is known as Glucose (fig-1), a type of sugar found in human plasma 2. They are the major and primary synthesized by plants via photosynthesis. They have both structural as well as functional roles to play in all biological systems. Some examples of the naturally occurring carbohydrates are Cellulose (in plants) and Chitin (exoskeleton component in insects).
BASED ON FUNCTIONAL GROUPS
A sugar contains aldehyde as a functional group is said to be Aldoses (eg. Glucose, galactose, ribose and glyceraldehyde). Similarly a sugar having ketone as a functional group is called as Ketoses (eg. Fructose, ribulose, erythrulose). A sugar contains a hemiacetal or hemiketal groups is said to be reducing sugars (eg. Sugars include, glucose, galactose, fructose, maltose, lactose). All aldoses are reducing sugars. Many ketoses are also considered as reducing sugar as they can isomerise to aldose.1 A sugar having no hemiacetal group is said to be non-reducing sugar (eg. Sucrose and all polysaccharides are in this group).3
BASED ON NUMBER OF CARBONS
Sugars can be classified based on the number of carbon atoms present in the chain. They are as follows,
No of carbon atoms Common name Examples
5 Pentose Ribose, Ribulose, Xylulose.
6 Hexose Glucose, Galactose,
9 NanoseNeuraminic acid, also called as Sialic acid.
Table 1: Classification of Carbohydrates.
Lehninger, Albert L. Principle of Biochemistry. Delhi: Publishers & Distributors, 1984.
Amit Arora. Textbook of sugar chemistry. Delhi: Publishers & Distributors, 2011.