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Classification of Carbohydrates and its Structure


Carbohydrates are a distinct group of organic compounds which occurs in living tissues and food in the form of sugar, starch and cellulose. The ratio of oxygen and hydrogen in carbohydrates is the same as water i.e., it is 2:1. It is typically broken down in the animal body to release energy. Cn(H2O)m is the generic formula for all the existing carbohydrate groups. This formula is only valid for simple sugars, which are made up of the same amount of carbon and water.

Prior to this, the term carbohydrate was used to describe compounds that were literally carbohydrates, because they had the empirical formula CH2O. Carbohydrates have been classified in recent years on the basis of carbohydrate structures, not their formulae. Such aldehydes and ketones are now known as polyhydroxy. Cellulose, starch, and glycogen are amongst the compounds that belong to this family. Chemically, carbohydrates are defined as the optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis.

The substance referred to as sugar is the sucrose disaccharide, which is extracted from both sugar cane or beets. Sucrose is the disaccharides that is known for its sweetness. It is approximately three times sweet as maltose and six times sweet as lactose. In the recent decade in many consumer products, sucrose has been replaced with corn syrup. Corn syrup is obtained when the polysaccharides in cornstarch are broken down. Corn syrup is primarily glucose and it is as sweet as sucrose for only about 70%.

According to the classifications, carbohydrates were defined as compounds with the empirical formula of Cn(H2O)m. The common sugars such as glucose, fructose or sucrose fit this formula, but recently, the convention is to regard as a carbohydrate, polyhydroxy aldehydes or a polyhydroxy ketone with the classical formula, a molecule closely related to it, oligomers or polymers of such molecules. Their study evolved as a separate discipline within organic chemistry for practical reasons. They are water soluble and difficult to crystallize so that their manipulation demanded different sets of skills from classical natural products such as steroids, alkaloids, terpenes and so on.


The term monosaccharide refers to a carbohydrate derivative possessing a single carbon chain. However, disaccharide and trisaccharide refer to molecules containing two or three such monosaccharide units joined together by acetal or ketal linkages respectively. Oligosaccharide and polysaccharide refer to larger such aggregates, with a few and many monosaccharide units, respectively. Current usage seems to draw the distinction between few and many at around 10 units.

In the middle of the nineteenth century, a group of relatively pure carbohydrates such as sucrose, cellulose, starch, glucose, fructose, mannose and lactose were known to the chemists of Europe, especially in Germany. In 1878, Emil Fischer synthesized phenyl hydrazine for his thesis at the University of Munich. In 1884 he further discovered that carbohydrates gave crystalline phenylosazone in which two phenyl hydrazine reacted with the aldehyde group and the carbon adjacent to the aldehyde group.

Classification of Carbohydrates

The different types of carbohydrates can be classified on the basis of their behavior on hydrolysis. They are mainly classified into three groups as follows:

Monosaccharides: Monosaccharide carbohydrates are the simplest carbohydrates that cannot be hydrolyzed further to give simpler units of polyhydroxy aldehyde or ketone. If a monosaccharide contains an aldehyde group, then it is called aldose and alternatively if it contains a keto group then it is called a ketose.

Disaccharides: Upon hydrolysis, disaccharides yield two molecules of either the same or different monosaccharides. The two monosaccharide units are joined by oxide linkage which is formed by the loss of water molecules and this linkage is called glycosidic linkage. Sucrose is one of the most common disaccharides which on hydrolysis gives fructose and glucose. Maltose and Lactose is also known as milk sugar are the other two important disaccharides. In maltose, there are two α-D-glucose and similarly in lactose, there are two β-D-glucose which are connected by an oxide bond.

Polysaccharides: Polysaccharides are compounds that contain long monosaccharide units joined together by glycosidic linkage. Most of them act as food storage for instance, starch. Starch is the main storage polysaccharide for plants. It is a polymer of α-glucose and consists of two components namely Amylose and Amylopectin. Cellulose is also one of the polysaccharides that are significantly found in plants. It is composed of β-D-glucose units joined by a glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.





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