Classification of Carbohydrates and Its Structure

What are Carbohydrates? - Examples and Structure of Carbohydrates


(CH 2O)n is the general empirical structure of carbohydrates. Monosaccharides are simple sugars which serve as fuel molecules and fundamental constituents of living organisms and these are the simplest carbohydrates required as the energy sources. Glucose and fructose are the most commonly known carbohydrates.

The saccharides or carbohydrates are simply defined as ketones or polyhydroxy aldehydes or the compounds that produce units of such type on hydrolysis.

Few common types of carbohydrates are milk, bread, popcorn, potatoes, maze, etc.

The carbohydrates are largely distributed in both plant and animal tissues. Carbohydrates occur mainly in the form of glycogen and glucose in animal cells and as cellulose and starch in the plant cells.

Classification and structure of carbohydrates:

Generally, carbohydrates are classified into three major groups. They are as follows:

  • 1. Monosaccharides:

  • Monosaccharides are simple sugars. They cannot be hydrolyzed into a simpler form. The simplest carbohydrates are the three carbon dihydroxyacetone and trioses glyceraldehyde. They are further classified into glucose, fructose, galactose, and mannose. 

  • Glucose:

  • Glucose can be seen generally in the fruit juices and formed in the body by hydrolysis of cane sugar, starch, lactose, and maltose. Glucose is said to be the sugar of the body. Glucose structure can be depicted in the form of a ring or chain. It is found in blood, fruits, honey and under abnormal conditions, in urine.

  • Fructose:

  • Fructose can be seen naturally in honey, tomatoes, and apples. Hydrolysis of cane sugar in the body can also give up fructose. C6H12O6 is the molecular formula for fructose. Generally, fructose is the sweetest monosaccharide and is prepared by sucrose hydrolysis.

  • Galactose:

  • An element of glycoproteins and glycolipids is the galactose. It is produced in the mammary glands and hydrolyzed to make the lactose of milk.

  • Mannose:

  • On the hydrolysis of plant gums and mannosans, mannose is obtained. A constituent of the prosthetic polysaccharide of albumins, mucoproteins, and globulins is a mannose. Hexoses and pentoses exist in both ring and open chain forms.

    According to the number of carbon atoms they possess, simple sugars might be further divided into tetroses, trioses, hexoses or heptoses, pentoses and as ketoses or aldoses based on whether the ketone or aldehyde groups are present. For example:
    (C3H6O3) Trioses GlyceraldehydeDihydroxyacetone
    (C4H8O4) TetrosesErythrose Erythrulose
    (C5H10O5) PentosesRiboseRibulose
    (C6H12O6) HexosesGlucoseFructose

  • a. Trioses:

  • Trioses are formed throughout the metabolic breakdown of the hexoses in the body. Example: dihydroxyacetone and glyceraldehydes.

  • b. Pentoses:

  • Pentoses are vital constituents of many coenzymes and nucleic acids. They are also formed as transitional throughout certain metabolic processes. Example: nucleic acids and coenzymes NAD, Ribose that is a structural element of ATP and flavor proteins: Arabinose, ribulose, and xylose.

  • 2. Disaccharides:

  • Disaccharides comprise of two monosaccharides connected by a glycosidic linkage (C-O-C). Cn (H2O)n-1is the general formula for disaccharides. The most common disaccharides forms are lactose, sucrose, and maltose. 

  • Maltose:

  • Maltose is formed as a transitional product of the action of amylases on starch and it contains two glucose residues in 1, 4 linkages. It can be seen in a detectable amount in many germinating tissues and seeds where starch is being broken down.

  • Lactose:

  • Lactose can be found in milk. On hydrolysis, it produces D-galactose and is a reducing disaccharide, as it has a free anomeric carbon on the glucose residue.

  • Sucrose:

  • Cane sugar or sucrose is a disaccharide of fructose and glucose. The hydrolysis of sucrose to D-glucose and D-fructose is often known as inversion as it is accompanied by a net change in optical rotation from dextro to levo as the equimolar mixture of fructose and glucose is formed and this mixture is known as invert sugar. Certain enzymes like invertases catalyze this reaction. Sucrose is tremendously abundant in the plants and is commonly known as table sugar.

  • Trehalose:

  • Trehalose possesses two D-glucose residues and it is a non-reducing disaccharide like that of sucrose. It is the main sugar that can be seen in many of the insects' haernolymph.

  • 3. Trisaccharides:

  • Plenty of trisaccharides occur free in nature. Raffinose can be seen abundantly in many higher plants and sugar beets. Melezitose can be seen in the sap of a few coniferous trees.

  • 4. Polysaccharides:

  • Majority of the carbohydrates that can be found in nature take place as polysaccharides of high molecular weight. Polysaccharides are complex carbohydrates that are formed by the method of polymerization of a huge number of monosaccharide monomers. The other name for polysaccharides is also known as glycans.

    They are lengthy which might be unbranched or branched. After the completion of hydrolysis with specific enzymes and acid, the polysaccharides give up simple monosaccharide derivatives and/ or monosaccharides. Depending upon the composition, polysaccharides can be classified into two types: Homopolysaccharides and heteropolysaccharides.

  • a. Homoglycans or homopolysaccharides are the complex carbohydrates that are formed by the method of polymerization of only one type of monosaccharide monomers. Example: glycogen, starch, and cellulose are composed of a single type of monosaccharide known as glucose.

  • On the basis of monosaccharide unit, the polysaccharide is known as glucan which is made of glucose, xylan which is having xylose, fructan that is made of fructose, galactan that is formed of galactose, araban that is made of arabinose, etc.

  • b. Heteroglycans or heteropolysaccharides are the complex carbohydrates that are made by the process of condensation of either one type of monosaccharide monomer or monosaccharide derivatives. Example: agar, chitin, arabanogalactans, peptidoglycan, arabanoxylans, etc.

  • Examples of carbohydrates:

    Fruits: whole fruit, fruit juice
    Legumes: beans, plant-based proteins
    Dairy products: ice cream, yogurt, milk
    Starchy vegetables: potatoes, corn
    Grain: wheat, cereal, rice