Classification of carbohydrates

Classification of carbohydrates

Carbohydrates are classified according to the number of saccharide (sugar) groups present. They are broadly classified as simple carbohydrates and complex carbohydrates. The simple carbohydrates includemonosaccharides(Singlesugar)and disaccharides(Doublesugars). Complex carbohydrates include starch,glycogen and fibers. The classification of carbohydrates is schematically represented below:

Simple Carbohydrates

A) Monosaccharides

They have one saccharide group and are the simplest form of carbohydrates. All carbohydrates are reduced to this state before absorption and utilization. They contain 3-6 carbon atoms and are accordingly termed triose, tetrose, pentose or hexose.

1.      Biose: C₂H₄O₂ (e.g.)Glycolic aldehyde

2.      Triose: C₃H₆O₃ (e.g.)Glyceraldehyde and Dihydroxyacetone. They occur in plant and animal tissues in small amounts and are derived from the breakdown of glucose.

3.      Tetroses: C₄H₈O₄ (e.g.) Erythrose, Threose

4.      Pentoses: C₅H₁₀O₅(e.g.) Arabinose, Xylose, Ribose and Deoxyribose.

5.      Hexoses: C₆H₁₂O₆ .They are further sub-divided into 2 groups(i.e) Aldoses or sugars containing aldehyde group(e.g.) Glucose, galactose and mannose as well as Ketoses or sugars containing ketone group(e.g.) Fructose.

Major monosaccharides

a.       Glucose (Dextrose or grape sugar): It serves as the main source of energy in the body. It is abundantly found in nature. It is found in sweet fruits such as grapes, berries, oranges in vegetables like sweet corn and carrots. It is less sweet than cane sugar. It is the end product in the digestion of disaccharides and polysaccharides and is the form of carbohydrate circulating in the blood.

b.      Fructose(Levulose or fruit sugar): It is much sweeter than cane sugar and is found in honey,ripe fruits and some vegetables.It is also a product of the hydrolysis of sucrose.

c.       Galactose: It does not occur in the free state, but occurs as a constituent of lactose present in milk.

B) Disaccharides

They are formed by the combination of 2 monosaccharides. The disaccharides of nutritional importance are sucrose, maltose and lactose.

a. Sucrose(Cane sugar, beetsugar, tablesugar): It occurs in sugarcane (10-12%) and beetroot(12-18%).In the intestine, sucrose is broken down into monosaccharides -glucose andfructose by the enzyme sucrase present in the intestinal juice and then absorbed.

b. Lactose(Milk sugar): It occurs in the milk of mammals. Cow’s milk and buffalo’s milk contain 4% of lactose, while human milk contains about 7% of lactose. Lactose is hydrolysed to glucose and galactose by the enzyme lactase present in the intestinal juice

c. Maltose (Malt sugar): It is found in all sprouted and malted products.It is an intermediate product formed in the process of conversion of starch into glucose. Maltose is hydrolysed to 2 molecules of glucose by the enzyme maltase present in the intestinal juice. Sprouted cereals and beer contain large amount of maltose.

Complex Carbohydrates

These are complex compounds with high molecular weights. Their  structural

formula is (C₆H₁₀O₅)n , where n>2.They are formed by a combination of more than 2 molecules of a monosaccharide. Unlike the sugars,which contain 3 monosaccharides – Glucose, fructose and galactose in different combinations, the polysaccharides – Starch and Glycogen are composed entirely of glucose. They differ from each other only in the nature of the bonds that link the glucose units together.

1. Starch : It is a long, straight or branched chain of hundreds of glucose units linked together. The important sources of starch are cereals and millets(65-85%) and roots and tubers(19-35%). Starch is a polysaccharide formed in nature by the condensation of large number (4000-15000) of glucose molecules. It consists of a mixture of 2 components called amylase and amylopectin.It is the storage form of carbohydrate in the plant kingdom. Cooking facilitates the digestion of starch. Boiling causes swelling of the starch granules and rupture of the cell walls, allowing better digestion. The enzyme amylase present in the salivary and pancreatic juices, converts starch into maltose which is subsequently broken into glucose and absorbed.

2. Dextrin: It is not found in direct form in nature. They are polysaccharides formed by the partial hydrolysis of starch by acids or amylase. They are composed of large number of glucose molecules.

3. Glycogen: It is made up of chains of glucose,which are more highly branched than starch molecules.It is the storage form of carbohydrates in human beings and animals. It is formed by  the  condensation of large number (5000-10000) of glucose molecules.When required by the body, glycogen is converted to glucose to give energy.