Brief notes on the composition and classification of Carbohydrates




Carbohydrates are made up of carbon, hydrogen and oxygen. The rate between the numbers of hydrogen and oxygen atoms in the molecules is always 2:1 i.e. the same as water. It is because of this peculiar composition that these compounds are called carbohydrates, which means hydrates of Carbon, Starch and Cellulose are complex carbohydrates. The energy value of Carbohydrates is 4 calories of energy per gram.


1. Monosaccharide's:

Carbohydrates are classified, according to the number of single carbohydrate units found in each chemical structure as:

Monosaccharide's or simple sugars are compounds that cannot be hydrolyzed to simpler compounds. Glucose, fructose and galactose are three nutritionally important Carbohydrates in this category.

Glucose also known as dextrose, grape sugar or cow sugar is mostly found in nature. In plants, it is found in sweet than cane sugar and is abundant in grapes, berries and oranges and in some vegetables like sweet corn and carrots. It is also prepared commercially as corn syrup, honey and molasses. In the animal body, it is the end product of the digestion of starch, sucrose, maltose and lactose.

Fructose, called as fruit sugar is a highly soluble sugar and does not readily crystalize. 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.

Galactose does not occur freely in nature. It is produced in the body during the digestion of disaccharide lactose.

2. The Disaccharides:

The disaccharides or double sugar results when- two hexodes are combined with the loss of one molecule of water. They are water soluble and can split into simple sugars by acid hydrolyses or by digestive enzymes. Sucrose, Maltose and Lactose are disaccharides present in the food.

Sucrose is the table sugar we commonly use in our homes. This is produced from sugar cane and sugar beets. Many fruits and vegetable also contain considerably small amount of sucrose.

Lactose or milk sugar is produced by mammals and is the only dietary carbohydrate of animal origin. Human milk contains 6.8% of lactose, whereas, cow's milk contains 4.9%. When lactose is hydrolised, galactose and glucose are formed.

Maltose or malt sugar does not occur in the average diet. It occurs in sprouting grains, malted cereals and malted milk. Among sweetening agents, this is found in corn syrup and corn sugar. It is also used with dextrins as the sources of carbohydrate for some infant formulas.

3. The Polysaccharides:

Polysaccharides are complex compounds with a relatively high molecular weight. They contain as many as 2000 simple carbohydrate units arranged in long chains of either straight or branched structure. They are amorphous rather than crystalline, are not sweet and are insoluble in water. Starches, dextrin's, glycogen are nutritionally important polysaccharides.

Starch is one of the most important carbohydrates in the human diet. It is the storage form of carbohydrate in the plants. The seeds of plants like corn millet, rye, rice and wheat are the richest store house of starch, as much as 70%. The starch is found in the dried seeds of leguminous plants as beans, peas etc. is about 40%. Starch is insoluble in water but when boiled with water forms a paste. Cooked starch is more palatable and easily digestible.

Dextrin is intermediate products in the hydrolysis of starch and consists of shorter chains of glucose units. Some dextrin is produced when flour is browned or bread is toasted.

Glycogen the so-called 'animal starch’ is the stored polysaccharides in animals. It is found in the liver of all animals and in small aquatic animals. On hydrolysis, glycogen also gets converted into glucose. Only about 350 gms or 3/4 lb of glycogen is found in the body, as reserve, as the body cannot store glycogen in the same way as it stores fat.

Cellulose is the most abundant organic compound in the world and is the structural constituent of the plant cell. It is indigestible polysaccharides and is not important from nutritional point of view. But it is significant in the human diet, as it provides a source of roughage in the intestinal tract, which helps in normal, easy bowel movements. Dried fruits, whole grain cereals, nuts, fresh fruits and vegetables are the best sources of cellulose.