Define carbohydrate in biochemistry?

CARBOHYDRATES

Carbohydrates are the most important organic molecule in nature. Carbohydrates are the polyhydroxy aldehyde or ketone. They are primarily composed of hydrogen, carbon and oxygen. The name carbohydrates mean hydrates of carbon. Carbohydrate possesses empirical formula (CH2O) n. They occur in all living organisms in every part of the cell. Some carbohydrates like cellulose, chitin forms cell wall in plants and microorganisms. When carbohydrates combine with proteins, they form glycoprotein and when carbohydrates combine with lipids then they form glycolipids. These are the conjugate compounds which play an important role in cell.  

CLASSIFICATION 

 Classification is based upon number of atoms. Carbohydrates are classified into three groups on the base of number of carbon atoms or subunits.

• Monosaccharide
• Oligosaccharide
• Polysaccharides 

MONOSACCHRIDES 

These are simple sugar and cannot be further hydrolysed into simple sugar. These have general formula (CH2O) n where n is number of carbons which may be equal to 3 or more than 3. Monosaccharide is further classified on the base of functional group and number of carbons.

Aldoses 

If the functional group in monosaccharide is an aldehyde, then they are known as Aldose (-CHO) e.g., glucose.

Ketoses 

If the functional group in monosaccharide is a ketone, then they are known as ketoses (-CO) e.g., glucose.

OLIGOSACCHRIDES 

The on hydrolysis produce 2 to 10 monosaccharides.

 For example:

•  Maltose (glucose + glucose)
• Sucrose (glucose + fructose)

In oligosaccharides, the monosaccharide is joint to each other through glycosidic linkage (H2O release).

POLYSACCHARIDES 

As the name indicates polysaccharides contain large number of monosaccharides. These have high molecular weight. They are usually tasteless. They serve as structural compound of the cell.

For Example 

• Starch
• Glycogen

STARCH

It is the reserve carbohydrate of plant kingdom. Starch is composed of amylose and amylopectin.

SOURCES

• Potatoes 
• Cereals 
• Other food grains

AMYLOSE 

Amylose is the straight chain polymer 15-20% of the starch, 60-300 glucose units that are joined by 1,4-glycosidic bonds.

AMYLOPECTIN 

Amylopectin is the branched chain polymer, 80-85% of the starch, composed of 300-6000 glucose units joined primarily by 1,4-glucosidic bonds and occasionally by 1,6-glucosidic bonds.

 

PHYSICAL PROPERTIES 

White amorphous powder having no taste and no odour. Its granules consist of two polysaccharides units that is analyse and amylopectin insoluble in water but swells up to form a gel in water.

CHEMICAL PROPERTIES 

Reaction with iodine Soluble Starch gives blue colour with iodine. The colour disappears on heating to 80 Celsius and reappears on cooling.

GLYCOGEN 

Glycogen is the carbohydrate reserve in animals called as animal starch. Glucose storage molecule of animals. Stored in granules in liver and muscle cells. Like amylopectin, is a nonlinear polymer of glucose units joined by 1,4 and 1,6-glycosidic bonds. More highly branched structure its branches are shorter. It gives red-brown colour with iodine.

USES OF GLYCOGEN 

• It act as storage food reserve in animals and help in glycolysis. 
• Provides energy when needed.

CELLULOSE 

Cellulose widely occurs in vegetables.A fibrous carbohydrate found inall plants where it serves as the structural component of the plant's  cell wall. A linear polymer of glucose units joined by 1,4-glucosidic bonds. Cotton 95% cellulose and wood  50% cellulose.

INDUSTRIAL APPLICATIONS :

Cellulose  employed in manufacture of cotton and linen. Cellulose finds an extensive application in manufacture of artificial silk.

CHITIN 

Similar to cellulose in both function and structure. Polymer of N-acetyl-D glucosamine bound by beta-1,4- glycosidic linkages.

FUNCTION 

Is to give rigidity to the exoskeletons of crabs, lobsters, shrimp, insects, and other arthropods.