CARBOHYDRATES, CLASSIFICATION OF CARBOHYDRATES AND ITS USES

CARBOHYDRATES

What are Carbohydrates?

The carbohydrates are widely distributed in both animals and plant tissues. In plants, carbohydrates are produced by photosynthesis while in the animal’s cells, carbohydrates are produced in the form of glucose and glycogen serves as an important source of energy for vital activities. Some carbohydrates have highly specific functions, e, g, Ribose in the nucleoproteins of the cells, glucose in certain lipids and lactose in milk.

Definition

Carbohydrates can  be defined as polyhydroxy alcohols with aldehydes or ketones and their derivatives.

Classification of carbohydrates

Types of Carbohydrates

Carbohydrates can be divided into 4 major groups, which are as follows

• Monosaccharide
• Disaccharides
• Oligosaccharides
• Polysaccharides

Monosaccharides

Monosaccharides are also called  simple sugars, which cannot be hydrolysed into a simpler form.

The general formula is Cn(H2O)n, these can be further subdivided as follows

Carbon atoms	Aldoses	Ketoses
trioses	Glyceraldehydes or glycerols	Dihydroxy-acetone
tetroses	Erythrose	Erythrulose
pentoses	Ribose, Xylose, Arabinose	Ribulose
hexoses	Glucose, Galactose, Mannose	Fructose
heptoses	Glucoheptose, glactoheptose	sedoheptulose

 

Simplest form of aldose – glyceraldehydes

Simplest form of ketose – dihydroxyacetone

Commonest aldose – glucose

Commonest ketose – fructose

 Disaccharides

These carbohydrates produce two molecules of the same or of different monosaccharide’s on hydrolysis. The general formula is Cn(H2O)n-1.

Examples are

Maltose = 1 MOL. Glucose + 1 MOL. Glucose

Lactose = 1 MOL. Glucose + 1 MOL. Galactose

Sucrose = 1 MOL. Glucose+ 1 MOL fructose

Maltose

 it does not occur in the body

The sources of it are germinating cereals and malt, and it is the intermediary product of the  breakdown of starch by amylase in the alimentary canal.

It is hydrolysed to glucose by enzyme maltase and the product is absorbed.

Lactose (milk sugar)

It is present in milk and formed in the lactating mammary gland

It is hydrolysed to glucose and galactose by the enzyme lactase in the alimentary canal and the product are absorbed

Sucrose

It does not exist in the body, but occurs in cane sugar, pineapple, carrot roots, sweet potatoes and honey

It is hydrolysed to glucose and fructose, the enzymes' invertase in the alimentary canal. The product of hydrolysis is then absorbed in the body

Polysaccharides

These carbohydrates yield more than 10 molecules of monosaccharide on hydrolysis. 

These can be further classified into homo polysaccharides and hetero polysaccharides 

depending on the presence of either same monosaccharide or more than simple sugar 

in alternating repeating sequence

Cellulose

It is the main constituent of the supporting tissue of plants and forms a considerable part of our vegetable food, it does not occur in the human body.

It is of considerable human dietetic value only because it adds bulk to the intestinal contents, thereby stimulating peristalsis and elimination of food residues.

Glycogen

It is the reserve carbohydrate found in the liver and muscles of animals and human beings.

The glycogen content of the liver is more than that of muscle.

It is also found in plants, which have no chlorophyll system e.g., fungi and yeasts, but not in green plants.

Starch

It is the stored carbohydrates of chlorophyll containing plants. In plants the starch is laid down in the cell  granules.

It is the most important source of carbohydrates in our food and is found in cereals, potatoes in high concentration

Oligosaccharide

These carbohydrates yield two to ten monosaccharide’s units on hydrolysis for ex materials