LECTURE NOTES ON CARBOHYDRATES

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. Many, but not all, carbohydrates have the empirical formula (CH2O)n; some also contain nitrogen, phosphorus, or sulfur.

Carbohydrates are generally classified into 4 major groups :

1. Monosaccharides

2. Disaccharides

3. Oligosaccharides and

4. Polysaccharides

(the word “saccharide” is derived from the Greek

sakcharon, meaning “sugar”).

 Monosaccharides, or

Monosaccharides

simple sugars, consist of a single polyhydroxy aldehyde or ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar D-glucose, sometimes referred to as dextrose/Monosaccharides of four or more carbons tend to have cyclic structures

These are carbohydrates that cannot be hydrolysed into more simpler form. These are otherwise known as simple sugars. The general formula is Cn(H2O)n. They may be subdivided into trioses, tetroses,pentoses and hexoses depending upon the number of carbon atomsthey contain and also subdivided as aldoses and ketoses depending upon the presence of aldehyde or ketone groups (eg). Monosaccharides are colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents.Most have a sweet taste. The backbones of common monosaccharides are unbranched carbon chains in which all the carbon atoms are linked by single bonds. In the open-chain form, one of the carbon atoms is double-bonded to an oxygen

atom to form a carbonyl group; each of the other carbon atoms has a hydroxyl group. If the carbonyl group is at an end of the carbon chain (that is, in an aldehyde group) the monosaccharide is an aldose; if the carbonyl group is

at any other position (in a ketone group) the monosaccharide

is a ketose. The simplest monosaccharides are the two three-carbon trioses: glyceraldehyde, an aldotriose, and dihydroxyacetone, a ketotriose

Simplest form of aldose - Glyceraldehyde

Simplest form of ketose - Dihydroxy acetone

Commonest aldose - Glucose

Commonest ketose – Fructose

Functions

1. Ribose is a structural element of nucleic acids and also of some coenzymes.

2. Glucose on oxidation yield energy which is required for various metabolic activities.

3. Fructose is found in fruits, honey etc.which are responsible for sweetness and can be converted to glucose and utilised in the body.

4. Galactose is a component of milk sugar-lactose, glycolipids and glycoproteins

5. Mannose is a constituent of mucoproteins and glycoproteins which are essential for the body.

Disaccharides

These are carbohydrates that yield two molecules of same or

different types of monosaccharides on hydrolysis. The general formula

is Cn(H2O)n-1 (eg) Lactose, Maltose and Sucrose. The monosaccharide

units are united by a glycosidic linkage.

Functions

1. Lactose is otherwise called as milk sugar. It is present in milk and

is made up of monosaccharides - glucose and galactose.

Glucose + Galactose Lactose

2. Maltose is otherwise known as ‘malt sugar’ and is present in

germinating cereals, malt etc.It is the intermediate product in the

hydrolysis of starch by amylase in the alimentary canal. It is made

up of 2 molecules of glucose.

Glucose + Glucose Maltose

3. Sucrose is otherwise called as ‘table sugar’ or ‘cane sugar’. It is

the common sugar and is widely distributed in all photosynthetic

plants. It does not exist in the body but occurs in sugarcane,

pineapple, sweet potato and honey. It is made up of glucose and

fructose.

Glucose + Fructose Sucrose

Oligosaccharides consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. Oligosaccharides

These are carbohydrates that yield 2-10 monosaccharide units on

hydrolysis.eg. Maltotriose.

In cells, most oligosaccharides  consisting of three or more units do not occur as free entities  but are joined to nonsugar molecules (lipids or proteins) in glycoconjugates.

Polysaccharides

These carbohydrates yield more than 10 monosaccharide units on hydrolysis. They are further classified into homopolysaccharides and heteropolysaccharides

The polysaccharides are sugar polymers containing more than 20 or so monosaccharide units; some have hundreds or thousands of units. Some polysaccharides, such as cellulose, are linear chains; others, such as

glycogen, are branched. Both glycogen and cellulose consist of recurring units of D-glucose, but they differ in the type of glycosidic linkage and consequently have

strikingly different properties and biological roles.

Homopolysaccharides

These on hydrolysis yield same type of monosaccharide units. (eg). starch, glycogen, cellulose, inulin, pectin and hemicellulose yield only glucose on hydrolysis.

Heteropolysaccharides

These on hydrolysis yield a mixture of different types of monosaccharides. The heteropolysaccharides situated in extra cellular matrix are called as mucopolysaccharides.

(eg). hyaluronic acid, heparin, keratan sulphate and chondroitin sulphate.

Hyaluronic acid is made up of glucuronic acid and N-acetyl glucosamine

Heparin is made up of glucosamine and glucuronic acid

Chondroitin sulphate is made up of either glucuronic acid (A and C type) or Iduronic acid ( B type) and 2-N-acetyl amino galactose.

Keratan sulphate consists of N-acetyl galactosamine, galactose and

sulphuric acid

Functions

1. Starch is made up of repeated units of glucose moiety. It is the most important source of carbohydrate in our food. Such a compound which produces only glucose on hydrolysis is called a glucosan, and is found in cereals, potatoes, legumes and other

vegetables.

2. Glycogen is the major carbohydrate reserve in animals and is often called animal starch. It is stored in liver and muscle of animals. It is also found in plants which have no chlorophyll system [eg. fungi and yeasts] but not in green plants.

3. Cellulose is widely distributed in plant sources. It occurs in the cell walls of plants where it contributes to the structure. It is the main consituent of the supporting tissues of plants and forms a considerable part of vegetables.

4. Pectin and hemicellulose are present in fruits of many plants andn serve as jelling agents.

5. Hyaluronic acid occurs in synovial fluid, in skin and in tissues. It acts as a cementing substance in tissues and also acts as a lubricant. It is also present in vitreous humor.

6. Heparin is used in medicine as an anticoagulant and prevents blood clotting.

7. Keratan sulphate is an important component of cartilage and cornea