Carbohydrates

The word carbohydrate represents hydrated carbons. They contain carbon, hydrogen and oxygen and the allowance of hydrogen to oxygen is the same as in water. Their general formula is Cn (H2O), where n is a whole number ranging from 3 to many thousands.

Chemically carbohydrates are characterised as

• polyhydroxy  aldehydes

or

• ketones.

OR

complex substances on hydrolysis yield

• polyhydroxy aldehydes

Or

• ketone subunits.

Carbohydrates in cells combine with proteins and lipid and the resultant compounds are called glycoproteins and glycolipids respectively which have structural roles in the extracellular matrix of animals an bacterial cell wall and are components of biological membranes.

Classification of carbohydrates

Carbohydrates are also called saccharides derived from Greek word sakcharon meaning sugar and are classified into three groups.

Monosaccharides:

Monosaccharides is derived from Greek word, mono means one and saccharide means sweet. These are simple sugars, are sweet in taste, easily soluble in water and cannot be hydrolysed into simpler sugars. All the carbon atoms in monosaccharide have a hydroxyl group except one which either has an aldehyde group or are ketone group.

The sugar with aldehyde group is called Aldo sugar and with the keto group as keto sugar.

Glyceraldehyde is the aldehyde form of triose and dihydroxyacetone is the keto form of triose.

In nature monosaccharides with three to seven carbon atoms are present. They are called trioses (3C e.g. glyceraldehyde), tetroses (4C e.g. erythrose), pentoses (5C e.g. ribose), hexoses (6C e.g. glucose and fructose) and heptoses (7C e.g. glucoheptose). They have a general formula (CH2O)n. The most important hexose is Glucose which is an aldose sugar.

Formation of ring structure

Many monosaccharides make a ring structure when present in solution. For example, ribose form of five cornered ring known as ribofuranose, where as glucose form a six cornered ring known as glucopyranose.

In Free State, glucose is found in all fruits mostly in grapes, figs and dates. Our blood have 0.08% glucose.

In combined form it is found in many disaccharides and polysaccharides Starch, cellulose an glycogen yield glucose on complete hydrolysis.

Oligosaccharides

Sugar that yield two to ten monosaccharides on hydrolysis are called or oligosaccarides.

These are comparably reduced sweet in taste and reduced soluble in water. Oligosaccarides yielding 2 monosaccharides are known as disaccharides, those yielding three are known as trisaccarides and so on. In the formation of oligosaccarides, monosaccharides are linked through a covalent bond known as glycosidic bond.

The important disaccharide is sucrose (cane sugar) which on hydrolysis give glucose and fructose, both are reducing sugars. Its molecular formula is C12H22O11. Sucrose is found in SAP in plants and is the transport and storage form in them. Fructose is the sweetest monosaccharide.

There is 1,2 glycosidic linkage  in sucrose and 1,4 glycosidic linkage in maltose.

Glucose + glucose = maltose.

Glucose + galactose = lactose

Glucose + fructose = sucrose

Polysaccharides

These are the

• most complex
• and the most plentiful carbohydrates in nature.

They are usually branched and tasteless, are formed by several monosaccharides units linked by glycocidic bonds, they have higher molecular weight and are only sparingly soluble in water.

Some important polysaccharides are 

• starch
• Glycogen
• Cellulose
• Dextrins
• Agar
• pectin
• and chitin.

Examples of polysaccharides

Starch

It is present in fruits, grains, seeds and tubers. On hydrolysis, it yields glucose molecules.

Types of starch

Starch are of 2 types

1. Amylose
2. Amylopectin

Amylose starch

Amylose starches contain Unbranched chains of glucose and dissolve in hot water.

Amylopectin starch

Amylopectin starches contain branched chains and not dissolve in hot or cold water.

Starches give blue colour with iodine. Plant cells stock glucose in the form of starch.

Glycogen

It is also called animals starch. It is the chief form of carbohydrates stored in animal body, is found abundantly in liver and muscles, though also found in other animal cells. It yields glucose on hydrolysis  and is insoluble in water.

Glycogen gives red colour with iodine. Animal cells stock glucose in the form of glycogen.

Cellulose

It is the most abundant carbohydrate in nature. It is the main constituent of cell wall of plants, is highly insoluble in water and yields glucose molecules on hydrolysis. The rings of glucose are arranged in flip flap manner (criss-cross arrangement) in cellulose. It is not digested in human digestive tract because the enzyme required for its digestion cellulase is absent in men.

In the herbivores, it is digested because of microorganisms (bacteria, yeasts and protozoa) in their digestive tract which secret enzyme cellulase for its digestion. Cellulose give no colour with iodine.

Examples of cellulose

Cotton is the pure form of cellulose.

Learn more

What is carbohydrates?

What is hydrated carbons?

Classification of carbohydrates

What is monosaccharides?

Formation of ring structure in monosaccharides

Examples of monosaccharides

What is oligosaccarides?

Examples of oligosaccarides

What is polysaccharides ?

Examples of polysaccharides

What is starch?

Types of starch

What is Amylose starch?

What is Amylopectin starch?

What is Amylose?

What is Amylopectin ?

What is glycogen?

Starch give which colour with iodine?

glycogen give which colour with iodine?

what is cellulose?

Cellulose give which colour with iodine?