Classification of carbohydrates

Classification of carbohydrates

Carbohydrates are also called ‘saccharides’ (derived from Greek word ‘sakcharon’ meaning sugar) and are classified into three groups;

1. Monosaccharides
2. Oligosaccharides
3. Polysaccharides

(i) Monosaccharides

These are simple sugars.

Features

1. They are sweet in taste
2. They are easily soluble in water and they cannot be hydrolyzed into simple sugars.

Composition

• Chemically they are either polyhydroxy aldehydes or ketones.
• All carbon atoms in monosaccharides except one, have a hydroxyl group.
• The remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called aldo-sugar e.g. glyceraldehydes and with the keto group as keto-sugar e.g. dihydroxyacetone.

These are indicated in the case of two trioses sketched below.

Structure of glyceraldehydes, a 3C Sugar (C3H6O3). The aldehyde form is glyceraldehydes, whereas ketonic form is dihydroxyacetone.

Types

In nature monosaccharides with 3 to 7 carbon atoms are found. They are called trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). They have general formula (CH2O)n. Where n is the whole number from three to seven.

Examples

• Two trioses mentioned above i.e. glyceraldehydes and dihydroxyacetone, are intermediates in respiration and photosynthesis.
• Tetroses are rare in nature and occur in some bacteria.
• Pentoses and hexoses are most common. Form the biological point of view most important hexose is glucose, which is an aldose sugar.

Structures of ribose and glucose is give below.

Structure of Ribose and Glucose

Ring formation

Most of the monosaccharides form a ring structure when in solution. For example ribosewill form a five-cornered ring known as ribofuranose, whereas glucose will form six-cornered ring known as glucopyranose.

Ribose and glucose form ring shaped structures.

Glucose a common example

• Glucose is naturally produced in green plants, which take carbon dioxide from the air and water from the soil to synthesize glucose.

• As indicated in the equation, energy is consumed in this process which is provided by sunlight. That is why the process is called photosynthesis.
• For synthesis of 10g of glucose 717.6 Kcal of solar energy is used. This energy is stored in the glucose molecules as chemical energy and becomes available in all organisms when it is oxidized in the body.
• In Free State, glucose is present in all fruits, being abundant in grapes, figs and dates.
• Our body normally contains 0.08% glucose.
• In combined form, it is found in many disaccharides and polysaccharides. Starch, cellulose and glycogen yield glucose on complete hydrolysis.

(ii) Oligosaccharides

Features

1. These are less sweet in taste.
2. These are less soluble in water.
3. On hydrolysis, they yield from two to ten monosaccharides.
4. The covalent bond between two monosaccharides is called Glycosidic linkage.

Types

• Those yielding two monosaccharides on hydrolysis are called disaccharides.
• Those yielding three are known as trisaccharides and so on.

Examples

• Physiologically important disaccharides are maltose, sucrose and lactose.

Most familiar disaccharide is sucrose (cane sugar), which on hydrolysis yields glucose and fructose, both of which are reducing sugars. Its molecular formula is C12H22O11. Its structural formula is given below.

A disaccharide. Note carefully the glycosidic linkage between the two monosaccharides.

(iii) Polysaccharides

Polysaccharides are the most complex and most abundant carbohydrates in nature.

Features

1. They are usually branched and tasteless.
2. They are formed by several monosaccharide units linked by glycosidic bonds.
3. They have high molecular weights.
4. They are sparingly soluble in water.

Examples

Some biologically important polysaccharides are starch, glycogen, cellulose, dextrins, agar, pectin and chitin etc.