Monosaccharides

Monosaccharides

Monosaccharides are defined as polyhydroxy aldehydes or ketones, which cannot be further hydrolysed to simple sugars. Monosaccharides are divided into two groups according to their functional groups (Fig. 4.2).

They are also classified based on the number of carbon atoms present in the monosaccharides

 1. Aldotriose eg: Glycerose 1. Ketotriose :eg.Dihydroxy acetone

2. Aldotetrose eg : Erythrose 2. Ketotetrose : eg : Erythrulose

3. Aldopentose eg : Ribose 3. Ketopentose : eg. Ribulose

4. Aldohexose eg: Glucose 4. Ketohexose : eg. Fructose

1. Aldoses are sugars containing aldehyde group

eg : glucose, galactose, mannose.

2. Ketoses are sugars containing ketone group

eg : fructose and sorbose.

Carbohydrates posses asymmetric carbon atoms. A carbon atom to which four different atoms or groups of atoms are attached is said to be an asymmetric carbon.

Vant Hoff’s Rule of ‘n’

The number of possible isomers of any given compound depends upon the number of asymmetric carbon atoms the molecule posses.

According to this rule, 2n equals the possible isomers of that compound, where ‘n’ - represents the number of asymmetric carbon atoms in a compound.

Hexoses

Hexoses are monosaccharides containing 6 carbon atoms. The molecular formula of hexose is C₆H₁₂O₆. Aldohexoses contain asymmetric carbon atoms at position 2,3,4 and 5. Hence an aldohexose can exist in 16 isomeric forms. (2ⁿ = 2⁴ = 16).

The ketohexoses contain 3 asymmetric carbon atoms at position 3,4 and 5. Hence, it exist in 8 isomeric forms (2ⁿ = 2³ = 8).

Structure of glucose and fructose

Glucose is a simple sugar. It is a monosaccharide. It cannot be hydrolysed further.

Glucose is an important sugar of blood.

Human blood contains 60-100 mg of glucose in 100 ml of blood in fasting. It serves as the major metabolic fuel in cells and tissues. Oxidation of glucose quickly provides energy for the cells.Hence, glucose is described as the chief source of energy.

1. The empirical formula of glucose is CH₂O and the molecular formula is C₆H₁₂O₆. The molecular formula of fructose is C₆H₁₂O₆ . Glucose has aldehyde group whereas fructose has ketone group.