Introduction

• Polyhydroxy aldehyde and ketone that yield such compounds on hydrolysis.
• C:H:O is 1:2:1 . Eg: Glucose C₆H₁₂O₆ e. (CH₂O)₆

Types of Carbohydrate

a) Monosaccharide:

• Incudes Glucose, Fructose, Galactose.
• Simplest form of carbohydrate.
• Are generally crystalline solids and have high melting point and are soluble in water.
• They are more or less sweet in taste.
• Either aldehyde or ketone with one or more hydroxyl groups.
• If carboxyl group is at an end of the carbon chain, it is aldehyde and is called aldose.
• If carboxyl group is at any other position, it is called ketone.
• Glucose and fructose have 5 hydroxyl group.
• “n” Chiral centers can have 2ⁿ stereoisomers i.e. Aldohexoses have four chiral centres. So, 2⁴ = 16 isomers.
• C₄ à tetroses, C₅ à Pentoses , C₆à Hexoses, C₇à
• Most of the hexoses found in living organism is D-isomer.
• If -OH group is present at right side, it is D-group and on left-side, it is L-group.

• When two sugars differ only in configuration around one carbon atom, they are called epimers.
• The Cyclic forms of sugars are called pyranoses.
• Systemic name for the two ring forms of D-glucose are called α-D-glucopyranose and β-D-glucopyranose.

• An α and β forms of D-glucose interconvert in aqueous solution by a process called mutarotation. Hence, the solution has identical optical properties.
• The mixture has 1/3 α-D-glucose and 2/3 β-D-glucose and very small amount of linear form.
• The hemiacetal or carboxyl carbon atom is called anomeric carbon.
• Aldohexoses also exists in cyclic forms having 5 membered rings called furanoses.

Note: Six membered aldopyranoses is much more stable than Aldo furanose ring.

b) Disaccharides:

• Includes Maltose, Sucrose, Lactose and Trehalose.
• Consists of two monosaccharide joint covalently by an O-glycosidic bond.
• Formed when hydroxyl group on one sugar reacts with anomeric carbon on the other.
• Trehalose occurs in fungi and in insect blood.
• Lactose occurs in 5% of milk.
• Maltose is an important product of the enzymatic digestion of starch.

c) Oligosaccharide:

• Raffinose and Stachyose

d) Polysaccharide:

• Most of the carbohydrate found in nature.
• Oligosaccharide (5-20 sugar residues).
• Both Polysaccharide ad starch have similar structure except the -O- group in D-Glucose.