Monosaccharides

Monosaccharides are basic unit of carbohydrate. Greek word Mono means single and sacchar refers to sugar residues. They are as simple as crystal solids in structure. It is soluble in water, colorless. Most of the monosaccharides are sweeter. When two residues are linked together it is referred as disaccharides. When more than two sugar residues build together, building blocks result in the formation of polysaccharides. For instance glucose or dextrose, fructose or levulose, xylose, ribose and galactose are monosaccharides, sucrose is disaccharide and cellulose ad starch is examples in polysaccharide. Except first and last carbon atom is capable of accepting hydroxyl group they end up in chiral structure. This chirality can form many number of isomers having same chemical formula. For instance aldohexoses like glucose and galactose are of same isomer but they got different physical and chemical structural properties.

Structure of Monosaccharides

The general structure of monosaccharides has chemical formula C_X (H₂O_Y). The chemical structure of monosaccharides H (CHOH) _n C=O (CHOH) _m H. Possible isomers from this chemical structure are

• Aldehyde
• Ketone

When subscript m and n is zero it results in the formation of aldehyde and monosaccharides are called aldose. When subscript m and n are not zero it results in the formation of ketone and monosaccharides are called ketose. Most of the monosaccharides have functional groups like aldehyde, ketone, hydroxyl and or non carbonyl groups.

Monosaccharides form a chiral structure in aqueous solution. Depending on the presence of aldehyde or ketone they may be either hemiacetals or hemiketals respectively. For instance glucose can easily form hemiacetal linkage between 1 carbon atom and 5 oxygen atoms resulting in the formation of cyclic monosaccharide called pyranoside. The same reaction occurs between 1 carbon atom and 4 oxygen atom results in the formation cyclic monosaccharide called furanoside.

Nomenclature of Monosaccharide

They are broadly classified based on the number of carbon atoms as triose with three C atoms, tetrose with four C atoms, pentose with 5 C atoms, hexose with 6 C atoms and heptose with 7 C atoms. The other classification on the basis of carbonyl group they are of two types namely aldose with aldehyde group and ketose with ketone group.