Isomers are compounds having same molecular formulae but different molecular structures. Monosaccharides show isomerism.
The presence of an asymmetric carbon atom (a carbon atom to which £our different groups or atoms are attached) make possible the formation of isomers of a compound. The isomers are of two types: structural isomers and sterioisomers.
Structural isomers have same molecular formula but different structures due to difference in chain length (chain isomers), position of substituent groups (positional isomers) or different functional groups. Glucose and fructose are structural (functional) isomers as both have same molecular formula (C6H1206) but have different functional groups, aldehyde and ketone respectively.
Sterioisomers have same molecular formula; same structures but differ only in spatial configuration. Optical isomers are one type of stereoisomer where the isomers appear as mirror image of each other around the asymmetric carbon atom.
In glyceraldehydes the carbon atom number two is asymmetric. In D-glyceraldehydes the OH-group is to the right side and H-group is to the left. In L-glyceraldehydes the OH-group is to the left of the asymmetric carbon atom and H-group to the right. But there are sugars with two or more asymmetric carbon atoms.
In that case the orientation of - II and -Oil group around the lowermost asymmetric carbon atom (the penultimate carbon atom) is considered for determination of D and L forms. These D- and L-forms are called enantiomers.
The majority of monosaccharides found in human body are D-type. Some sugars do occur naturally as L-forms such as L-arabinose. The possible number of optical isomers formed by a monosaccharide depends upon the number of asymmetric or chiral carbon atoms. Glucose has 4 asymmetric carbon atoms hence can have 24 or 16 number of optical isomers.
But in all these enantiomers the D and L forms are designated depending upon the orientation of -H and - OH group around 5th carbon atom. So it will have 4x2 = 8 enantiomers.
In ring forms the carbon atom number 1 is called anomeric carbon atoms. The orientation of -H and -OH groups around carbon atom 1 will give rise to two isomers known as anomers. In one form the -OH group is below and this is the a-form (alpha) and in other form the -OH group is above and is called the b (beta) - form.
Many monosaccharides can rotate plane polarized light to right (D, dextrorotatory, +) or left side (L, levorotatory, -). Some monosaccharides are known as derived monosaccharides as they are derivatives of monosaccharides.
Deoxyribose sugar found in deoxyribonucleic acid is an aldopentose with one oxygen atom less than ribose. Amino sugars are monosaccharides having amino group (-NH2). Glucosamine is an amino sugar which forms chitin found in fungal cell wall.
There are sugar acids like glucuronic acid and galacturonic acid which occur in mucopolysaccharides. Sugar alcohols like glycerol are involved in fat or lipid formation. Mannitol is another sugar alcohol found as storage carbohydrates in some fruits and brown algae. Monosaccharides are also reducing sugars as they posses' free aldehyde or keto groups.