Which of the following statement is true-

Ans. is 'c' i.e., Glucose is a C4 epimer of galactose* Glucose is an aldose. Fructose is a ketose. Hence glucose and fructose are functional isomers (not stereoisomers)* Glucose and mannose are epimers at 2nd position* Glucose and galactose are epimers at 4th position* Ribose is a pentose and fructose is a hexose. Hence they are not isomers* Isomerism is a property by which two or more molecules have the same molecular formula but they have either different structural formula or they differ in spatial orientation of their groups* It is of three types# Functional Isomerism# Stereoisomerism# Optical isomerismFunctional Isomerism:* Here the two molecules have the same molecular formula but different functional groups. In carbohydrates, for every molecular formula, there is a set of aldoses and a set of ketoses. To be more specific, for a molecular formula of C6H1206, the well known aldose is glucose and the well known ketose is fructose.Stereoisomerism* Here the two molecules have the same molecular formula and the same structural formula, but they differ in spatial orientation of their groups* Stereoisomerism is dependent on the presence of asymmetric carbon atom* The number of stereoisomers present in a molecule with n number of asymmetric carbon atoms is 2nd.* In glucose, other than the functional carbon atom (Cl) and the last carbon atom (C6), the remaining 4 are asymmetric carbon atoms. But once they form a closed ring, even the functional carbon atom becomes asymmetric. Hence there are totally 5 asymmetric carbon atoms in glucose. The number of stereoisomers possible is 25 or 32 for glucose.* In fructose, other than Cl, C6 and the functional carbon atom (C2), remaining 3 are asymmetric carbon atoms. But once fructose forms a closed ring, even the functional carbon atom (C2) becomes asymmetric. Hence there are totally 4 asymmetric carbon atoms and the number of stereoisomers possible for fructose is 24 or 16.* Stereoisomerism is of four types :# Anomerism# Epimerism# Diastereoisomerism# Enantiomerism* Anomerism is a type of stereoisomerism in which two molecules have the same molecular formula and the same structural formula, but they differ in spatial orientation with respect to anomeric carbon atom or the functional carbon atom.# Hence anomerism in glucose is with respect to 1st carbon atom and in fructose, it is with respect to 2nd carbon atom.# In the anomeric or functional carbon atom, if OH lies above the plane of the ring, it is b form. In the anomeric carbon atom if OH lies below the plane of the ring, it is a form.* Epimerism is a type of stereoisomerism in which two molecules have the same molecular formula and the same structural formula but they differ in spatial orientation with respect to one of the asymmetric carbon atoms other than penultimate carbon atom.* In glucose, epimerism is with respect to 2nd or 3rd or 4th carbon atom (As 1st and 6th carbon atoms are symmetric and 5th is the penultimate carbon atom).* Epimer of glucose at 2nd position is mannose.* Epimer of glucose at 4th carbon atom is galactose.* Diastereoisomerism is a type of stereoisomerism in which two molecules have the same molecular formula and the same structural formula but they differ in spatial orientation with respect to one or more of the asymmetric carbon atoms other than penultimate carbon atom.# Mannose and galactose differ with respect to both 2nd and 4th carbon atoms. Hence they are diastereoisomers.* Enantiomerism is a type of stereoisomerism in which two molecules have the same molecular formula and the same structural formula but they differ in spatial orientation with respect to all the carbon atoms and they are named based on the orientation in the penultimate carbon atom. In the penultimate carbon atom, if OH is on the right side, it is D form, if OH is on the left side, it is L form. The other name for enantiomerism is Racemism.* Optical Isomerism is a type of stereoisomerism in which the two molecules have the same molecular formula but they differ in the way they turn the plane polarised light. If an isomer turns the plane polarised light to the right side, it is dextrorotatory form or d or + form. If an isomer turns the plane polarised light to the left side, it is levorotatory or 1 or - form.# D & L are not synonymous with d and 1 forms# Not all D forms are dextrorotatory and L forms are levorotatory# For some molecules, D forms are dextrorotatory and L forms are levorotatory, for some molecules D forms are levorotatory and L forms are dextrorotatory.# Hence they are not synonymous, but they complement each other