Classification of Carbohydrates and its Structure - Practice Questions & MCQ

These are polyhydroxy aldehydes or ketones or substances that form these on hydrolysis and possess at least one chiral atom. The (-OH) group is available in the form of hemiacetals or hemiketals. The carbohydrates are stored in the animal body as glycogen which is also known as animal starch because its structure is highly branched like amylopectin. It is found in liver, muscles and brain as well as in fungi and yeast. When the body requires glucose, the enzymes break glycogen to glucose.
Carbohydrates are indispensable for both plant and animal lives. These are utilised as the storage molecules in the form of starch in plants and glycogen in animals. The cell wall of bacteria and plants consists of cellulose. Carbohydrates are present in biosystem in combination with several proteins and lipids.

Classification
The carbohydrates can be classified into three categories:

1. Monosaccharides: They are the simplest carbohydrates which cannot be hydrolysed to smaller molecules. They are sweet and crystalline and are called sugars.
2. Oligosaccharides: These carbohydrates on hydrolysis give two to nine molecules of monosaccharides classified as di-, tri, tetra-saccharides, etc. For example, sucrose, maltose, lactose and raffinose, etc. They are also called sugars.
3. Polysaccharides: These carbohydrates, on hydrolysis, give a large number of monosaccharide, e.g., starch, cellulose, etc. They are also called non-sugars.

Reducing and Non-reducing Sugars
Those sugars which reduce Fehling's and Tollens solutions are called reducing sugars and those which do not reduce these reagents are called non-reducing sugars. All the monosaccharides and disaccharides, except sucrose, are reducing sugars, whereas all the polysaccharides are called non-reducing carbohydrates.

The CHO group of glucose either reacts with C-5 OH group or C-4 OH group to give hemiacetalic linkage and forms stable six- and five- membered cyclic rings, respectively. The reaction of CHO group with C-6 OH group or with C-3 OH does not occur as the formation of seven- membered or four-membered ring respectively is not favourable due to angle strain theory.

Haworth representation: Cyclic structure of glucose was established by English chemist W.N. Haworth. The cyclic structure of glucose are depicted below.

It also exists in two cyclic forms which are obtained by the addition of —OH at C5 to the (C=O) group. The ring, thus formed is a five-membered ring and is named as furanose with analogy to the compound furan. Furan is a five-membered cyclic compound with one oxygen and four carbon atoms. The cyclic structures of two anomers of fructose are represented by Haworth structures as given below.

Anomers
Anomers are diastereomers that differ in the configuration at the acetal or hemiacetal C atom of a sugar in its cyclic form. In other words, anomers are epimers whose conformations differ only about C-1. For example, α-D(+) and β-D(+) glucose are anomers.

Epimers
Diastereomers with more than one stereocentre that differ in the configuration about only one stereocentre are called epimers.

1. D-Glyceraldehyde and L-glyceraldehyde are epimers
2. D-Erythrose and L-threose are epimers.
3. Epimerisation of glucose at C-2 gives mannose.
4. Epimerisation of glucose at C-3 gives allose.
5. Epimerisation of glucose at C-4 gives galactose.

Mutarotation
The change in specific rotation of an optically active compound in solution with time, to an equilibrium value, is called mutarotation, or it is the change in the optical rotation occuring by epimerisation, i.e., the change in the equilibrium between two epimers, when the corresponding stereocentres interconvert. During mutarotaion, the ring opens and then ring recloses either in the inverted position or in the original position giving a mixture of α and β forms. All reducing carbohydrates, i.e, monosaccharides and disaccharides undergo mutarotation in aqueous solution. Mutarotation proves the existence of anomers and cyclic structures.