Classification and structure of carbohydrates

Carbohydrates, also called carbohydrates, are the most abundant organic substances on Earth. Their molecules are made up of oxygen, carbon and hydrogen atoms. From a chemical point of view, these are polyhydroxyaldehydes' and polyhydroxyketones. They contain functional aldehyde or keto groups in their molecule, as well as a larger number of hydroxyl groups.

Carbohydrate Classification
'According to the number of units in the molecule, we distinguish:
 * monosaccharides - cannot be further hydrolyzed into simpler units;
 * oligosaccharides - they form 2-10 units of monosaccharides by hydrolysis;
 * polysaccharides' - hydrolyzing into more than 10 monosaccharides.
 * Monosaccharides and oligosaccharides are generally called sugars. A synonym for polysaccharide is the word glycan.

'Monosaccharides are divided according to:


 * Number of C-atoms: trioses, tetroses, pentoses, hexoses.
 * Functional groups: aldoses and ketoses.

'Polysaccharides are divided into:
 * Homopolysaccharides': these are polymers made up of the same type of monosaccharide. Examples are starch, glycogen or cellulose.
 * Heteropolysaccharides': they are polymers made up of more than one type of monosaccharide. An example is hemicellulose.

Carbohydrate Structure
'The structure of a saccharide molecule can be expressed by different formulas:
 * Linear (Fischer') formula;
 * Cyclic (Haworth's) formula that results from the formation of a heterocyclic structure.


 * Cycle can be:
 * six-membered - pyranose - by similarity to six-carbon pyran;
 * five-membered - furanose - by similarity to five-carbon furan.


 * The Tollens formula describes the creation of a cyclic structure from a linear formula. It shows the reaction of hydroxyl with a carbonyl group to form a so-called ``semiacetal (hemiacetal)'' structure.

Isomerism
It is a condition where compounds with the 'same general formula have a different structural arrangement'' of atoms in the molecule. The following types of isomerism are found in carbohydrate molecules.


 * - and - series
 * Indicated by the position of the &minus;OH group' on the last chiral carbon. Assignment to the respective series is based on similarity with the starting compound of the carbohydrate series – glyceraldehyde. The &minus;OH group in Fischer's formula is on the right for - and on the left for - isomers.


 * - and - isomers are mirror images of the so-called enantiomers' - 'optical isomers. They differ in the sign of optical rotation, or the direction in which they rotate the plane of polarized light.


 * However, it is not generally true that the - series is right-handed and the - series is left-handed.
 * Equimolar mixture of enantiomers is called a racemic mixture, or mixture, and it does not show optical activity.
 * The - isomers' are more common in nature.




 * Pyranoses and furanoses
 * They are designated according to the similarity of the cyclic form of the respective monosaccharide 'with the cycle of pyran or furan. Glucose in solution is more than 99% in the form of gluco-pyranose, the rest of the molecules, less than 1%, then appears in the form of gluco-furanose.


 * α- and β- anomers
 * They are designated according to the position of the hemiacetal' or hemiketal &minus;OH in the cycle. "Hemiacetals" are formed by the reaction of aldehyde and alcohol groups, hemiketals by the reaction of keto and alcohol groups.


 * If the &minus;OH group is oriented to the same side as the &minus;OH group indicating belonging to the - or - series, it is an α-anomer. If the &minus;OH group is oriented to the opposite side, it is a β-anomer'.

Anomers differ in optical rotation.

Examples are glucose' and mannose.
 * Epimers
 * They differ from each other by the position of one &minus;OH group in the molecule'.




 * Aldoses and ketoses
 * They are designated by the different functional group on the 1st and 2nd carbons of the molecule.