Carbohydrates

Carbohydrates are the biomolecules containing C, H and O. These are also known as ‘saccharides’. Word saccharide is derived from Greek word σάκχαρον (sákkharon), meaning "sugar".
Carbohydrates are the ‘hydrate’ of ‘carbon’ with empirical formula C_m(H₂O)_n, where ‘m’ and ‘n’ are same number for monosaccharides whereas, different for polysaccharides. Carbohydrates are polyhydroxyalcohol or polyhydroxyketone or the polymers that yield these upon hydrolysis.
D-glucose is the most common sugar.

Classification

Classification of carbohydrates

Carbohydrates are classified on the basis of hydrolyzable product.

1. Monosaccharides: These are simple carbohydrates consisting only of single sugar unit. Monosaccharides are sweet-tasting.
2. Oligosaccharides: These are polymers consist of two to ten monosaccharides. These are also sweet-tasting.
3. Polysaccharides: These very long-chain polymers consist of more than hundreds of monosaccharides. These not sweet-tasting and hence, also known as non-sugar.

Monosaccharides

Monosaccharides are simple from of carbohydrates. These cannot be further hydrolysed. Depending upon the number of carbon atom present, monosaccharides are of various types. These monosaccharides are named after the number of carbons with suffix ‘ose’.

Examples:

Carbohydrates with 3 carbon atoms are trioses (Glyceraldehyde and dihydroxyacetone).
Carbohydrates with 4 carbon atoms are tetroses (Erythrose, arabinose)
Carbohydrates with 5 carbon atoms are pentose (Ribose, xylose)
Carbohydrates with 6 carbon atoms are hexose (Glucos, fructose)
Carbohydrates with 7 carbon atoms are heptose (Sedoheptulose)

Monosaccharides with functional group- aldehyde are called as ‘aldoses’.
Monosaccharides with functional group- ketose are called as ‘ketoses’.
The atom of carbon is numbered with smallest near the functional group.

Optical isomers/stereoisomers/enantiomers

Monosaccharides containing chiral carbon or asymmetrical carbon are optically active. These monosaccharides can exist in two steric configurations, which are non-superimposable upon each other. These are called are called as optical isomers or stereoisomers or enantiomers. Steric configuration, where -OH group is at right side of chiral carbon, known as D-form, whereas, -OH group at left side of chiral carbon is known as L-from.

Epimers

Monosaccharides differing only at single carbons are called as ‘epimers’. D-glucose is epimer of D-mannose at C-2 and of D-galactose at C-4.

Cyclic forms of monosaccharides

Aldo and keto group of 5 and 6 carbon monosaccharides react with alcohol (-OH) to form hemiacetal and hemiketal, respectively. This result in the formation of cyclic planner ring. The linear structure of carbohydrate is also known as Fischer structure or Fisher projection. In aqueous solution, fisher structure become converted in to cyclic form.
In glucose (aldohexose) aldo group (-CHO) at the C-1 forms bond with the -OH at C-5 and exists in cyclic form. Similarly, In fructose (ketohexose) keto group (C=O) at C-2 forms bond with the -OH at C-5 and become cyclic in aqueous solution. The 5-C skeleton ring of glucose is similar with the pyran and 4-C skeleton ring of fructose is similar with the furan.

Anomers

The C-1 of cyclic forms of monosaccharides can have two configurations. If -OH lies at upper side of the plane of ring, it is known as β, where as if -OH lies at lower side of plane of ring, it is said to be α. In aqueous solution of D-glucose, -OH can rotate from α to β configuration. This phenomenon is known as mutarotation. The C, at which mutarotation occurs is said to be anomeric carbon. The two forms differing in configuration of -OH at anomeric carbon are said to be anomers.
In aqueous solution, at equilibrium, 36 % of the D-glucose is in α anomeric forms, 63 % occurs as β anomers and <1 % occur as linear form.

Disaccharides and glycosidic bonds

A disaccharide consists of two monosaccharide units joined by the formation of o-glycosidic bond. Glycosidic bonds are formed by the condensation of OH group present at anomeric carbon of one monosaccharide and OH group of other monosaccharide. In the reaction one molecule of H₂O is displaced per o-glycosidic bond.
Glycosidic bonds are named from left to right.

Maltose is a disaccharide consists of α-D-glucose and β-D-glucose joined by α (1-4) glycosidic bond.
Sucrose is a disaccharide consists of α-D-glucose and β-D-fructose joined by α (1-2) glycosidic bond.

Reducing and non-reducing sugars

Sugars capable of reducing the ferric ion (Fe⁺⁺⁺) into ferrous ion (Fe⁺⁺) or cupric ion (Cu⁺⁺) into cuprus ion (Cu⁺) are called as reducing sugars. Those sugars which can not reduce metallic ions are called non-reducing sugars. It is the presence of -OH at right most C atom in a sugar, which makes a sugar reducing.

Monosaccharide, glucose as well as disaccharide matlose have -OH group at right most (anomeric) carbon atom, hence these are reducing sugars.
Diasaccharide fructose, however, does not have -OH at right most C, hence it is non-reducing sugar.

Polysaccharides

Polysaccharides are consists of hundreds or thousand monosaccharide units. These are also called as glycans. Polysaccharides are not sweet-tasting, hence known as non-sugars.
Based on the nature of monomeric units, polysaccharides are classified into homopolysaccharides (consist of single type of monomeric unit) or heteropolysaccharides (consist of two or more types of monomeric units).
Based on the function polysaccharides are divided into structural polysaccharides and storage polysaccharides. Structural polysaccharides provide mechanical strength, whereas, storage polysaccharides serve as reserve food material. Structure and properties of some important polysaccharides are mentioned below.

Starch

Starch are linear, branched chain of consists of amylose and amylopectin. Both the amylose as well as amylopectin are consist of repeating unit of α-D-glucose. Amylose is linear chain, whereas, amylopectin is branched. In amylopectin, branching occurs at 25 to 30 α-D-glucose residue. Branching takes by the formation of α (1-6) glycosidic linkage.
Function: Starch is reserve food material in plants.

Glycogen

Structurally, glycogen is similar to that of starch. However, it is much branched than the starch. Branching in amylopectin occurs at every 8 to 10 glucose residue.
Function: Glycogen is the reserve food materials in animal and most of the fungi. In animal, it is chiefly found in the muscles and liver.

Cellulose

Cellulose in linear and unbranched homopolysaccharide, which consists of repeating units of β-D-glucose joined by β (1-4) glycosidic linkage.
Function: Cellulose is the chief component of cell wall in plants. Humans lack enzyme to hydrolyze β (1-4) glycosidic linkage, therefore, cellulose is non-digestible in humans, however it provide roughage in the intestine.

Chitin

Chitin is a linear, unbranched homopolysaccharide consists of repeating units of N-acetyl-D-glucosamine, joined by β (1-4) glycosidic linkage.
Function: Chitin is chief component of cell wall of most of the fungi and exoskeleton of insects and crustaceans.

Peptidoglycan

Peptidoglycans are also called as "mucopeptide" or "murein". It is a linear, unbranched heteropolysaccharide consists of repeating alternative units of N-acetyl-D-glucosamine (NAG) and N-acetlymuramic acid (NAM), joined by β (1-4) glycosidic linkage.
Function: Peptidoglycan is the chief component of cell wall in bacteria and blue-green algae (cyanobacteria).