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The twenty one outstanding features or characteristics of the carbohydrates are listed below:
1. Three major classes of carbohydrates are monosaccharide’s, oligosaccharides and polysaccharides.
2. The simplest of carbohydrates are monosaccharide’s from which disaccharides, olgosaccharides and polysaccharides are formed.
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3. Monosaccharide’s are colourless, crystalline solids which are freely soluble in water and insoluble in nonpolar solvents.
4. Two families of monosaccharide’s are aldoses and ketoses.
5. Monosaccharide’s except dihydroxyacetone have asymmetric centres since they contain one or more asymmetric or chiral carbon atom and hence exist in optically active isomeric forms.
A simple example is of the aldose, glyceraldehyde, having a chiral centre (the middle carbon atom) hence possess two different optical isomers or enantiomers (Fig. 37.1.), shown by ball and stick model. It can also be represented by Fischer projection formulas and perspective formulas.
Three dimensional sugar structures on paper can be represented by Fischer projection formula as below:
6. The common monosaccharide’s occur in cyclic forms. In-fact the monosaccharide’s with five or more carbon atoms in the backbone usually occur in aqueous solution as cyclic or ring structures.
7 .Haworth perspective formulas are generally used to show the ring forms of monosaccharide’s. The six membered pyranose rings is not really planar, as Haworth perspectives suggest but tends to assume either the boat or chair conformation.
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However, two forms of molecules are inter-convertible without the breakage of any bonds. But two configurations can be interconverted only by breaking a covalent bond-in the case of a and p configurations the bonds involving the pyranose oxygen atom. Three dimensional conformations of the monosaccharide units are important in determining the biological properties and functions of some polysaccharides.
8. A variety of hexose derivatives occur in organisms.
9. Simple monosaccharide’s are reducing agents.
10. Disaccharides contain a Glycosidic bond.
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11. Most of the carbohydrates found in nature occur as polysaccharides or the polymers of high molecular weight. Polysaccharides are also known as glycans.
12 The polysaccharides may be homopolysaccharides having single type of monomeric units and heteropolysaccharides having two or more different kinds of monomeric units.
13. Important storage polysaccharides in nature are starch in plant cells, and glycogen in animal cells.
14. Cellulose and chitin ate structural homopolysaccharides.
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15. Bacterial cell wall has heteropolysaccharide.
16. Glycosaminoglycan’s and proteoglycans are components of extracellular matrix.
17. Most proteins secreted by eukaryotic cells are glycoproteins.
18. The oligosaccharides of glycoproteins have biological functions.
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19. Glycolipids and lipopolysaccharides are the components of cellular membranes.
20 The structure of oligosaccharides and polysaccharides is investigated by a combination of specific enzymatic hydrolysis to determine stereochemistry and produce simple fragments for further analysis.
21. Methylation analysis can be exploited to locate the glycosidic bonds and high resolution NMR spectroscopy to establish sequences and confirm configurations.
