CARBOHYDRATES
Introduction
Carbohydrates are a group of compounds which constitute the basic components of animal and plant tissues. The basic chemical unit represented by CnH2nOn, is used by living cells to make complex primary and secondary metabolites in tissues.
Carbohydrates serve as:
i. Sources of energy, e.g. sugars ii. Stores of energy, e.g. starch and glycogen iii. Constituents of shells e.g. chitin iv. Plant supportive tissue, e.g. cellulose
The hierarchical complexity can be summarized as follows:
Monosaccharides
Disaccharides
Trisaccharides, etc.
Oligosaccharides
Polysaccharides
The word “saccharide” was derived from the Greek word sacharr which means sugar.
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Monosaccharides
The monosaccharides are known as the simple sugars.
The smallest simple sugar is D-Glyceraldehyde:
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This is a 3 carbon hydroxyladehyde and is called an aldose or a triose. The isomer with a ketone group on the second carbon is termed a ketose. Thus the simple monosaccharides for a congeneric series of aldoses or ketoses with increasing molecular weight.
Triose has 3 carbon atoms Tetrose has 4 carbon atoms Pentose has 5 carbon atoms Hexose has six carbon atoms Heptose has 7 carbon atoms Octose has 8 carbon atoms Nonose has 9 carbon atoms Decose has 10 carbon atoms Etc.
Thus an aldose is an aldehyde with a terminal –CHO functional group. A ketose is a monosaccharide with a ketone, >C=O functional group.
TETROSES
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D-Ribose
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Aldo-hexose Keto-hexose
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There two types of projections used to represent sugars:
Fischer projection
Alpha-D-Glucose
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Harworth Projection
Alpha-D-Glucose
CYCLIC FORMS OF CARBOHYDRATES
The cyclic structures of sugars can be in two stereochemical forms, the alpha and beta anomers:
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The cyclic forms of sugars can also be found in the six membered ring form, called the pyran structure, or in the 5-membered form called pyran structure:
MUTAROTATION OF SUGARS
Mutarotation of Glucose
Mutarotation is the process whereby there is a change in the specific rotation of plane polarized light when it is passed through an aldohexose, a monosaccharide with six carbon atoms and an aldehyde group. Mutarotation refers to the conversion of a pure anomer of the aldohexose into an equilibrium mixture of two anomers.
Pure Alpha-D-Glucose
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DISACCHARIDES
Disaccharides are formed from the condensation of two simple sugars.
Sucrose is formed from condensation of Glucose and Fructose:
Sucrose is found in sugar cane.
Lactose (milk sugar) is formed from galactose and glucose:
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Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com) Maltose is formed from condensation of two glucose units
FORMATION OF GLYCOSIDES
A sugar reacts with an alcohol to form a glycoside
Therefore the disaccharides are in fact glycosides.
OLIGOSACCHARIDES
An oligosaccharide is a sugar polymer which contains a small number of component simple sugars, usually three to ten sugar units.
Fructo-oligosaccharides are found in many fruits which contain short chains of fructose units.
Inulin is another oligosaccharide but much more complex than fructo-oligosaccharides.
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Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com) The oligosaccharides are therapeutically useful to human because they undigested in the GIT and provide nutrition for the intestinal microflora thereby reducing the population of harmful bacteria.
Galacto-oligosaccharides also occur naturally and consist of short chains of galactose units and these are sometimes metabolized.
POLYSACCHARIDES
The polysaccharides are complex carbohydrates and are polymers formed from monosaccharides which are joined together by glycosidic bonds. They very large and branched and are referred to as macromolecules. They are amorphous, insoluble in water and they have no taste.
When a polysaccharide is formed from only one type of monosaccharide it is called a homopolysaccharide, but when more than one monosaccharide combine to constitute a polysaccharide, it is termed a heteroploysaccharide.
Examples of polysaccharides are:
i. starch and glycogen which are storage polysaccharides. ii. Cellulose and chitin are structural polysaccharides. iii. Inulins are polysaccharides which occur in plants and are of both nutritional and medical significance. In biochemistry, inulin is used in the measurement of Glomerular filtration rate (GFR) in humans. iv. Dextrans are formed by microorganisms from sucrose.
IDENTIFICATION TESTS FOR CARBOHYDRATES
1. Molisch Test
2. Iodine test
3. Benedict’s Test
4. Fehling’s Test
5. Barfoed’s Test
6. Seliwanoff’s Test
7. Anthrone Test
8. Osazone Test
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