Carbohydrates are classified into three main groups depending upon the number of saccharides contained:
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1. Monosaccharides:
These are simplest carbohydrates having only one sugar or saccharide molecule in their constitution.
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They cannot be hydrolysed into small molecules. They are further classified, according to the number of carbon atoms they contain, into trioses, tetroses, pentose, hexoses and-heptoses and so on.
(i) Trioses:
These are smallest molecules of carbohydrates which contain three carbon atoms in their molecules, e.g., glyceraldehyde and dihydroxyacetone. The emperical formula is C3H602.
(ii) Tetroses:
Monosaccharides containing four carbon atoms are called tetroses, e.g., erythrose and threose. The emperical formula is C4H804.
(iii) Pentoses:
Monosaccharides containing five carbon atoms in their molecules are termed as pentoses, e.g., xylose, ribose and arabinose. The emperical formula is C5H10O5.
(iv) Hexoses:
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These contain six carbon atoms in their molecules, e.g., glucose, fructose and galactose.
(v) Heptoses:
Heptoses contain seven carbon atoms in their molecules.
Monosaccharides are the most active carbohydrates as they form the main respiratory substrate of all living cells.
The common monosaccharides are the pentoses and hexoses. They play fundamental role in cellular nutrition.
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The important hexoses are glucose, dextrose, fructose and galactose. These are sweet and neutral in nature. They can be crystallized and are soluble in water. They can also undergo alcoholic fermentation.
The important pentoses are ribose, deoxyribose and ribulose. Ribose is the important constituent of the ribonucleic acid (RNA) and of certain coenzymes as nicotinamide adenine dinucleotide (NAD), NAD phosphate (NADP), adenosine triphosphate (ATP) and coenzyme A (CoA).
Deoxyribose is one of the constituents of deoxyribonucleic acid (DNA). Ribulose is a pentose sugar which is essential for photosynthetic mechanism.
(vi) Glucose:
It occurs in nature in fruits, such as grapes and in honey along with fructose. It is found in the blood, in the intestines during digestion and in the urine jf the diabetics.
It occurs in the body in combination with other substances, e.g., as esters of glucose and hexose phosphates and as lactose in milk in combination with galactose.
It is a white crystalline solid readily soluble in water and sweet in taste. With yeast it is converted into alcohol with liberation of CO2 in bubbles.
(vii) Mannose:
It occurs in various plant seeds. It is a colourless substance readily soluble in water but slightly in alcohol. It is sweet in taste.
(viii) Fructose:
It is found in sweet fruits and in honey. Although it is termed d-fructose but it is strongly laevo-rotatory and hence called evulose.
Chemically it is a ketose. It is readily converted into glycogen in the animal body. It is absorbed slowly in the intestine than other monosaccharides.
It’s excess, in the body, may be converted into fat, starch and glycogen and may pass out in the urine as levulose.
(ix) Galactose:
Galactose occurs in nature in combined form as a constituent of lactose, and in certain complex proteins and lipids.
It is rarely found in plants but in mammals mammary glands prepare it from the blood glucose. It with excess of glucose forms lactose or milk sugar. It is generally used in the body in the same manner as glucose. It is absorbed rapidly in the body.
(x) Pentoses:
Normally they are found in nucleic acid molecules in animal tissues. They are also found as a component of Vit B2 (1) or riboflavine.
They are widely distributed in plants as pentosome (complex polysaccharides) yielding pentoses on hydrolysis. They are all reducing sugars and do not ferment with pur? yeast. They yield acids on oxidation and alcohols on reduction.
Chemical properties of monosccharidcs:
The various chemical properties the monosaccharides exhibit can be studied under the following heads:
1. Reactions is acid solution:
When monosarciuHies (pen¬tose) are treated with a strong mineral acid, the prcruuet furfural is obtained. This is the characteristic reaction of sugars and is known as dehydration.
With hexose, the analogos reaction leads to for mation of 5- hydroxymethyl furfural, which on further heating is transformed to levulinic acid.
If the fiirfurals are treated with variety of phenolic compounds, coloured products are produced. These reactions are important for qualitative and quantitative tests of carbohydrates.
Molish Test:
Take 5 ml sugar solution. Add 2 ml of Molisch reagent (5% a-naphthol in alcohol) and 3 ml of concentrated sulphuric acid to sugar solution. A reddish-violet zone appears between the acid and sugar layer.
2. Reaction in alkaline solution:
When the sugars are treated with dilute alkaline solution, they change to cyclic a-and /3-forms (i.e., mutarotation).
If the above mixture is heated at 37°C, the acidity increases as a result of which a series of enols called ‘endiols’ are formed.
Sugars with strong alkali solution produce decomposition products and yellow and brown pigments develop.
3. Reduction:
When monosaccharides are treated with H2 gas under pressure in the presence of a metal catalyst, or with an active metal, such as Ca, in water, the carbonyl group is reduced to an alcoholic hydroxyl group, yielding a polyhydric alcohol.
D-Glucose, under these circumstances-yields sorbitol (D-gluci- tol), which is also a product of the reduction of L-sorbose.
The reduction products of mannose and galactose are termed, respectively, mannitol and dulcitol.
4. Oxidation:
Monosaccharides are readily oxidized in acids in the presence of mild oxidizing agents. Under this condition when only the aldehyde group is oxidized, an aldomc acid is formed.
When primary alcohol is oxidized, uromc acid is formed. When ootn the groups are oxidized, saccharic acids are formed.
5. Ester formation:
Simple sugars or monosaccharides on treatment with an acid anhydride form sugar acetate and benzoates, etc. During this reaction hydroxyl groups are esterified.
6. Osazone formation:
When monosaccharides are treated with phenylhydrazine, yellow crystalline compounds are formed which are called osazones. The osszone compounds of sugars arc coloured and possess characteristic crystalline forms, melting points and precipitation times.
During this reaction, one molecule of phenylhydrazine first reacts with one molecule of an aldose or ketose sugar to form a hydrazone.
The hydrazone, thus formed, is oxidized in the presence of excess phenylhydrazine to form glucosazone and phenylhydrazine is reduced to form aniline and ammonia.