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The following points highlight the top three classifications of carbohydrates. The classifications are: 1. Monosaccharides 2. Disaccharides 3. Polysaccharides.
Carbohydrates: Classification # 1. Monosaccharides:
Trioses and Tetroses:
Phosphates of the two trioses, glyceraldehyde and dihydroxyacetone, are formed from fructose-1, 6- diphosphate by glycolysis. 3-phosphoglyceraldehyde and erythrose-4- phosphate are formed by the hexose monophosphate shunt.
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Pentoses:
Ribose and Deoxyribose are important constituents of nucleic acids and many coenzymes. The structures are given at the right. Deoxyribose is lacking in one atom of oxygen from carbon no. 2.
Heptoses:
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Sedoheptulose is a ketoheptose found in plants. Its phosphate is important as an intermediate in the hexose monophosphate shunt and has been identified as a product of photosynthesis.
Important Chemical Reactions of Monosaccharides:
(i) lodo Compounds:
An aldose sugar, when heated with concentrated hydriodic acid (HI), loses all of its oxygen and is converted into an lodo compound (glucose to iodohexane, C6H13I).
(ii) Ester Formation:
Sugars, by virtue of the alcohol groups, readily form esters with acids. All the free -OH groups are replaceable. The greater biochemical significance is the ester with phosphoric acid and, to a lesser extent, with sulphuric acid. Pentose phosphates are involved in the formation of nucleic acids.
(iii) Acetylation:
The acetylation with acetyl-chloride indicates the presence of -OH group present in the sugar. The presence of 5 OH groups of glucose results in a penta-acetate.
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(iv) Oxidation:
Oxidation of the aldehyde group forms “al-donic acids”. If the aldehyde group remains intact and the primary alcohol group is oxidized “uronic acids” are formed. Oxidation of galactose with concentrated HNO3 yields the dicarboxylic mucic acid. This compound crystallizes readily.
(v) Reduction:
The monosaccharides are reduced to their corresponding alcohols by reducing agents such as sodium amalgam.
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Thus, Glucose yields sorbitol (Protects liver).
Galactose yields dulcitol.
Mannose yields mannitol.
Fructose yields mannitol and sorbitol.
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Mannitol:
(a) It is neither reabsorbed nor secreted in tubules.
(b) It is used as force diuretics i.e. to increase the volume of urine.
(vi) With Strong Mineral Acids:
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There is a change of hydroxyl groups towards and of hydrogen away from the aldehyde end of the chain.
Reaction products with acids will condense with certain organic phenols to form compounds having characteristic colours.
(vii) Heat:
Gluconic acid on heating produces lactones. These are cyclic structures which resemble pyranoses and furanoses.
(viii) With Alkali:
Monosaccharides react in various ways:
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(a) In dilute alkali the sugar will change to the cyclic alpha and beta structures, with an equilibrium between the 2 isomeric forms.
On standing, a rearrangement will occur which produces an equilibrated mixture of glucose, fructose and mannose through the enediol form.
(b) In concentrated alkali, sugar produces a series of decomposition products. Yellow and brown pigments develop, salts may form, many double bonds between carbon atoms are formed, and carbon-to-carbon bonds may rupture.
(ix) Osazone Formation:
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It is nothing but the formation of crystalline derivatives of the sugars which are valuable in the identification of sugars.
These crystals are obtained by adding a mixture of phenyl hydrazine hydrochloride and sodium acetate to the sugar solution and heating in a boiling water bath.
The carbonyl group (i.e. aldehyde or ketone group) and the next adjacent carbon are involved in this reaction. With an aldose the reaction is shown Fig. 3.17.
The hydrazone then reacts with two additional molecules of phenyl hydrazine to form the osazone. The ketoses also show similar reaction.
From the comparison of their structures it may be noted that glucose, fructose and mannose form the same osazone; whereas galactose forms a different osazone because the part (carbon 4) in the structure of galactose which differs from that of glucose, fructose and mannose remains unaffected in osazone formation.
(x) Other Reactions:
Various other reactions take place due to the presence of aldehyde or ketone groups of the sugars which are important for analytical purposes. The best-known tests are reduction of metallic hydroxides together with oxidation of the sugar.
The alkaline metal is kept in solution with sodium potassium tartrate (Fehling’s solution) or sodium citrate (Benedict’s solution). Other metallic hydroxides may be used (Bismuth, Ammoniacal silver, Tollen’s test, Nylander’s test).
Barfoed’s test distinguishes between monosaccharides and disaccharides. The copper acetate in dilute acid is reduced in 30 seconds by monosaccharides; whereas reduction of the same takes several minutes by disaccharides.
Carbohydrates: Classification # 2. Disaccharides:
The disaccharides are composed of two monosaccharide units united by a glycosidic linkage.
The physiologically important disaccharides are maltose, lactose and sucrose:
Maltose = 1 mol. glucose + 1 mol. glucose.
Lactose = 1 mol. glucose + 1 mol. galactose.
Sucrose = 1 mol. glucose + 1 mol. fructose.
a. Maltose (Malt sugar):
(i) It does not occur in the body.
(ii) The sources of it are germinating cereals and malt and it is the intermediate product in the breakdown of starch by amylase in the alimentary canal.
(iii) It is hydrolysed to glucose by the enzyme maltase and the products are absorbed.
(iv) It has one free aldehyde group and hence shows mutarotation and the final rotation of the solution is +130°. It can exist in α or β forms.
(v) It can reduce Fehling’s and Benedict’s solutions since it is a reducing sugar but cannot reduce Barfoed’s solution.
(vi) It forms an osazone with phenyl hydrazine.
b. Lactose (Milk Sugar):
(i) It is present in milk and formed in the lactating mammary gland. It may occur in urine during pregnancy.
(ii) It is hydrolysed to glucose and galactose by the enzyme lactase in the alimentary canal and the products are absorbed.
(iii) It has free aldehyde group and hence shows mutarotation and the final constant specific rotation of the solution is +55.2°. It can also exist in α or β forms.
(iv) Since it is a reducing sugar it can reduce Fehling’s and Benedict’s solutions but cannot reduce Barfoed’s solution.
(v) It forms an osazone with phenyl hydrazine.
c. Sucrose (Cane Sugar):
(i) It does not exist in the body but occurs in cane sugar, pineapple, carrot roots, sweet potato and honey.
(ii) It is hydrolysed to glucose and fructose by the enzyme invertase (sucrase) in the alimentary canal. The products of hydrolysis are absorbed.
(iii) It has no free aldehyde or keto group because the linkage is between the aldehyde group of glucose and keto group of fructose. Hence it is a non-reducing sugar. It does not exhibit mutarotation and cannot exist in α or β forms.
(iv) Since it is a non-reducing sugar it does not reduce Fehling’s or Benedict’s solutions. It cannot reduce Barfoed’s solution too.
(v) It cannot form osazone with phenyl hydrazine.
(vi) The specific rotation of sucrose solution is +66.5°. During hydrolysis this rotation changes to -19.5°; since the laevorotation of fructose is greater than the dextrorotation of glucose. The product of the hydrolysis is used to be referred to as “invert sugar”.
Carbohydrates: Classification # 3. Polysaccharides:
Polysaccharides are classified as homopolysaccharides and heteropolysaccharides.
The physiological important homopolysaccharides are Cellulose, Glycogen, Starch, Dextrins, and Inulin.
a. Cellulose:
(i) It is the main constituent of the supporting tissues of plants and forms a considerable part of our vegetable food. It does not occur in the animal body.
(ii) It is made up of β-glucose molecules which are linked by 1:4 linkages.
(iii) Owing to the difference in chemical structure, cellulose is not acted upon by amylases present in the digestive juices.
(iv) It is of considerable human dietetic value only because it adds “bulk” to the intestinal contents, thereby stimulating peristalsis and elimination of food residues.
(v) It is insoluble in ordinary solvents and gives no colour with iodine.
(vi) Cotton and filter paper are nearly pure cellulose.
b. Glycogen (Animal Starch):
(i) It is the reserve carbohydrate found in liver and muscles of animals and human beings. The glycogen content of liver is more than that of muscle.
(ii) It is also found in plants which have no chlorophyll system (e.g., fungi and yeasts), but not in green plants.
(iii) The molecular weight of glycogen obtained from different sources may range from 105 to 108 and each molecule contains from 5,000 to 10,000 glucose molecules.
(iv) It has a branched structure with straight chain units of 12- 18-α-D-glucopyranose [in a 1 -4 glucosidic linkage] with branching by means of a [1-6]-glucosidic bonds.
(v) It is non-reducing, readily soluble in water and gives a red colour with iodine.
c. Starch:
(i) It is the stored carbohydrate of chlorophyll-containing plants. In plants, the starch is laid down in the cells in granules. The microscopic form of the granules is characteristic or the source of the starch.
(ii) It is formed by an α-glucosidic chain. Such a compound—which produces only glucose on hydrolysis—is called a glucosan.
(iii) It is the most important source of carbohydrate in our food and is found in cereals, potatoes, legumes and other vegetables in high concentrations.
(iv) It is a mixture of two substances – amylose and amylopectin – both are composed of glucopyranose units.
In the amyloses, the glucose units are joined by 1, 4 – α links to form un-branched chains which are in the form of a helix with six glucose units per turn.
Their molecular weights are about 60,000 which are equivalent to about 300 – 400 glucose units and are responsible for the development of blue colour with iodine.
Amyiopectins have much larger molecular weights of about 500,000 and the chains have at least 80 branches; each branch is at an interval of 24-30 glucose units. The point of branching is the sixth carbon atom of glucose.
(v) Raw starch is insoluble in cold water owing to the resistance of the outer cellulose layer of the granule.
When this is ruptured by heating in water, starch is soluble. Concentrated solutions gelatinise on cooling and are used as an adhesive-starch paste.
d. Dextrins:
(i) Dextrins are formed by the partial hydrolysis of starch by an enzyme (salivary amylase), dilute mineral acids or heat.
(ii) Amylodextrin, erythrodextrin and achrodextrin give blue, red-brown and no colour, respectively, with iodine, Achrodextrin being the simplest.
(iii) If they have reducing properties at all, they are very feeble.
(iv) They have a faint sweet taste.
(v) They form sticky solutions in water and are frequently used as adhesive, e.g., on postage stamps.
(vi) The final product of hydrolysis of starch by an amylase is maltose which is hydrolysed to glucose by maltase.
e. Inulin:
(i) It is found in tubers and roots of dahlias, and dandelions.
(ii) It is hydrolysed to fructose; hence it is a fructosan.
(iii) It does not produce any colour with iodine.
(iv) It is easily soluble in warm water.
(v) It is used in physiological investigation for the determination of the rate of glomerular filtration.
