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In this essay we will discuss about the simple and compound carbohydrates which has been classified according to the number of simple sugar present in the molecules.
Essay on Carbohydrates
Essay # 1. Simple Carbohydrates:
They contain only one unit of simple sugar. For instance, glucose, fructose, galactose, etc.
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Monosaccharides are further subdivided in two ways:
(1) According to the number of carbon atoms present in the molecule, viz., monose, diose, triose, tetrose, pentose, hexose, heptose, etc., containing 1, 2, 3, 4, 5, 6 and 7 carbon atoms respectively;
(2) According to the nature of the reducing group they contain, viz.
(a) Aldoses:
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E.g., glucose. Here, the reducing group is an aldehyde (-CHO) radicle.
(b) Ketoses:
These contain. Ketone (C=0) group, viz., fructose. The general formula of monosaccharides is Cn(H2O)n.
General Properties:
(a) They are colourless crystalline compounds having a sweet taste,
(b) Chemically they are derivatives of polyhydric alcohols. The aldoses are derived from primary alcohols and the ketoses from secondary alcohols.
(c) Formation of Esters:
By virtue of the alcohol group they easily form esters with acids, e.g., acetates, benzoates, etc. Such esters, having great physiological importance, are those of phosphoric acid, viz., hexose phosphates, which play an important role in carbohydrate metabolism; pentose phosphates, present in the nucleic acid and such others,
(d) Reducing Power:
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By virtue of the aldehyde (-CHO) or the ketone (C=O) groups, the simple sugars are powerful reducing agents. They easily reduce alkaline copper, bismuth or silver solutions,
(e) Isomerism:
Due to the presence of asymmetric carbon atom in the molecule, monosaccharides may remain in different isomeric forms,
(f) Optical Rotation:
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Simple sugars rotate the plane of polarised light and therefore may exist in either dextro or laevo forms,
(g) Condensation:
Simple sugars condense and form bigger carbohydrate molecules, viz., polysaccharides,
(h) Formation of Glycosides:
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When replaceable hydrogen atom of a hydroxyl group from sugar is substituted with other radicles it is ‘called a glycoside; that formed from glucose is a glucoside, from galactose a galactoside and the like. A good number of glycosides occur in leaves, roots, etc. of plants and are bitter solids. Phlorizin (glucose-phloretin), digitonin (galactose+xylose+digitogenin), indican (glucose + indoxyl) are-a few examples of them,
(i) Sugar Acids:
Either carbon atom one or six in a hexose molecule may be oxidised to carboxyl group. That formed from oxidation of number one is named hexonic acid (glucose → gluconic acid), and from six called uronic acid (glucose → glucuronic acid). Many drugs and a number of hormones combine with glucuronic acid in the body and are excreted as glucuronides.
(j) Formation of Hexosamine:
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Replacement of hydroxyl group in hexose sugar with amino one produces amino sugar or hexosamine, that formed from glucose is glucosamin. They occur in some complex polysaccharides.
(k) Osazone Formation:
All reducing sugars condense with phenylhydrazine and produce osazone compounds. The crystalline forms of those osazone compounds are so characteristic that they can be used for the identification of the particular sugar. However glucose, glucosamine and fructose form similar osazones.
(l) Ring Structures of the Sugars:
Sugars not only remain as straight chain compounds but may also remain in the form of rings. This ring may include six members (five carbon atoms and one oxygen atom) or five members (4 carbon atoms and 1 oxygen atom). Haworth has suggested all sugars having six-membered rings to be called pyranoses (from their relation to pyran) and those forming five-membered rings, furanoses (from furan) (vide below), and
(m) Fermentation:
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Sugars, in general, undergo fermentation by yeast and other microorganisms.
Physiologically Important Monosaccharides:
i. Trioses (Three-Carbon Sugars):
Two trioses are of great physiological importance, glyceric aldehyde and dihydroxyacetone. The corresponding phosphoric acid esters-phosphoglyceraldehyde and dihydroxyacetone phosphate are of great physiological value.
Formation of these triosephosphates is an essential step in the metabolism of sugar.
ii. Pentoses (Five-Carbon Sugars):
(a) Two different pentoses are present in two nucleic acids, RNA and DNA. Sugar ribose is present in the RNA, whereas that present in DNA is deoxyribose. In the Free State both the sugars exist in the pyranose form. In combination with nucleic acids they exist as deoxyribofuranose and ribofuranose.
(b) Arabinose:
It is also a pentose commonly found in gum Arabic—chiefly as l-arabopyranose.
iii. Hexoses (Six-Carbon Sugars):
The following hexoses are of physiological importance:
a. Glucose (Dextrose, Grape Sugar):
It is found in grapes, dextrorotatory and is an aldose. Glucose is found in nature in ‘free form’, in the form of disaccharides (maltose, sucrose, etc.), in the form of polysaccharides (starch, glycogen, etc.) and in combination with proteins (glycoproteins). It may remain both as a straight chain compound and as well as, in various ring forms, a- and p-glucose are the isomeric forms.
The optical reaction of glucose is peculiar. As soon as dextrose is dissolved in water it gives a specific rotation of +112°. On standing the rotation gradually diminishes and finally remains constant at +52.5°. This phenomenon is called mutarotation. If however, dextrose be first recrystallised from boiling pyridine, its solution in water at first gives a rotation of +19° and not +112°.
This solution on standing also shows mutarotation in which the specific rotation gradually increases and finally becomes constant at the same value, +52.5°. This shows that when in solution one particular variety changes into the other, until an equilibrium mixture of the two forms is produced with a constant rotation of +52.5°.
Due to asymmetrical nature of carbon atom 1 in ring form of glucose, it can exist in two forms. The a-glucose has a rotation of +112° and the P form, +19°. A solution of glucose is an equilibrium mixture between the straight chain and both a and P-ring forms, the percentage of p being more than a. Glucose forms esters with phosphoric acid.
b. Fructose (Laevulose, Fruit Sugar):
It is laevorotatory and is a ketone. Free fructose is present in honey and nearly in all sweet fruits. In combination it is found in cane sugar and inulin. In the human body normally it does not occur in Free State and rapidly converted into glucose in the liver and intestine. In contrast with other sugars, fructose is soluble in hot absolute alcohol. Like glucose it also forms phosphoric acid ester which plays an important role in carbohydrate metabolism.
c. Galactose:
It does not occur free in nature. In the body it is found as a constituent of lactose (milk sugar) and cerebrosides (galactolipids). As polysaccharide (galactan) it is present in lichens, mosses, sea-weeds, etc.
d. Mannose:
It does not occur free in nature. It is found as a constituent of certain animal proteins and converted to glucose in the body. It is also found as a polysaccharide known as mannan specially found in the ivory nuts.
iv. Heptoses (Seven-Carbon Sugars):
In the metabolic pathway of pentose phosphate by hexose monophosphate shunt, a seven-carbon keto sugar, sedoheptulose is formed as an ester with phosphoric acid (vide Metabolism).
Essay # 2. Compound Carbohydrates:
These are made up of from two to one thousand or more than thousand monosaccharide units either with or without non-carbohydrate units. Thus, on hydrolysis these may give rise to monosaccharide and non-carbohydrate units.
These are principally of two types:
i. Simpler Compound:
The simpler compound carbohydrates contain only a few monosaccharide units and are crystalline, water-soluble and give sweet taste. These simpler compounds are generally called oligosaccharides. Oligosaccharides on hydrolysis give rise to at least two to ten mono-saccharide units. Thus oligosaccharides are composed of disaccharides (two monosaccharide units), trisaccharides (three monosaccharide units) and tetrasaccharides (four monosaccharide units).
ii. More Complex Compound:
These carbohydrates include glycogen (glucan), cellulose, starch and also dextrins which are composed of ten or more monosaccharide units. These are mostly tasteless (dextrins are slightly sweet) and are amorphous solid substance. As these compounds are composed of many monosaccharide units, these are called polysaccharides (glycans) which are composed of ten or more monosaccharide units, held together by glycosidic linkages.
Those polysaccharides are composed of monosaccharides are called homoglycans (e.g., glycogen (glucan), starch, cellulose, etc.) whereas those are made up of two or more different types of monosaccharides are named as heteroglycans (e.g., mucopolysaccharides).
Simpler compound carbohydrates:
Oligosaccharides include:
(a) Disaccharides;
(b) Trisaccharides;
(c) Tetrasaccharides.
(a) Disaccharides:
These can be regarded as condensation products of two monosaccharide units with the elimination of one molecule of water. Their general formula is Cn(H2O)n-1. During union the active groups (aldehyde or ketone) become engaged. In lactose only one aldehyde group becomes engaged and one aldehyde group (reducing group) remains free.
Similar is the case with maltose. But in cane sugar both the active radicles (-CHO of glucose and -CO of fructose) become engaged. For this reason lactose and maltose can reduce alkaline copper (although much less than monosacchrides), form characteristic osazone crystals and exhibit mutarotation. Cane sugar, on the other hand, in which both the active radicles are engaged, show no reducing power, does not form any osazone nor shows any mutarotation. In nature about 16 disaccharides are present.
Of these only the following are of physiological importance:
i. Lactose (Milk Sugar):
Composed of one molecule of glucose and one of galactose, found in the milk of mammals.
ii. Maltose (Malt Sugar):
Composed of two glucose units. It is an intermediate product in the digestion of starch.
Sucrose (Cane Sugar):
On hydrolysis it gives rise to one molecule of glucose and one of fructose. It is the chief form of sugar taken in diet. It is widely distributed in many plant juices, such as sugar cane, sugar maple, and pine apple and also in sugar beets.
Sucrose is dextrorotatory but on hydrolysis the resulting mixture becomes laevorotatory owing to the liberation of fructose. The laevorotation of fructose is greater than the dextro rotation of glucose. For this reason the mixture is known as invert sugar and the enzyme (sucrase) which hydrolyse sucrose is also called invertase.
iii. Trehalose (Composed of Two Units of Glucose):
It is present in the haemolymph of insects as the principal sugar, also occurs in yeast and fungi.
(b) Trisaccharides:
These types of oligosaccharides on hydrolysis give rise to two monosaccharide units.
These trisaccharides are:
i. Mannotriose giving rise to two molecules of galactose and one molecule of glucose on hydrolysis.
ii. Robinose on hydrolysis gives rise to galactose and two molecules of rhamnose.
iii. Rhamninose on hydrolysis gives rise to galactose and two molecules of rhamnose.
iv. Raffinose on hydrolysis gives rise to fructose, glucose and galactose.
v. Gentianose on hydrolysis gives rise to fructose and two molecule of glucose.
vi. Melibiose (melezitose) on hydrolysis gives rise to fructose and two molecules of glucose.
Among six trisaccharides described, the first three are reducing and the rest three are non-reducing. The general formula of trisaccharides is Cn(HzO)n-2.
(c) Tetrasaccharides:
Only two tetrasaccharides are known.
These are:
(1) Stachyose which is composed of D-glucose, galactose and fructose found in Stachys tuberifera, and
(2) Scorodose in the bulbs of garlic and onion.
More Complex Compound Carbohydrates (Polysaccharides):
Polysaccharides are made up of a large number of monosaccharide units. During condensation all the active radicles become engaged, so that they do not show any reducing power, do not produce any osazone and are generally not sweet to taste. They are soluble in water excepting cellulose. The molecules are large and hence, show colloidal properties. The empirical formula of polysaccharides is (C6H10O5)n.
Polysaccharides are classified according to the nature of the constituent units. For instance, those made up of pentose units are called pentosans, those of hexose unit’s hexosans, etc., more specifically they are described after the name of the particular units of which they are composed.
For instance, glucosans (dextrans) are made of glucose, galactans of galactose, fructosans (laevans) of fructose and such others. Starch, glycogen and cellulose are glucosans. Inulin is afructosan. Agar is a galactosan (galactan). Gums are mixtures of both pentosans and hexosans.
i. Starch:
This is the chief form of carbohydrate taken in diet. It is manufactured by the plants and plays the same role in them as glycogen does in animals, i.e., an easily available sugar store. It is the main constituent of food grains. Starch from different sources can be identified from their microscopic peculiarities. It is insoluble in cold water due to the presence of another cellulose layer around the granule.
On boiling the insoluble cellulose covering ruptures and starch enters into a colloidal solution. It is a glucosan and on hydrolysis breaks down into glucose. It gives blue colour with iodine. It has got no reducing power and is tasteless. Starch is a mixture of two substances having similar structure – amylose (mol. wt. 60,000) and amylopectin (mol. wt. 300,000) both being composed of chains of 24 to 30 glucopyranose units.
ii. Glycogen:
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It is called animal starch; because it is in this form that glucose remains stored in the liver and muscles of animal body. Glycogen is also found in those plants which do not possess any chlorophyll, such as yeast, fungi, etc., but not-in green plants. There is probably more than one type of glycogen.
When rabbit is fed with glucose, it synthesises a glycogen containing 12 glucose units, but if fed with galactose it forms a glycogen with 18 glucose units. But curiously enough, mixtures of glycogens with 12 and 18 units are not found. Glycogen is soluble in water, makes an opalescent solution and gives reddish colour with iodine. The glycogen molecule contains many glucose units. The units are joined by linkage between carbons 1 of one unit to carbon 4 of the next one; branches involving 1, 6 linkages are frequently present.
iii. Dextrins:
They do not occur naturally, but are the split product of starch resulting from hydrolysis. For this reason they ought to have been described as derived carbohydrates. The term dextrin is a group name including several varieties of dextrins. The earlier products of hydrolysis of starch are all called dextrins. The names of the different dextrins are given according to their colour reaction with iodine.
The first dextrin formed is amylodextrin giving blue colour with iodine, the next product is called erythro-dextrin giving red colour with iodine and the next product gives no colour with iodine and is called achroodextrin. As these products become smaller and smaller, they develop more and more the characteristic properties of monosaccharide, such as the reducing power, a sweet taste and the others.
iv. Cellulose:
It is a stable, insoluble compound, found in the plants and never present in the animal body. Cellulose is taken with vegetable food. It cannot be digested by the human beings. Herbivorous animals can digest cellulose with the help of bacteria. Although indigestible, yet cellulose is of considerable importance.
In human dietetics because it adds ‘bulk’ to the intestinal contents, stimulate peristalsis and thereby help in the formation and evacuation of faeces. Filter papers, commonly used in the laboratories, are almost pure cellulose.
v. Inulin:
It is a polysaccharide composed of D-fructose units. It is a white crystalline powder and is readily soluble in hot water. It does not give any such characteristic colour with iodine.
Other polysaccharides (heterosaccharides) containing carbohydrate and non-carbohydrate units are:
(a) Heteropentosans (mucilages, gums, peptic substances, etc.).
(b) Heterohexosans (hignocellulose, pectocellulose, etc.).
(c) Mucopolysaccharides (mucin and mucoids) are composed of two to six different monosaccharide units including amino sugar and uronic acid.
Many important biological compounds are included in this group; they are hyaluronic acid, chondroitin sulphate, heparin, blood group substances, etc. The first three are called acid mucopolysaccharides due to their acidic character. The mucopolysaccharides in tissues are usually present in combination with protein and are called glycoproteins (mucoproteins, mucoids).
