Carbohydrates contain carbon, hydrogen, and oxygen atoms with the empirical formula Cx H2y Oy or Cx(H2O)y. Hydrogen and oxygen are found in the 2:1 ratio as in water.
Carbohydrates can be classified into three main groups
- Oligosaccharides (disaccharides)
Monosaccharides – Structure, and Examples
Monosaccharides are simple sugars that can be hydrolyzed into smaller units under reasonably mild conditions. They contain a single carbon chain and are classified with the help of a number of carbon atoms,
C3 sugars are trioses,
C4 sugars are tetroses,
C5 sugars are pentoses,
C6 sugars are hexoses,
C7 sugars are heptoses.
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Some important examples of monosaccharides are as follows –
D-ribose is an aldopentose and is one of the components obtained on the hydrolysis of RNA.
2-deoxy-D-ribose is found in DNA which carries the hereditary traits.
D-xylose occurs in wood, straw, and other fibrous materials in combined forms from which it is obtained on hydrolysis.
D-glucose, dextrose, starch sugar, or corn sugar is the most abundant of hexoses. It is found in ripe fruits, sweet corn, and honey where it is associated with fructose. It occurs in many large carbohydrates.
D-fructose, the most abundant ketohexose in nature is often called levulose or fruit sugar and is widely distributed with D-glucose. It is also associated with several larger carbohydrates. Fructose is the sweetest among naturally occurring sugars.
D-galactose is found largely in combined forms and occurs with D-glucose in disaccharide lactose.
D-mannose, principally as a Component of mannans, is found in fruits like berries, vegetable ivory, and endosperm of nuts of the low growing palm of the tropics.
Oligosaccharides – Structure, and Examples
Two or more monosaccharide units of different or identical types joined end to end form oligosaccharides. If two units of monosaccharides make up sugar, it is called a disaccharide, if three monosaccharide units make up sugar it is called trisaccharide and if four monosaccharide units are used, it is called tetrasaccharide.
Disaccharides – Sucrose, maltose, lactose, and cellobiose
Trisaccharides – Gentianose and raffinose
Tetrasaccharide – Stachyose.
It is formed as a result of linking glucose and fructose units together and eliminating a water molecule.
Though both units are reducing sugars yet sucrose is not a reducing sugar because of the fact that reducing groups of both the simple sugars get engaged in linking together to form sucrose and the oxygen bridge is formed between carbon-I of glucose and carbon-2 of fructose resulting in the elimination of free carboxyl groups of glucose and fructose In sucrose unit, glucose occurs as a six-membered ring (pyranose ring) and fructose as a five-membered ring, (furanose ring) hence sucrose may be called α-glucopyranosido-ß-fructo-furanoside, Sucrose is commonly known as cane sugar, beet sugar or saccharose.
It is most widely distributed in free form as compared to other sugars of this group and occurs in most green plants.
The commercial sources of this sugar are sugarcane, beet, palm, maple, sorghum cane, etc.
Sucrose is the storage product of photosynthesis in sugarcane and sugar beet.
Maltose and cellobiose –
They are the products of partial degradation of polysaccharides such as starch and cellulose.
Both are reducing sugars. Maltose consisting of two units of D-glucose joined together in α(1-4) links while cellobiose consists of two D-glucose units joined together by β (1-4) links.
Maltose is found in small amounts in cell sap and in greater quantities in germinating malted grains.
Lactose is also known as milk sugars, is found in the milk of mammals and is a disaccharide of both galactose and glucose.
Stachyose is an example of a tetrasaccharide consisting of one unit each of glucose and fructose and two units of galactose. This tetrasaccharide sugar has been reported from several tree species.
Polysaccharides – Structure, and Examples
Polysaccharide is the polymerized form of simple sugars (monosaccharides) and consists of many carbon atoms. If polysaccharides consist of similar sugar units, these are called homopolysaccharides and if of dissimilar units, these are called heteropolysaccharides. Polysaccharides made up of pentoses are known as pentosans while those of hexoses are hexosans.
Homopolysaccharides – Starch, cellulose, and glycogen
Heteropolyyccharides – Pectins, chitins, and hemicellulose
Starch is found abundantly in rice, wheat, and other cereals, grams, legumes, potato, tapioca, and bananas. It is a hexosan polysaccharide made up of a large number of glucose units.
The glucose units are arranged in two components, e.g. amylose and amylopectin. In general, starch contains 20-30 % of amylase while the rest is amylopectin.
In some varieties of maize and other cereals, starch consists entirely of amylopectin whereas the wrinkled surface of certain varieties of a pea may have as much as 98% amylose.
Both amylose and amylopectin are formed by the condensation of α-D-glucose (pyranose form). Amylose is in the form of a straight-chain which maybe consisting of 2000 α-D-glucose units. Glucose units are linked together at 1-4 α-linkages.
Amylopectin contains, besides a straight chain, several side chains which may be branched further. During the formation of a side chain, the carbon-1 of the first unit of glucose is linked to the C-6 (sixth carbon of glucose) of the straight chain. The number of α-D-glucose units in amylopectin is about 100,000.
Starch is stored in the form of grains. There may be a number of grains in each chloroplast, but it is temporary or assimilation starch. However, there is only one large grain per amyloplast. It is a reserve or stored starch.
Cellulose – It is the most abundant organic compound found in the biosphere. It is the chief constituent of the cell wall of autotrophic plants. Dried wood contains about 65 % cellulose, 30% lignin, and 1 to 2% pentosans. Cellulose exists in several forms of which alpha form is the most common (alpha not related to ring form). The pure alpha form is commercially obtained from cotton linters, flax, and wood. Cellulose (4-β-glucopyranosido-glucose) is a glucogen made up of 500 to 20,000 β-glucose units. The glucose units are linked together by oxygen bridges with 1-4 β linkages.
Cellulose occurs in the form of microfibrils, each consisting of two parts, crystalline core and non-crystalline cover (paracrystalline). The crystalline part consists entirely of pure cellulose while the non-crystalline may have many substances including hemicellulose.
Cotton fiber contains about 90% cellulose while wood contains only about 20-50%. The artificial fiber RAYON is manufactured by dissolving cellulosic materials in alkali and by extruding and coagulating the filaments. By treatment with other chemicals, cellulose is converted into cellulose nitrate which is used in propellant explosives, cellulose acetate used in fabrics, cellulosic plastic, and shatterproof glasses, and carboxymethyl cellulose which is added to ice creams, cosmetics, and medicines to emulsify and to give a smooth taste.
Hemicellulose – Hemicelluloses are water-insoluble carbohydrates consisting of L-arabinose, D-galactose, D-glucose, and D-xylose. Uronic acids (glucuronic and galacturonic acids) are also found present in them.
On the basis of the presence and absence of uronic acids, two types of hemicelluloses are recognized:
1) pentosans (contain no uronic acids),