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Carbohydrates are organic compounds containing carbon, hydrogen and oxygen. Since the proportion of hydrogen and oxygen in carbohydrates is the same as that in water they were thought to represent 'hydrated carbon', hence the name 'carbohydrate' was given to these compounds. Carbohydrates are called saccharides or compounds containing sugar. Based on the number of sugar molecules contained in a carbohydrate we have different types of carbohydrates such as monosaccharides, disaccharides, oligosaccharides and polysaccharides.
Monosaccharides are simple sugars containing a number of carbon atoms. Glucose, for example, is a monosaccharide containing six carbon atoms.

Ribose is a monosaccharide containing five carbon atoms. According to the number of carbon atoms they contain, monosaccharides are classified into
  1. Trioses (C3H6O3);
  2. Tetroses (C4H8O4);
  3. Pentoses (C5H10O5); hexoses (C5H12O6), and septoses (C7H14O7).
Examples of monosaccharides are glyceraldehyde (triose), erythrose (tetrose), ribose (pentose) and glucose (hexose). Hexoses and pentoses exist in both open chain and ring forms.

Hexose sugars such as glucose, fructose and galactose are white crystalline, sweet tasting substances and are extremely soluble in water. 

Glucose is the most important sugar occurring in animals and fructose is the common sugar found in fruits. Deoxyribose is the pentose sugar occurring in deoxyribonucleic acid (DNA).

Monosaccharides exhibit two important chemical properties. Those sugars which have a few aldehyde or ketone group can reduce cu+ to cu+. Such sugars are the reducing sugars and this property of reduction of copper is made use of in Benedicts' and Fehlings' tests used for detection of glucose in urine. The other property of simple sugars is that they can be linked to other compounds; the aldehyde or ketone group of a monosaccharide reacts with an alcoholic group of another compound to form a bond between the two molecules. 

This bond is called the glycoside-linkage. Cane sugar (sucrose) for example is a disaccharide formed by the linkage between a glucose molecule to a fructose molecule. Maltose is another disaccharide formed by the glycoside-linkage between two glucose molecules. Such glycoside-linkage between many glucose molecules result in the formation of what are called polysaccharides such as glycogen which on hydrolysis liberates the individual glucose units.

These compounds are soluble in both water and fats and therefore serve a vital role in the cell by binding water soluble compounds (i.e. proteins) and lipid-soluble compounds together. Lecithin is a key structural material in the cell membrane, because it can maintain continuity between the aqueous and lipid phases of the inside and outside of the cell. Many phospholipid molecules may arrange themselves in a double-layered membrane (lipid bilayer) in aqueous media. Such lipid bilayers are the basic components of the cell membrane. Glycolipids are another class of lipids which contain one or more simple sugars.

Functions of Small Carbohydrates

Small carbohydrates, especially monosaccharides, are important intermediaries in cellular metabolism; trioses, pentoses and heptoses are intermediates in the photosynthetic pathway in plant cells. The process of photosynthesis results in the formation of hexoses. During cellular respiration glucose is oxidized to generate energy which is used for various cellular activities. Small carbohydrates are, thus, the principal source of energy for the body. Glucose is the blood sugar in many animal species. Mammary glands synthesise a disaccharide, lactose which contains one molecule of glucose and one molecule of galactose. Ribose and deoxyribose are important constituents of nucleic acids. Glucose is also utilised for the synthesis of fats and amino acids. Polysaccharides such as glycogen and cellulose are formed by the polymerisation of monosaccharide units. Polysaccharides form important structural components of the cell; cellulose and lignocelluloses which are found in plant cell wall. Cellular membranes contain many oligosaccharides. Starch and glycogen are polysaccharides which are important food storage materials in the cell.

The various polysaccharides differ from one another not only in their sugar composition but also in molecular weight and other structural characteristics. Some of the polysaccharides are linear polymers and others are highly branched. In all cases the linkage that unites the monosaccharide units is glycosidic bond.

Chemically, polysaccharides are inactive and do not ionise. Consequently they are ideal substances as storage compounds. A great many polysaccharides are found in both animal and plant cells. Starch, glycogen, dextrins, cellulose and vegetable mucilages are some of the common polysaccharides found in the cell. Polysaccharides do not form clear solutions in water. They are optically active and are not sweet to taste.

Types of Polysaccharides

Polysaccharides may be of two types namely, homopolysaccharides and heteropolysaccharides. Homopolysaccharides contain the same type of individual sugar units (monomers) of monosaccharides and heteropolysaccharides contain different types of monosaccharides in their structure. Starch, dextrins, glycogen, cellulose and lignin are examples of monopolysaccharides. Important heteropolysaccharides include chitin, hyaluronic acid, and glycoproteins or mucoproteins. Polysaccharides can also be classified according to their function in the cell, that is, as
  1. Food storage polysaccharides, and
  2. Structural polysaccharides.

Food Storage Polysaccharides

These include starch and glycogen.

Fig. 2.3.1. Arrangement of Glucose Molecules in Polysaccharides. Betaglucose is found in cellulose, Alpha Glucose in Amylose and Amylopectin

Starch is a complex polysaccharide formed by the condensation of amylose and amylopectin. Amylopectin is a branched polysaccharide with shorter chains (Fig.2.3.1).

It is found in abundance in plants, seeds, fruits, and tubers. Starch is formed during photosynthesis and is an important energy storage material.

Glycogen is the main storage polysaccharide in animals, and fungi. Glycogen is made up of molecules like amylopectin but with more numerous side chains (Fig. 2.3.2). On hydrolysis by dilute acids glycogen liberates the individual glucose molecules of which it is made of.

The advantages of storing carbohydrates in the form of polysaccharides are: 
  1. Since many water molecules are removed from monosaccharides in the formation of polysaccharides they become more condensed and the bulk of the material can be stored. 
  2. Because of their chemical properties they are relatively easy to be stored and where required, the individual monomer units of monosaccharides could be liberated on hydrolysis.

Fig.2.3.2. Arrangement of Alpha Glucose Molecules in Glycogen

Structural Polysaccharides

Important structural polysaccharides include substances such as cellulose, lignin and chitin.

The cell walls of plants and the external coats of all grains is composed of the homopolysaccharide cellulose. It contains long chains of β-glycosides and many contain thousands of monosaccharide units in a single molecule. These long chains may be further strengthened by hydrogen bonding from one chain to another. It is absolutely insoluble in water and resistant to the action of dilute acids and alkali. Strong acids hydrolyse it to glucose. It is also hydrolyzed by the enzyme cellulose into cellobiose and eventually to glucose. Wood and cotton are two important sources of cellulose.

Cellobiose - Structure

Cellulose is the most abundant organic compound in the biosphere. Because of its many properties it has become one of prime importance to man. Man has used cellulose containing materials for a long time, for shelter, fuel, tools, clothing, etc. Fibres of cotton, linen, and jute are used for textile and ropes. Artificial rayon is a product of cellulose manufactured by dissolving cellulose materials in alkali and by extruding and coagulating the filaments. Another product of cellulose, cellulose acetate, is used in fabrics, cellulose plastics, and shatter-proof glass. Cellulose nitrate is another compound widely used in propellant explosives. Carboxyl methyl cellulose is used in the manufacture of ice-creams, cosmetics and medicines. Cellulose can be hydrolysed to soluble sugars and microbial action on these solutions results in the formation of ethanol, butanol, acetone, methane and other useful materials.

The bulk of the human diet is cellulose containing materials which is a must for the proper functioning of the digestive system. Human beings cannot digest cellulose because of the absence of the enzyme cellulose form the system. Ruminant animals such as cattle digest cellulose with the help of the microorganisms living in their stomach. These microorganisms break up cellulose into digestible materials. Other animals such as snails and termites also depend on microorganisms for cellulose digestion.

It is a structural polysaccharide forming the cuticle of insects. It also forms the cell walls of fungi. The chitin molecule is made up of long chains of monosaccharide units containing amino groups (N-acetyl glucosamine). Chitin is soft and leathery; it becomes hardened when impregnated with calcium carbonate or proteins.

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