USA: +1-585-535-1023

UK: +44-208-133-5697

AUS: +61-280-07-5697

Polysaccharides

 

They have 10 or more monosaccharide units and are usually insoluble in  water or they may form colloidal suspension. They are tasteless amorphous solids and are called  glycans, e.g., glucans are polymers of glucose, fructans  of fructose, etc. Depending upon the types of monomers found in polysaccharides, they are of two types:

 

(i)                 Homo-polysaccharides: Consisting of one type of monomers (about 95%,     rarely in pure form), e.g., glucans, like starch, glycogen, cellulose that are  consisting of glucose, chitin that is made up of N-acetyl glucosamine (amino derivative of glucose) that are linked by β-1.4-linkage (glucan). Some important homo-polymers are:

 

(a)    Glycogen: It is found as reserve food in animals (liver and muscles), bacteria, fungi, etc. Structurally, it is profusely branched (branches after 8-10 glucose) with α-1,4-glycosidic linkage between a-D-glucose units). It is a non-reducing  carbohydrate and gives red

 

 

 

 

(a)    Starch: It is found as reserve food in plants. It consists of branched chains of   glucose (branches after about 25 Glucose units), where the glucose (a-D-Glucose) units in chains are linked by α-1,4-glycosidic linkage and at the point of branches the bond is α-1,6- glycosid.ic linkage type. It consists of amylose (linear and soluble in hot water) and amylopectin (branched and remains as such in hot water) present in 1 : 4 ratio (Fig. 2.3).

Starch upon hydrolysis by a-amylase yields glucose and maltose. The partial hydrolysis is of starch by a.-amylase produces dextrins, which by the action of the enzyme dextrinase gives rise to glucose.

 

(c) Cellulose: It is found in plant cell wall and consists of unbranched chains of glucose. The glucose microfibrils are held together by hydrogen bonds. It is insoluble in water and highly resistant to hydrolysis. It can not be digested by human beings. The bacteria present in the rumen of cattle, termites, some fungi contain the enzyme cellulase that helps in the digestion of cellulose. Various β-D-glucose units are linked by β-1 ,4-linkages (Fig. 2.4 ).

(d) Chitin: It is found in fungal cell wall, hard exoskeleton of arthropods (insects and  crustaceans), etc. It is a polymer of N-acetyl glucosamine that are linked by β-1,4 glycosidic bonds. The chains are held together by hydrogen bonds (Fig. 2.5)

(e) Inulin: It is present as reserve carbohydrates in many plants and is the polymer of fructose (furanose) (fructan) that are linked by β-1,2 glycosidic bonds (Fig. 2.6).

(f) Mannans: These are polymers of mannose and are found in yeast cell wall. The mannose units are linked by a-1,6 linkages and at branches the bonds are a-1,5 linkage or a-1,3 linkage type.

 

(g) Xylans: These are polymer of D-xylose tbar are linked by β-1,4linkage. It is a type of

hemicellulose.

 

(ii) Heteropolysaccharides: They contain more than one types of monomers (generally 2).   e.g., pectins, hemicellulose, some gums, mucilage. agar, etc. Some important  eteropolysaccharides are:

 

(a) Agar: It is made up of D-galactose and L-galactose (galactan) and is used in bacteriological medium. It is obtained from certain sea weeds (red algae).

 

(b) Pectins: It is found in cell wall, intercellular spaces of plants and consists of arabinose, galactose and galacturonic acid (acid of galactose).

 

(c) Hemicellulose: It is found in the cell wall (in addition to cellulose) of plants and is made up of xylose, L-arabinose. D-galactose. L-rhamnose, D-mannose and D-glucuronic acid.

 

(d) Gums: It is extruded from plants on mechanical injury or after infection and seals the

wound.

 

(e) Murein: It is found in cell wall of many bacteria and consists of a polysaccharisde  consisting of alternate units of N-Acetyl glucosamine or NAG and N-Acetyl mm·amic acid or NAM) and short peptides (tetrapeptide of L-Ala, D-Glu, L-Lys. D-Ata and pentapeptide of L-Gly) that is known as peptidoglycan. Lysozyme enzyme attacks on the peptidoglycan by hydrolysing NAG-NAM bonds.

 

Role of carbohydrates

 

(i) It is found as reserve food, e.g., starch, glycogen, etc.

 

(ii) Cellulose, hemicellulose, pectins, chitin, murein are present in cell walls of many

organisms and, thus, act as structural components.

 

(iii) Heparin prevents blood clotting in blood vessels.

 

(iv) Receptor sites help in responding to external stimuli.

 

(v) In many pathogens, they impart pathogenicity, e.g., capsules in many bacteria

(Diplococcus pneumoniae).

 

(vi) Antibodies, blood group substances, fibrinogen, albumin, plasma proteins are                glycoproteins.

QUESTIONS

 

1. Explain what are carbohydrates?

2. Write short notes on:

(i) Monosaccharides           (ii) Disaccharides       (iii) Polysaccharides.