Derivatives of Monosaccharides | Biology for ACT PDF Download

Derivatives of Monosaccharides

(1)  Amino Sugars 

Formed by the displacement of hydroxyl group from second carbon atom by amino group e.g. Glucosamine, Galactosamine.

(2)  Sugar Alcohol

Aldehyde  group (–CHO) of the sugar is changed to primary alcohol (–CH₂OH). Sorbitol and Mannitol are respectively formed from glucose and mannose.   

(3)  Sugar Acids

They are formed by the oxidation of terminal –CHO or –CH₂OH group of sugar to produce carboxyl group –COOH e.g. Glucoronic acid, Galacturonic acid.     (4)  Glycoside – They are compounds formed by condensation reaction between a sugar (eg. glucose) and hydroxyl group of another substance which may be a sugar, a sterole, methanol in presence of dry HCl. They are acetal which can be hydrolysed by strong reagents like HCN, NH₂OH, C₆H₅NHNH₂. They cannot be hydrolysed in acidic condition. Streptomycin is a glycoside.

Oligo – Saccharides

Oligo – Saccharides are those carbohydrates which on hydrolysis yield 2 to 10 monosaccharide units (monomers). In oligosaccharides, monosaccharides are linked together by glycosidic bonds. Aldehyde or ketone group of one monosaccharide reacts with alcoholic group of another monosaccharide to form glycosidic bond. One molecule of H₂O eliminates during glycosidic bond formation (dehydration synthesis). Direction of glycosidic bonds is usually 1'.4''.

Types of Oligosaccharides

(i)   Disaccharides – composed of two monosaccharide units e.g. Maltose, Sucrose, Lactose, Trehalose.

 All disaccharides are water soluble and sweet in taste, so they are known as sugar.

Maltose: is commonly called malt sugar. It is intermediate compound in starch digestion. Maltose has 1'-4'' glycosidic linkage between α-D glucose and α-D glucose.

Lactose: is milk sugar with b-1'-4'' glycosidic linkage between glucose and galactose. It is least sweetest sugar.

Maximum % of lactose = Human milk » 7%

Sucrose: In plants transport of sugar is present in form of sucrose. Sucrose is also known as invert sugar. Sucrose is called Cane Sugar or Table Sugar or Commercial Sugar.  Sucrose is composed of α-D Glucose and fructose.

Trehalose: is present in haemolymph of insects. It has glycosidic linkage between two anomeric carbon (α-glucose and β-Glucose).

(ii)   Trisaccharides –       e.g    Raffinose (Galactose + Fructose)

(iii)  Tetrasaccharides –   e.g    Stachyose (Gal. + Gal. + Glu. + Fructose)

(iv)  Pentasaccharides  – e.g    Barbascose (Gal + Gal + Glu + Glu + Fructose)          

Raffinose and Stachyose occur in phloem and may be employed for translocation carbohydrates.

Polysaccharides

1.  Poly saccharides composed of large number of monosaccharide units.
2. Suffix '–an' added in their names and they are known as glycans.
3. Pentose polysaccharides are called pentosans for e.g. xylan, arabinoxylan.
4. Araban (from L-arabinose), xylan (from D-xylose), all these are found in cell wall.
5. Hexose polysaccharides are called ''hexans''. for e.g. mannan (from mannose) cellulose, starch etc.
6. Polysaccharides are insoluble in water and do not taste sweet.
7. All polysaccharides are non-reducing.
8. According to function, they are classified as nutritive and structural.
9. On structural basis polysaccharides are of two types.

(I) Homopolysaccharides

(a)   Cellulose : - Linear polymer of β-D-glucose units (6000 to 10,000) . It has b 1'-4'' linkage. Partial digestion yields a cellobiose units (Disaccharide). Cellulose is main component of plant cell wall. In wood, cellulose is 50% and in cotton, it is 90%.  Most abundant organic molecule on earth.

In urochordates, animals their occur cellulose like material and it is called ''Tunicine'' It is also called animal cellulose.  It is also used to form Rayon fibre (Artificial silk).

(b)  Starch – It is main stored food in plants. Starch is polymer of α-D-glucose units. Starch consists of two types of chains:

(i)   Amylose – 250-300 glucose units are arranged in an unbranched chain by a 1'-4'' linkage.

(ii)  Amylopectin – A branched chain molecule. Approximately 30 glucose units are linked by α-1' 4'' and α-1', 6'' linkage.    

•  Amylose gives blue colour with iodine.
•  Amylopectin gives red colour with iodine.
•   Starch present in potato contains 20% amylose and 80% amylopectin.

(c)  Glycogen – Storage form of carbohydrate in animals, Storage region of glycogen is liver and muscles. Storage of glycogen: liver > muscle. Glycogen is also called as animal starch. Glycogen is highly branched polymer of α-D-glucose.

•  Glycogen is formed by the 1'-4'' bond linkage at long chain and 1', 6'' bond linkage at branching point.
•  Glycogen gives red colour with iodine.
•  Glycogen is store food of fungi.

(d) Chitin – Linear polymer of N-acetyl-D-glucosamine with β-1', 4''-linkage.

•  N-acetyl D-glucosamine is an amino acyl (-NH-CO-CH₃) derivative of β-D-glucose.
•  Chitin is an important component of exoskeleton of Arthropods and cell walls of fungi.
•  Second most abundant organic molecule on earth.
•  It is also called Fungal cellulose. 

(e)  Inulin – Linear polymer of fructose units linked with β-1', 2'' bonds. Insulin is found in roots of Dahalia and Artichoke. It is water soluble polysaccharide and it is used to know the glomerular filteration rate. It is smallest storage polysaccharide.  

(f)  Dextrin – Dextrin is an intermediate substance in the digestion of glycogen and starch. By hydrolysis of dextrin, glucose and maltose are formed. It also occurs as stored food in yeast and bacteria.

(II) Heteropolysaccharide

Composed of different monosaccharide units.

(a)  Hyaluronic acid – Found in vitreous humour, umbillical cord, joints and connective tissue in the form of lubricating agent. It also occurs in animal cell coat as binding material (Animal cement).

• Hyaluronic acid is made up of D-glucouronic acid and N-acetyl – D-glucosamine arranged in alternate orders. These different monosaccharides have β-1'-4'' bonds and such disaccharides have β -1'- 4" bonds.

(b) Chondriotin – D-glucuronic acid + N-acetyl galactosamine.

•  Chondriotin occurs in connective tissue.
•  Sulphate ester of chondriotin is main structural component of cartilages, tendons and bones.

(c)  Heparin – It is anticoagulant of blood. Heparin is made up of D-glucuronic acid and N-sulphate glucosamine arranged in alternate order.

(d) Pectins – Methylated galacturonic acid + galactose + arabinose.

•  Pectin found in cell wall where it binds cellulose fibrils in bundles.
•  Salts of pectin i.e Ca and Mg-pectates form middle lamella in plants.           
•   It is also called Plant cement. 

(e) Hemicellulose  – Mannose + Galactose + Arabinose + Xylulose.

• Store material – Phytalophus (Ivory palm). Hemicellulose which is obtained from this plant is white, hard and shiny and it is used to form billiard ball and artificial ivory. 

Mucopolysaccharides

Slimy polysaccharides with capacity to bind proteins and water are called mucopolysaccharides. In plants, mucilage is a common mucopolysaccharide formed of galactose and mannose units.

Hyaluronic acid, chondriotin, heparin are other examples.

Special Points :

1.   Peptidoglycan – Present in cell wall of bacteria.

– Composed of N-acetyl Glucosamine + N-acetyl muramic acid + peptide chain of 4-5 amino acids.

2.   Agar-Agar –  It is a mucopolysaccharide which is obtained from some red algae – Gracilaria, Gelidium, Chondrus. Its is composed of D-galactose and L-galactose unit and after every 10^th unit a sulphate group is present it is used for preparing culture medium (1, 3 linkage).