Classification of Carbohydrates & structure of carbohydrates
Carbohydrates
These are composed of carbon(C), Hydrogen (H) and Oxygen (Hydrogen and oxygen ratio is 2:1).General formula of carbohydrate is Cn H2n On (Exception of this formula is Deoxyribose sugar, Formula is-C5 H10 O4) where value of n is 3 or more. Basic component of carbohydrate is Sugar hence carbohydrate is also called Saccharides. Carbohydrates may have an aldehyde (aldose) or ketone (ketoses) group. Simple Carbohydrates having free aldose or ketose group are called reducing sugars because they can reduce cupric ion (Cu++) to cuprous state (Cu+) (Fehling or Bennedict’solution).
Classification of Carbohydrates
Carbohydrates are classified into 3 main categories on the basis of number of saccharides in hydrolysis
I-Monosaccharides
II-Oligosaccharides
III-Polysaccharides
Monosaccharides
ü These are simplest sugars which cannot be further hydrolyzed into smaller unit. In these sugars numbers of carbon and oxygen atoms are same.
ü All monosaccharide are found “L”, “D” form except Dihydroxy acetone.
ü On the basis of number of carbon atoms monosaccharide are further classified into following small unit.
i-Trioses (C3 H6 O3) –
e.g.Glyceraldehyde (“L” and “D”form) and Dihydroxy acetone. PGAL and DHAP are precursors of all other carbohydrates.
ii- Tetroses (C4 H8 O4) –
The aldotetroses have two chiral centers (asymmetric carbon atoms) and so 4 different stereoisomers are possible. There are 2 natural stereoisomers which are the enantiomers of erythrose and threose that have D configuration but not the L enantiomers. The ketotetroses have 1 chiral center and hence 2 possible stereoisomers: erythrulose (L- and D-form).
e.g.Erythrose, Erythrulose.
iii-Pentoses (C5 H10 O5)-
These have 5 carbon atoms. The aldopentoses have 3 chiral centers and therefore 8 different stereoisomers are possible. e.g. Ribos, Ribulose, Arbinose,Xylulose, , Deoxyribose.
The 2-ketopentoses have two chiral centers, and therefore four different stereoisomers are possible.
Ribose and deoxyribose occur in nucleotides and nucleic acids. Xylose is non-nutritive sweetener, available as xylitol.Arabinose occurs in Gum arabic.
iv-Hexoses(C6H12O6) e.g.Glucose,Fructose,Galactose,Mannose,Rhamnose(C6 H12 O5).
ü Fructose (laevulose) is known as fruit sugar. It is sweetest of all sugar and present in honey.
ü Glucose (Dextrose) is known as grape sugar because found in grape and also called blood sugar. It is main respiratory substrate and hence immediate source of energy .While other hexoses are converted into glucose by liver.
ü Glatctose is a constitute of agar-agar. It is also called brain sugar.
ü Mannitol is alcohol of mannose.
v-Heptoses (C7 H14 O7) – e.g.Sedoheptulose.
Derivative of Monosaccharides
1-Amino sugar:-
Formed by the displacement of hydroxyl group from second carbon atom by amino group .e.g. Glucosamine, Galactosamine.
Glucosamine is raw material of chitin and fungal wall.
2-Sugar alcohol:-
Aldehyde group (-CHO) of the sugar is changed to primary alcohol.
e.g. Sorbital is formed from Glucose and Mannitol from Mannose.
3-Sugar acids:-
These are formed by the oxidation of terminal –CHO (aldehyde) or CH2OH group of sugar to produce carboxyl group (-CHOH). E.g.Glucoronic acid,Galacturonic acid.
Ascorbic acid(vitamin-c) is sugar acid.
Oligosaccharides
These sugars are formed by condention of 2-10 units of monosaccharides.In Oligosaccharides , aldehyde or ketone group of one monosaccharide are linked with alcoholic group of another monosaccharide to form Glycosidic bond(C-O-C).During this reaction one molecule of water H2O eliminates hence glycosidic bond formation is called Dehydration synthesis. Direction of glycosidic bond is 1’ -4’’.
When another monosaccharide unit is fructose then the direction of linkage is 1’-2’’. (Non reducing sugar). For example sucrose.
Types of oligosaccharides
1-Disaccharides:- malt sugar. It is intermediate compound in starch digestion. Maltose has 1’ -4’’ glycosidic linkage between α-D glucose and α-D glucose.
ü Lactose is milk sugar with β-1’-4’’ glycosidic linkage between glucose and galactose.Lactose is least sweetest sugar.Percent of lactose in human milk is 7%.
ü In plant transport of sugar is present in form of sucrose (Invert sugar). Sucrose is also known as Invert sugar or Cane sugar or Table sugar or Commercial sugar.Sucrose composed of α-D Glucose and fructose.
ü Trehalose is present haemolymph of insects.It has glycosides linkage between two anomeric carbon (α-glucose and β-glucose).
2-Trisaccharides:– e.g. Raffinose(Galactose + Glucose +Fructose).
3-Tetrasaccharides:– e.g. Stachyose(Galactose + Galactose +Glucose +Fructose).
4-Pentasaccharides:– e.g. Barbascose(Galactose +Galactose +Glucose +Glucose +Fructose).
Raffinose and stachyose occur in phloem and may be employed for translocation of carbohydrate.
Polysaccharides
ü These are composed of large number of monosaccharides units. These are also called glycans.Polysaccharides are insoluble, tasteless and without osmotic effect.
ü All polysaccharides are non-reducing but in a polysaccharides chain one end are reducing and another end is non-reducing.
Types of polysaccharides
v According to structure polysaccharides are classified into two category
i-Homopolysaccharides:–
(Homo- means same)These are formed of one type of monosaccharides.Depending upon monosaccharides homopolysaccharides are two types
- Hexogan e.g. i-Glucan or Glucosan
ii-Galactan or Galactosan
iii-Fructan or Fructosan
- Pentosan e.g. i-Araban
ii-Xylan
ii-Heteropolysaccharides:–
These are formed by condention of more than one type of monosaccharides or monosaccharides derivatives, e.g. chitin, pectin, hemicellulose.
v According to function polysaccharides are classified into three category
i-Storage polysaccharides:–
These are reserve foods which on hydrolysis produce monosaccharides sugars. E.g. Glycogen, Starch, Inulin.
ii-Structural polysaccharides:–
These are fibrous polysaccharides forming exoskeleton in arthopods, cell walls of fungi and plants.
e.g. Chitin, Fungus cellulose, cellulose.
iii-Mucopolysaccharides:–
These are mucilage producing polysaccharides. E.g. Pectin, Phycocolloides, Hyaluronic acids.
Cellulose:-
It is fibrous polysaccharides that form structural component of plant cell wall and some primitive fungi. Wood is 20-50% cellulose. Cellulose content of cell wall is 20-40%.Cotton fiber is 90% cellulose. Cellulose is digested by microbes present in gut of ruminants and white ants. Linear polymer of β-D-glucose units (6000-10000).It has β 1’-4’’ linkage. Partial digestion gives a cellobiose units (Disaccharides).It is most abundant organic molecule on earth. In urochordate animals cellulose like materials is found that is called Tunicine.hence it is called as animal cellulose. Cellulose is also used to form Rayon Fibers (Artificial silk).
ü Cellulose acetate is used in photographic films, shatter proof glass.
ü Cellulose nitrate is used as Gun cotton and Propellant explosive.
ü Corboxymethyl cellulose is extra-whitening agent of detergent and emulsifier for ice cream, medicine, cosmetics.
Starch:-
Starch is reserved food of plant. Starch is polymer of α-D-glucose units. It contain two types of chains amylose (250-300 glucose units arranged in an unbranched chain by α 1’-4’’ linkage) and amylopectin (30 glucose units link by α1’-4’’ and α-1’ -6’’ linkage).Amylose gives blue color with iodine while Amylopectin gives red color with iodin.Starch present in potato contains 20% amylase and 80% amylopectin.
Iodine Test:-Starch gives Blue color with iodine because Starch form helical structure so starch trap iodine molecules in the helical portion. While Cellulose do not give iodine test due to linear structure of cellulose. Glycogen gives Red color with iodine.
Glycogen:-
Glycogen is storage form of carbohydrate in animals and fungal cells. In animal it is stored in Liver and muscle (Ratio is 3:1).Hence glycogen is called Animal Starch. It 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.
Chitin:-
Also called fungal cellulose. Chitin is linear polymer of N-acetyl-D-glucosamine (it is an amino acyl, NH-CO-CH3, derivative of β-D-glucose) with β-1’-4’’ linkage. Chitin is component of exoskeleton of Arthropod and Cell wall of fungi. It is second most abundant organic molecule on earth.
Insulin:-
Linear polymer of fructose units linked with β-1’-2’’ bonds. It is found in root of Dahalia and Artichoke. It is water soluble polysaccharide and it is used to know glomerular filtration rate.
Heparin:-
It is anticoagulant of blood. Heparin is made up of D-glucuronic acids and N-sulphate glucosamine arranged in alternate order.
Pectin:-
It is Methylated galacturonic acids +galactose +arabinose.Pectin is called plant cement and salt of pectin (Ca and Mg pectate) form middle lamella in plants.
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