Biomolecules'

BIOMOLECULES
Carbohydrates
All living organisms are made up of basic six essential bio molecules i.e., Carbohydrates,
proteins, nucleic acids, lipids, vitamins and mineral salts. Any food which contains proper
proportional ratio of theses essential bio molecules, then that food is known as nutrition food.
Carbohydrates are the optically active organic compounds which are poly – hydroxy aldehydes or
ketones or compounds which on hydrolysis gives these compounds.

These carbohydrates which are sweet in taste are known as sugar, thereby carbohydrates are also
called as saccharides.
Classification of carbohydrates based on their hydrolyzed products:
Monosaccharide: Poly hydroxy aldehyde or ketones, which do not under go any hydrolysis.
Example: glucose, fructose, galactose.
Oligosaccharide: Poly hydroxy aldehyde or ketones, which on hydrolysis gives 2 – 10 units of
Monosaccharides.
Disaccharide: they give two units of monosaccharide on hydrolysis. Example: sucrose,
lactose, maltose.
Polysaccharide: Poly hydroxy aldehyde or ketones, which on hydrolysis gives more than 10 units of
monosaccharides. Example: starch, cellulose, glycogen.
Classification of carbohydrates based on chemical nature towards Tollen’s reagent:
Reducing sugar: Sugar which reduces Tollen’s and Fehling’s solutions. These sugars will have free aldehyde
or ketonic group in their structures. These sugars are sweet in taste, water soluble and having crystalline
structure. Example: glucose, fructose, lactose, maltose etc.
Non – reducing sugar: Sugar which will not reduces Tollen’s reagent and Fehling’s solutions. These sugar will
not have free aldehyde or ketonic group in their structures. Example: sucrose.
Classification of monosaccharides:
These are classified into two types: (1) based on presence of functional group
(2) based on number of carbon atoms in the structure.
Based on functional group monosaccharides are classified into two types
(1) Aldose or aldo sugar: monosaccharides which are having aldehydic group in their structure.
Example: glucose.
(2) Ketose or keto sugar: monosaccharides which are having ketonic gruoup in their structure.
Example: fructose.
Based on number of carbon atoms present in the structure monosaccharides are classified as follows:
 If the structure of monosaccharide contains three carbon atoms then they called as triose
 If the structure of monosaccharide contains four carbon atoms then they called as tetrose
 If the structure of monosaccharide contains five carbon atoms then they called as pentose
 If the structure of monosaccharide contains six carbon atoms then they called as hexose.
Preparation of glucose:

, From sucrose: Sucrose on acid hydrolysis i.e., Boiled with dil. HCl in alcoholic solution, gives glucose
and fructose.
C12H22O11 + H2O + H+ —> C6H12O6 + C6H12O6
From starch: when starch is boiled upto 393K in the presence of dil. H2SO4 it undergoes hydrolysis giving
rise to glucose.
(C6H10O5 )n + nH2O + H+ —> nC6H12O6
STRUCTURE ELUCIDATION OF GLUCOSE:
1. Elementary analysis of glucose shows that it is having the molecular formula C6H12O6
2. Nature of carbon chain: on reduction with HI it gives n – hexane, showing linear structure.
C6H12O6 + HI —> C6H14
3. Nature of oxygen atom:
(a) presence of carbonyl group:
 it reacts with hydroxyl amine to form oxime
CH2OH (CHOH)4CHO + NH2OH —> CH2OH (CHOH)4CH = NOH + H2O
 it reacts with hydrogen cyanide to give an addition product i.e., Cyanohydrin.
CH2OH (CHOH)4CHO + HCN —> CH2OH (CHOH)4CH(OH)CN
(b) Confirmatory test for aldehydic group:
 it reacts with bromine water gets oxidized to gluconic acid.
CH2OH(CHOH)4CHO + Br2 water —> CH2OH (CHOH)4COOH
4. Presence of five – OH groups: one molecule of glucose give penta acetyl derivative when react with
acetic anhydride.
CH2OH(CHOH)4CHO + 5(CH3CO)2O —> CH(CHCOCH3)4CH2COCH3 +5H2O
Since penta acetyl derivative is stable which shows that each carbon atom in glucose contains one – OH
each.
5. Presence of one primary hydroxylic group:
Glucose undergo oxidation with nitric acid to give dicarboxylic acid called saccharic acid, which is obtained
when gluconic acid is subjected to oxidation using nitric acid.
CH2OH(CHOH)4CHO + nitric acid —> COOH (CHOH)4COOH
Glucose saccharic acid

CH2OH (CHOH)4COOH + nitric acid —> COOH (CHOH)4COOH
Gluconic acid saccharic acid
From the above series of test the structure of glucose may be
OH OH OH OH OH
| | | | |
CHO — C — C — C — C — C — H
| | | | |
H H H H H
Fischer studied the structure of glucose, gluconic acid and saccharic acid and found that the proper stereo
structure of glucose as follows


This glucose molecule is optically active in nature i.e., they turn plane of plane polarized light towards right or
left i.e., Dextro rotatory or leavo rotatory compound. To indicate its optical nature (+) is added before the name