These are hydrates of carbon, hydrogen and oxygen.
Carbohydrates provide 55-65% of total energy required.
Based on hydrolysis carbohydrates can be classified as below:
Simplest form of sugars
Do not undergo hydrolysis but oxidize to carbon dioxide and water
These are soluble in water but less soluble in alcohol and insoluble in ether.
Number of carbons is 3 - 7
Suffix -ose is used in nomenclature
General Formula: Cn H2n On
A six membered ring is known as pyranose and five membered rings is furanose as shown below:
Structure of Pyranose
Structure of Furanose
Based on the nature of functional group, monosaccharides are classified in two groups:
They have aldehyde group (-CHO) as functional group . Suffix -ose.
Examples: Glyceraldehyde, Erythrose, Ribose
They have ketone group (-C=O) as functional group. Suffix -ulose.
Examples: Fructose, Tagatose
Isomerism in Carbohydrates
Monosaccharides have at least one asymmetric carbon atom (except dihydroxyacetone), thus exist in different isomeric forms.
(a) D and L Isomerism
The D and L Isomers are mirror images of each other.
As show in the Fig. the orientation of the -OH group on the carbon atom adjacent to terminal primary alcohol carbon if on the right side is D-form and if on the left side is L form.
With reference to the Fig. as shown below if two monosaccharides differ from each other in their configuration around a single carbon atom other than anomeric atom, they are regarded as epimers of each other.
Epimerases is the enzyme which catalyses the interconversion of epimers and the process is thus called as epimerization.
As shown in the Fig. α and β cyclic forms of D-glucose are known as anomers with respect to configuration only around hemiacetal carbon known as anomeric carbon.
The configuration of α anomer comprises of OH group held by anomeric carbon on the opposite side of the group -CH2OH of sugar ring.
When 2-10 molecules of monosaccharide undergo condensation, oligosaccharide is formed or we can say these are group of compound which on hydrolysis yield two or more molecules of same or different monosaccharide units.
The linkage involved is glycosidic linkage. Aldehyde or ketone group of one monosaccharide reacts with alcoholic group of another monosaccharide to form glycosidic bond.
One molecule of water is eliminated during bond formation.
Examples of Oligosaccharides are : Sucrose, Raffinose etc
Sucrose is also known as invert sugar, cane sugar, Table sugar or commercial sugar.
Sucrose is composed of α-D Glucose and fructose as shown in figure.
In plants transport of sugar is present in the form of sucrose.
When two monosaccharides join by glycosidic linkage a disaccharide is formed.
It is the smallest and commonest oligosaccharide.
Example of Disaccharide:
lactose is milk sugar with β-1'-4'' glycosidic linkage between glucose and galactose shown in Fig.
Lactose is least sweet sugar.
In human milk 7% of lactose is present.
When three monosaccharides are joined by two glycosidic linkages a trisaccharide is formed.
Example of Trisaccharide:
Raffinose is a sugar with α-D-galactopyranosyl-(1 -> 6)-α-D-glucopyranosyl-(1 -> 2)-β-D-fructofuranoside linkage.
Raffinose is composed of galactose, glucose and fructose.
Sugar b: D- Fructose, Sugar c: D-glucose, Bond 1: α 1-6 glycosidic bond and Bond 2: α 1-2 glycosidic bond.
Polysaccharides can be regarded as complex carbohydrates as they are formed by polymerisation of 11 to thousands of monosaccharide monomers.
General formula (C6H10O5)n.
In a polysaccharide chain (like glycogen), the right end is called the reducing end and the left end is called non-reducing end.
(a) Homoglycans or Homopolysaccharide
It is composed of only one type of monosaccharide monomer e.g. starch.
In plants food is stored mainly in the form of starch. Starch is composed of monomers of α-D Glucose.
Amylose and amylopectin together comprise starch.
Amylose is the unbranched chain of 250-300 molecules of glucose linked by α 1' - 4'' linkage while as amylopectin is a branched chain molecule in which approximately 30 glucose units are linked by α-1'-4'' linkage and α-1'-6'' linkage.
Amylose with iodine gives blue colour while as amylopectin gives red colour with iodine.
(b) Heteroglycans or Heteropolysaccharide
It is composed of more than one type of monosaccharide monomers by condensation e.g. agar
It is composed of D-galactose and L-galactose units and after every 10th unit a sulphate group is present.
It is used for preparing culture media. It is obtained from red algae- Gracilaria, Gelidium, Chondrus.
5. Based on reducing properties carbohydrates are classified as:
|Reducing Sugar||Non-Reducing Sugar|
|These are sugars with free aldehyde or keto groups.||These sugars do not have free aldehyde or keto groups|
|They can reduce Cu2+ to Cu+ ion.||They cannot reduce Cu2+ to Cu+ ion|
|Reducing sugars reduce Fehling's solution and Benedict reagent||Reducing sugars cannot reduce Fehling's solution and Benedict reagent|
|Example: all monosaccharides||Example: Sucrose|