Carbohydrates
The body gets energy from three main classes of food carbohydrates, proteins,
and fats, carbohydrates are the most important source of energy. When digesting food,
the body begins to digest carbohydrates first.
Carbohydrates are classified into three groups based on the number of sugar
units.
1.
Monosaccharide: contain carbon atoms (3-8), examples: glucose, galactose,
ribose and fructose.
2.
Oligosaccharides: liberate two to ten monosaccharide molecules on
hydrolysis.
They
classified
as
disaccharides
(two-monosaccharide
units),
trisaccharides (three-monosaccharide units), etc. based on the number of
monosaccharide units.
Common examples of disaccharides are sucrose (glucose–fructose) table
sugar; maltose (glucose–glucose) corn syrup; and lactose (galactose–glucose) milk
sugar, example of trisaccharides: raffinose ( glucose, fructose and galactose(.
3.
Polysaccharides: contain large number of monosaccharide molecules. They
are usually amorphous, insoluble in water and tasteless. They are sub-divided into
homopolysaccharides and heteropolysaccharides.
A – Homopolysaccharides: They possess only a single type of monosaccharide units.
Examples: Starch, cellulose and glycogen.
B - Heteropolysaccharides: They possess two or more types of monosaccharide
units.
Examples: Heparin and chondroitin sulphate.
Molisch's test
•
It is a general test for all carbohydrates.
•
Monosaccharides give a rapid positive test but Disaccharides and
polysaccharides react slower .
•
Molisch reagent dehydrates pentose to form furfural but dehydrates hexose
to form 5-methyl furfural.
Pentose
•
Hexose
The furfurals further react with α-naphthol present in the test reagent to
produce a purple product.
Steps:
1. Put 2 ml of carbohydrates in a clean test tube .
2. Add few (5) drops of alcoholic α- naphthol and mix well.
3. Add carefully 1 ml of concentrated sulfuric acid (H2SO4) on the side of test
tube.
4. Appearance of the violet ring at the junction of the two liquids indicates the
presence of carbohydrates. On shaking, the whole solution turns violet in
color.
Test
Observation
Result
Molisch's test
Violet color ring
Carbohydrates
Note: In Molisch's test we use concentrated sulfuric acid as the dehydrating acid, this
acid dehydrates all carbohydrates, so the test is used to distinguish between
carbohydrates and non-carbohydrates, the dehydrated product result from the reaction
of the sulfuric acid condense with α-naphthol to yield a violet (purple) product.
Note: concentrated sulfuric acid is toxic and corrosive and can cause severe burns so
do not place your thumb over the open end of a test tube when mixing its contents and
avoid it's ingestion.