Carbohydrates are a class of naturally occurring organic compounds of carbon, hydrogen
and oxygen which are primarily produced by plants. They are extremely widespread in plants
comprising upto 80% of dry weight. These are ultimate source of our food. In higher animals the
simple sugar glucose is an essential constituent of blood and occurs in a polymeric form as
glycogen in the liver and muscle.
In the green plants, carbohydrates are produced by a process called photosynthesis. This
process involves the conversion of simple compounds CO
2
and H
2
O into glucose (C
6
H
12
O
6
) and
is catalysed by green colouring pigment chlorophyll present in the leaves of plants. The energy
required for this conversion is supplied by sun in the form of sunlight.
Carbohydrates are very useful for human beings. They provide us all the three basic
necessities of life i.e., foog (starch containing grain), clothes (cellulose in the form of cotton ,
linen and rayon) and shelter ( cellulose in the form of wood used for making our houses and
furniture etc.). Carbohydrates are also important to the economy of many nations. For example,
sugar is one of the most important commercial commotidies.
The term carbohydrates arose because the general formula for most of them could be
written as C
x
(H
2
O)
y
and thus they may be regarded as hydrates of carbon. However, this
definition was not found to be correct e.g., rhamnose, a carbohydrate, is having the formula
C
6
H
12
O
5
while acetic acid
having formula
C
2
H
4
O
2
is not a carbohydrate. Simple carbohydrates
are also known as sugars or saccharides (Latin: Saccharum; Greek : Sakcharon, Sugar) and the
ending of the names of most sungars is –ose. Examples: glucose, fructose, sucrose, maltose,
arabinose, etc.
Chemically, carbohydrates contain mainly two functional groups, carbonyl group (
aldehyde or or ketone) and a number of hydroxyl groups. Accordingly carbohydrates are now
defined optically active polyhydroxy aldehydes or polyhydroxy ketones or the compound that
can be hydrolysed to either of them.