Introduction

The carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the
compounds which produce such units on hydrolysis.

Carbohydrates are primarily produced by plants and form a very large group of naturally ccurring organic
compounds. Some common examples of carbohydrates are cane sugar, glucose, starch, etc. Most of
them have a general formula, Cx(H2O)y, and were considered as hydrates of carbon from where the
name carbohydrate was derived. For example, the molecular formula of glucose (C6H1206) fits into this
general formula, C6(H2O)6. But all the compounds which fit into this formula may not be classified as
carbohydrates. For example acetic acid (CH3COOH) fits into this general formula, C2(H2O)2but is not a
carbohydrate. Similarly, rhamnose, C6H1205 is a carbohydrate but does not fit in this definition. A large
number of their reactions have shown that they contain specific functional groups. Carbohydrates are
probably the most abundant and widespread organic substances in nature, and they are essential
constituents of all living things. Carbohydrates are formed by

Green plants from carbon dioxide and water during the process of photosynthesis. Carbohydrates serve
as energy sources and as essential structural components in organisms; in addition, part of the structure
of nucleic acids, which contain genetic information, consists of carbohydrate.

Classification of Carbohydrates

Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly
divided into following three groups.

Monosaccharides: A carbohydrate that cannot be hydrolysed further to give simpler unit of

polyhydroxy aldehyde or ketone is called a monosaccharide. About 20 monosaccharides are
known to occur in nature. Some common examples are glucose, fructose, ribose, etc.
Oligosaccharides: Carbohydrates that yield two to ten monosaccharide units, on hydrolysis,
are called oligosaccharides. They are further classified as disaccharides, trisaccharides,
tetrasaccharides,
Etc., depending upon the number of monosaccharides, they provide on hydrolysis. Amongst
these the most common are disaccharides.

The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different.
For example, one molecule of sucrose on hydrolysis gives one molecule of glucose and one molecule
of fructose whereas maltose gives two molecules of only glucose.
 Polysaccharides: Carbohydrates which yield a large number of monosaccharide units on
hydrolysis are called polysaccharides. Some common examples are starch, cellulose,
glycogen, gums, etc. Polysaccharides are not sweet in taste, hence they are also called non-
sugars.
The carbohydrates may also be classified as either reducing or nonreducing sugars. All those
carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as
reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars.

Although a number of classification schemes have been devised for carbohydrates, the
division into four major groups-

Monosaccharides, disaccharides, oligosaccharides,

And polysaccharides-used here is among the most

Common. Most monosaccharides, or simple sugars, are found in grapes, other fruits, and
honey. Although they can contain from three to nine carbon atoms, the most common
representatives consist of five or six joined together to form a chainlike molecule. Three of
the most important simple sugars-glucose (also known as dextrose, grape sugar, and corn
sugar), fructose (fruit sugar), and galactose-have the same molecular formula, (C6H12O6),
but, because their atoms have different structural arrangements, the sugars have different
characteristics; i.e., they are isomers.

Slight changes in structural arrangements are detectable by living things and influence the
biological significance of isomeric compounds. It is known, for example, that the degree of
sweetness of various sugars differs according to the arrangement of the hydroxyl groups (-
OH) that compose part of the molecular structure. A direct correlation that may exist
between taste and any specific structural arrangement, however, has not yet been
established; that is, it is not yet possible to predict the taste of a sugar by knowing its specific
structural arrangement. The energy in the chemical bonds of glucose indirectly supplies
most living things with a major part of the energy that is necessary for them to carry on their
activities. Galactose, which is rarely found as a simple sugar, is usually combined with other
simple sugars in order to form larger molecules.

Two molecules of a simple sugar that are linked to each other form a disaccharide, or double
sugar. The disaccharide sucrose, or table sugar, consists of one molecule of glucose and one
molecule of fructose; the most familiar sources of sucrose are sugar beets and cane sugar.
Milk sugar, or lactose, and maltose are also disaccharides. Before the energy in disaccharides
can be utilized by living things, the molecules must be broken down into their respective
monosaccharides. Oligosaccharides, which c of three to six monosaccharide units, are rather
infrequently found in natural sources, although a few plant derivatives have been identified.

Polysaccharides (the term means many sugars) represent most of the structural and energy-
reserve carbohydrates found in nature. Large molecules that may consist of as many as
10,000 monosaccharide units linked together, polysaccharides vary considerably in size, in
structural complexity, and in sugar content; several hundred distinct types have thus far
been identified. Cellulose, the principal structural component of plants, is a

Complex polysaccharide comprising many glucose units linked together; it is the most
common polysaccharide. The starch found in plants and the glycogen found in animals also
are complex glucose polysaccharides. Starch (from the Old English word stercan, meaning
“to stiffen”) is found mostly in seeds, roots, and stems, where it is stored as an available
energy source for plants. Plant starch may be processed into foods such as bread, or it may
be consumed directly-as

In potatoes, for instance. Glycogen, which consists of branching chains of glucose molecules,
is formed in the liver and muscles of higher animals and is stored as an energy source.

The generic nomenclature ending for the monosaccharides is -ose; thus, the term pentose
(pent five) is used for = monosaccharides containing five carbon atoms, and hexose (hex =
six) is used for those containing six. In addition, because the monosaccharides contain a
chemically reactive group that is either an aldehyde group or a keto group, they are
frequently referred to as aldopentoses or ketopentoses or aldohexoses or ketohexoses. The
aldehyde group can occur at position 1 of an aldopentose, and the keto group can occur at a
further position (e.g., 2) within a ketohexose. Glucose is an aldohexose-i.e., it contains six
carbon atoms, and the chemically reactive group is an aldehyde group.

Preparation of Glucose

1. From sucrose (Cane sugar): If sucrose is boiled with dilute HCI or H2SO4 in alcoholic
solution, glucose and fructose are obtained in equal amounts.

C12H22O11+ H2O

Sucrose

C6H12O6+ C6H12O6

Glucose

Fructose

2.From starch: Commercially glucose is obtained by hydrolysis of starch by boiling it with

dilute H2SO4 at 393 K under pressure.

(C,H,Os), + nH,O Starch or cellulose

H 393K: 2-3 atm

nC6H12O6

Glucose

Structure of Glucose

Glucose is an aldohexose and is also known as