Carbohydrates are the most abundant organic

constituents of plants.

They serve as the major source of chemical energy

for living organisms (e.g. sugars and starch), as well as

important constituents of supporting tissues (e.g.

cellulose).

Carbohydrates are usually defined as:

“Polyhydroxy aldehydes or ketones, or

substances that hydrolyze to yield polyhydroxy

aldehydes or ketones”.

Simple carbohydrates are also known as “sugars or

saccharides” (latin: saccharum = sugar).

Monosaccharides

These consist of only one saccharide or sugar

unit and they are non-hydrolysable.

They are subclassified according to:

1-The number of carbon atoms present in their

molecule and,

2-The type of carbonyl group they contain.

Thus, a monosaccharide containing three carbon atoms is

called a triose and that containing five is called a pentose

and so on.

A monosaccharide containing an aldehyde group is called

an aldose and one containing a keto group is called a

ketose.

These two classifications are frequently combined: e.g. a

five-carbon aldose, for example, is called an

aldopentose, a six-carbon ketose is called a ketohexose

Oligosaccharides

These consist of 2 and up to 10 molecules of

simple sugars and are hydrolysable.

They are sub classified into di-, tri- and

tetrasaccharides etc…, according to the number

of molecules of simple sugars they yield on

hydrolysis.

Polysaccharides

Polysaccharides are high molecular weight

polymers of monosaccharides of very complex

nature.

They are hydrolysable and yield a large number

of monosaccharides.

Different groups of polysaccarides are distinguished

according to the final products of hydrolysis:

1-The homosaccharides (or holosides)which yield

on hydrolysis similar simple sugar units (units of

the same monosaccharide). Those, which yield

pentose sugar units on hydrolysis, are called

pentosans and similarly those yielding hexose

units are called hexosans.

2-The heterosaccharides (or heterosides), which yield

upon hydrolysis dissimilar sugar units e.g. mixture of

hexoses and pentoses and thus called pento-

hexosans.(put example)

3-The derived carbohydrates, which yield on

hydrolysis monosaccharides in addition to other

components such as uronic acids, sulfate esters or

amino sugars.

Physical Characters of CHO

1-Condition: monosaccharides and most disaccharides are white,

crystalline in shape and with sharp melting points.

2-Taste: Most of the simple and low molecular weight sugars have a

sweet taste.

3-Solubility

Monosaccharides are soluble in cold water and hot alcohol.

Starch, pectin, mucilages and glycogen are difficulty soluble

in cold water, but more soluble in hot water and insoluble in

alcohol.

4-Optical activity

Monosaccharides and water-soluble oligosaccharides are

optically active and determination of their specific rotation

is useful for their identification.

A compound is optically active when, in solution, it is capable

to rotate the plane of polarized light either to right

(dextrorotatory, + or d) or to the left (levorotatory, - or l)

and the two stereoisomers are called enantiomers. This optical

activity is due to presence of a chiral center in the molecule.

Mutarotation

When a sugar is dissolved in water, the specific rotation of

the solution gradually changes until it reaches to a

constant value, e.g. freshly prepared solution of glucose has

a specific rotation +112 o. When this solution is allowed to

stand (aged solution), the rotation falls to + 52.7 o, and remains

constant at this value.

This change in the specific rotation is known as the

mutarotation phenomenon. All reducing sugars (except a few

ketoses) undergo mutarotation.(why does it happen?)

Reactions of carbohydrates:

Reactions similar to carbonyl compounds

Glycoside formation ( acetal formation )

Each carbonyl group (aldehydic or ketonic) reacts with two

molecules of alcohol to give an acetal.

OROH ROH

OR

OH

OR

OR + H2O

Carbonyl compound Hemiacetal Acetal

Oxidation

These reactions produce different products according

to the reagent used.

i-Bromine water; is a mild oxidizing reagent. It selectively

oxidizes the -CHO group into -COOH, and converts

aldoses to the corresponding aldonic acids

ii-Nitric acid; is a strong oxidizing agent. It oxidizes both

the -CHO and terminal -CH2OH of an aldose to -COOH

groups, and these dicarboxylic acids are known as

aldaric acids.

CHO

OHH

HHO

OHH

OHH

CH2OH

COOH

OHH

HHO

OHH

OHH

CH2OHGlucose Gluconic acid

Br2/H2O

CHO

OHH

HHO

OHH

OHH

CH2OH

COOH

OHH

HHO

OHH

OHH

COOH

Glucose Glucaric acid

HNO3

iii-Controlled oxidation: this is carried out by first protecting the -CHO

group, followed by oxidation of the -CH2OH group, or in one step by the

aid of a specific enzyme to give alduronic acid, e.g. oxidation of

glucose into glucuronic acid.

Reduction

Aldoses and ketoses can be reduced with sodium

borohydride into the corresponding alditols.

Examples: glucose gives glucitol (-CHO is converted to –

OH) or sorbitol, mannose gives mannitol, galactose gives

dulcitol.

Specific chemical reactions: Reaction with phenylhydrazine (Osazone formation):

The carbonyl group of an aldose reacts with phenylhydrazine

to give a crystalline phenyl osazone.

Osazones are used in identification of mono- and

disaccharides as they differ in physical characters (e.g. m.p.

and shape of crystals).

Monosaccharides give the same crystalline osazone on hot,

while reducing disaccharides (lactose and maltose) give

different crystalline shapes and on cold.

Maltosazone Lactosazone

Glucosazone

Action of mineral acids

Treatment with hot concentrated mineral acid

(HCl or H2SO4) leads to dehydration of sugars.

Pentoses and methylpentoses give furfural and

methylfurfural, respectively by the action of hot

hydrochloric acid, while ketoses and aldoses give

hydroxymethylfurfural.

The formation of furfural derivatives is the basis of the

color reactions used for qualitative and quantitative

determination of monosaccharides by coupling with

phenolic compounds or amines.

Examples:

In Molisch’s test the phenol used is -naphthol.

In Seliwanoff’s test (rapid furfural) the phenol is resorcinol.

Reaction with oxidising cations

All monosaccharides and reducing disaccharides are

readily oxidized by metal ions such as Cu+2 (Fehling’s

reagent).

These reactions are used for identification and

quantification of reducing sugars.

Quantitative analysis of carbohydrates:

Phenol-Sulfuric Acid Method

Carbohydrates are destroyed by heat and acid. They

are particularly sensitive to strong acids and high

temperatures. Under these conditions, a series of complex

reactions take place, beginning with a simple dehydration

reaction.

Phenol-Sulfuric Acid Method

Continued heating in the presence of acid produces various furan

derivatives. They will condense with various phenolic compounds, such

as phenol, resorcinol, orcinol, to produce colored compounds that are

useful for carbohydrate analysis.

This method is simple, rapid, sensitive, accurate, specific for

carbohydrates, and widely applied. The reagents are inexpensive,

readily available, and stable. A stable color is produced, and

concentration could be obtained from a standard calibration curve. The

results are reproducible.

Specific Analysis of Mono and Oligosaccarides

High Performance liquid Chromatography is the method of

choice for analysis of mono- and oligosaccharides and can be

used for analysis of polysaccharides after hydrolysis.

HPLC gives both qualitative analysis(identification of the

carbohydrate) and also allows for quantitative analysis.

HPLC analysis is rapid, can tolerate a wide range of sample

concentrations, and provides a high degree of precision and

accuracy.

MonosaccharidesThe common monosaccharides belong to three groups: The pentoses,The hexsoses, and The deoxysugars (as deoxyribose in DNA).

Pentoses

These are simple sugars containing five carbon atoms.

They give furfural when warmed with dilute acids.

They are rarely present in the free state but generally occur as

pentosans or form the sugar moiety of glycosides.

Examples of pentoses: -D-Ribose

found in all plant and animal cells as the carbohydrate part of nucleic acids e.g. ribonucleic acid (RNA). 

-D-Xylose (or wood sugar)

prepared from corncobs (thick cylindrical part on which the grains grow), or any woody material.

-L-Arabinose (or pectin sugar ): found in gums, and forms the sugar part of several glycosides.

HexosesProperties

These contain six carbon atoms.

They occur free, combined as oligo- and

polysaccharides, or forming the sugar part of

glycosides.

Examples

Two groups of commonly occurring hexoses are

distinguished:

The aldohexoses such as glucose, mannose and

galactose.

The ketohexoses such as fructose.

OHO

HO

OHOH

OH

CHO

OHH

HHO

OHH

OHH

CH2OH

OHO

HOOH

OH

OH

OCH2OH

OCH2OH

OCH2OH

OHO

HO

OH

OH

OH

O

OH

HO

OHOH

OH

CHO

HHO

HHO

OHH

OHH

CH2OH

CHO

OHH

HHO

HHO

OHH

CH2OH

OHO

HO OH

OH

OH

OCH2OH

OCH2OH

CHO

HHO

OHH

OHH

HHO

CH2OH

-D-glucose -D-mannose

-D-galactose

-D-glucose

-L-galactose

-D-Glucose (dextrose, grape sugar, blood sugar) Source

D-glucose is found free in sugar juices or forms the sugar part of many

glycosides.

Glucose can also be obtained from honey, which contains a mixture of

glucose, fructose and sucrose, or by inversion of sucrose.

Properties

D-Glucose is white crystalline, sweet, readily soluble in water.

It reduces Fehling's and Barfoed's solutions.

It gives an osazone that crystallizes on hot. Glucosazone occurs in the

form of golden yellow, needle -shaped crystals.

Uses

It has a great pharmaceutical importance as

ingredient in i.v. injections, as nutrient, diuretic and

sweetening agent. It is used in ice-cream and candy

industries.

Ketohexoses

β-D-Fructose (levulose or fruit sugar)

Source

It is found free in honey and in fruits juices, or as constituent of

polysaccharide e.g. inulin.

Properties

Fructose occurs as crystals with intense sweet taste.

It reduces Fehling's, and Barfoed's solutions

It forms an osazone similar to that of glucose

It gives a positive Seliwanoff’s test for ketoses (rapid furfural).

Uses

Fructose is used as food for diabetics and in infant

feeding formulae (more easily digested than

glucose).

Diabetes is a disorder affecting the way the body produces and

uses insulin and how it handles blood glucose. Insulin is

essential for aiding glucose transport into cells. People with

type I diabetes do not produce insulin, whereas those with type

II diabetes either do not produce enough insulin or cannot

efficiently use the insulin their bodies make. Factors such as

overweight and obesity, lack of physical activity, and genetic

predisposition all increase the risk for type II diabetes.

People with diabetes must pay attention to the amount of all

carbohydrates—sugars and starches—they consume.

Because fructose does not require insulin, individuals with

diabetes can often tolerate it better than other sugars.

Disaccharides

According to the position of the linkage between the sugar

units, disaccharides are classified into non-reducing such as

sucrose and reducing such as maltose and lactose.

Non-reducing disaccharides

Sucrose (saccharose, cane sugar or beet sugar)

Source Sucrose is the most widely occurring disaccharide, it is found in many fruit

juices, seeds, leaves, roots and honey.

Properties It has a sweetening power more than glucose and less than fructose. On heating from 200 to 250oC, sucrose changes into an amorphous brown

substance known as caramel (a decomposition product widely used as flavoring and coloring matter).

It gives positive results with cobalt nitrate test (violet). It does not reduce Fehling’s solution. It does not form an osazone.

The enzymatic or dilute acid hydrolysis of sucrose is called “inversion”

due to the fact that: sucrose ([α]25D= +66.5o) is hydrolyzed to an

equimolecular mixture of D-(+)-glucose ([α]25D= +52.7o) and D-(-)-

fructose ([α]25D = -92o), because of the high negative rotatory power of

fructose, the final solution has [α]25D= - 20.4o .

The sign of rotation being changed from (+) in the original solution of

sucrose into (-) in the hydrolyzed solution, the process is called

inversion.

Uses of sucrose

In Pharmaceutical industries, sucrose is used in

syrup preparation and tablet manufacture. It is

used as nutrient and demulcent.

Reducing disaccharideMaltose, or malt sugar, is a disaccharide formed from two units of

glucose joined with an α(1→4) linkage.

It is the second member of an important biochemical series of glucose

chains. The addition of another glucose unit yields maltotriose; further

additions will produce dextrins (also called maltodextrins) and eventually

starch (glucose polymer).

Lactose (or milk sugar)

Source

Lactose is the principal sugar of mammalian milk and

is not present in higher plants.

Structure

It consists of galactose and glucose, linked by a β(1

— 4) linkage .

Preparation

Lactose is obtained from whey (a by-product from

cheese manufacture) after concentration, upon which

deposits of lactose crystallize out.

Uses

Lactose is used as nutrient in infant foods, since it is

less sweet than sucrose and more easily digested. It

is also used as inert diluent for other drugs.

Homosaccharides (Homopolysaccharides or

holosides)

Examples 

The most common holosaccharides are:

The glucans (glucosans) such as starch, dextrin ,

glycogen and cellulose.

The fructosans such as inulin

Starch

Source 

Starch is the most abundant and widely distributed

plant substance next to cellulose and hemicellulose.

It occurs as microscopic granules in the seeds, fruits,

tubers and roots of plants.

The most common commercial sources are the

graminaceous fruits (e.g. rice and wheat ), maize and

potato tubers.

Structure

It is an α-glucan ( or glucosan ) polysaccharide.

The final product of hydrolysis is glucose.

On heating starch with water, the starch granules

swell and produce a colloidal suspension from which

two major components can be isolated: amylose

and amylopectin.

Uses

Starch has a wide variety of applications in food and drug industries it is used as:

Antidote for iodine poisoning.Diluent in powders and tablets manufacture.Nutrient, demulcent, protective and adsorbent.Starting material in the manufacture of glucose, liquid

glucose, maltose, and dextrins.

The Mucopolysaccharides

Mucopolysaccharides are polysaccharides which on hydrolysis yields amino sugar units.

Amino sugars are derived from monosaccharides by replacement of a hydroxyl group by an amino group.

ex. Heparin

Source

Heparin is the powerful blood anticoagulant.

Uses

Heparin and heparin analogues are used as

anticoagulants.

They have multiplicity of biological activities such as,

anti-inflammatory, anti- cancer, antiviral, as well

as in treatment of Alzheimer’s disease.

Biologically Active Carbohydrates

According to recent researches carbohydrates promise to

be a major source of drug discovery. The diversity and

complexity of carbohydrates explain their wide range of

biological functions.

There are several established carbohydrates-based

products with “biopharmaceutical” application, as well as,

other new products with potential application in medicine,

e.g., development of specific cancer vaccines, new non-

steroidal anti-inflammatory drugs, and many other

examples.

I-Antibiotics

This class of useful and potential therapeutics has gained

attention with the appearance of complex nucleoside

antibiotics, which exhibit a variety of biological activities. Of

these are natural the thiosugars, aminoglycosides and

macrolide antibiotics produced by various species of bacteria

and fungi.

II-Antitumour agents 

Only some non-toxic antitumour polysaccharides

derived from bacteria, fungi, and algae have

demonstrated good antitumour activity.

Example (1-3)- β-D-glucans, such as lentinan

isolated from the fruiting body of Lentinus edodes.

Schizophyllan (Sonifilan®) is an extracellular of product of

the fungus Schizophyllum commune. It is an

immunostimmulant related to lentinan.

It is useful in combination with other antineoplastic

treatments in the management of carcinomas of the

lungs, stomach, uterus and breasts.

III-Laxatives:

Lactulose is used in treatment of chronic

constipation.

IV-Sucralfate is an aluminum hydroxide complex

of sucrose sulfate that is used in the therapy of

duodenal ulcers.

V-Sweeteners

Carbohydrates play an important factor in increasing the incidence of

diabetes, obesity and dental caries.

There is increasing need for other alternatives to sucrose as

sweetening agents for medical purposes, especially in case of

diabetes, and for diet improvement.

These agents should have high solubility in water, good stability, and

a relative sweetness close or equivalent to that of sucrose.

They should be safe, low caloric, and non-carcinogenic.

Sweeteners are either of “natural” or “artificial” origin:

Natural sweeteners

Natural sweeteners are classified into two broad classes:

The bulk sweeteners, having sweetening effect nearly similar to

sucrose.

The intense sweeteners having sweetening effect many times as that

of sucrose.

Bulk sweeteners

These are the traditional sweeteners such as sucrose,

glucose, fructose and the polyols or sugar alcohols e.g.

sorbitol, mannitol, xylitol and lactitol.

Intense sweeteners

These are either synthetic e.g. saccharin, aspartame,

cyclamate or natural e.g. steviol glycosides, glycyrrhizin.

Steviol glycosides

These are a group of diterpene glycosides obtained from

Stevia rebaudiana, Family Asteraceae.

The most important of which is stevioside. They are 100-

300 times sweeter than sucrose.

Glycyrrhizin

This is a triterpene glycoside isolated from Glycyrrhiza

glabra, Family Leguminosae.

It is an example of intense sweetener, sweetness 50 times

as sucrose.