CARBOHYDRATESCarbohydrates 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 in wood, cotton and flax).

The general name "Carbohydrates" is derived from that most of these compounds have the formula Cx (H2O)y and thus, appear to be "hydrates of carbon".

They usually contain hydrogen and oxygen in the same proportion as in water (2:1).

Metabolism Stored carbohydrates such as the starch of plant and

glycogen of animals are made available for energy production through oxidation to carbon dioxide and water.

C x (H2O) y + x O2 x CO 2 + + y H2O + Energy

The energy released is converted to heat, and much of it is conserved in a new chemical form .

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 sometimes called compound or complex sugars. • They are subclassified 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.

Configuration of MonosaccharidesD and L designations of monosaccharides The simplest monosaccharide, which contains only

one stereocenter, is glyceraldehyde.

D-Glyceraldehyde L-Glyceraldehyde

CHO

H

CH2OH

OH

CHO

H

CH2OHHO

These two forms of glyceraldehyde have been chosen as arbitrary standards for the D and L-series of monosaccharides in the sugar chemistry A monosaccharide in which the OH group attached to the carbon atom next to the CH2OH (farthest asymmetric carbon atom from the carbonyl group) is always to the right is designated as a “D-sugar” and that with the same OH to the left as “L -sugar”. D and L designations are like (R) and (S) designations in that they are not necessarily related to the optical rotations of the sugars to which they are applied. Thus, one may encounter sugars that are D (+) or D (-) and others that are L (-) or L (+).

Reactions of monosaccharides1-Reactions similar to alcohols Ether Formation: As alcohols, simple sugars form ethers. The methyl

derivatives (ethers) formed by reaction with methylating agents are useful in determination of both the ring size of monosaccharides and site of linkage in oligo- and polysaccharides.

2-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, e.g. glucose is transformed to gluconic acid.

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. Example, oxidation of glucose into glucaric acid (or saccharic acid) and galactose into mucic acid.

2-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, e.g. glucose is transformed to gluconic acid.

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. Example, oxidation of glucose into glucaric acid (or saccharic acid) and galactose into mucic acid.

Reaction with oxidising cations

• All monosaccharides and reducing disaccharides (i.e. all

sugars containing free hemiacetal or hemiketal groups) are

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

Benedict’s reagents), Bi+3 and Hg+2 in alkaline medium.

• These reactions are used for identification and quantification

of reducing sugars.

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

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.

Preparation • The best large-scale source of fructose is the “inversion” (hydrolysis) of sucrose by

acid or invertase enzyme.

Properties• Fructose occurs as colourless crystals with intense sweet taste. • It is freely soluble in water. • 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 (in emergencies of diabetic acidosis) and in

infant feeding formulae (more easily digested than glucose).

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; it can be isolated from

the mast cells of liver, heart and lung.

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.

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, non-cariogenic and non-

carcinogenic.

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

and or natural e.g. steviol glycosides.

.

Artificial sweeteners• These are synthetic sweeteners, non-caloric,

non-nutritive and must be used at very low concentrations.

• Examples: Cyclamate: 300 times sweeter than sucrose.

Aspartame: 1500 times sweeter than sucrose.

Saccharin: 3500 times sweeter than sucrose