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Carbohydrate can be defined as polyhydroxy aldehyde or polyhydroxy

ketone. Itis any of a large group of compounds in which hydrogen and oxygen,

in the proportions in which they exist in water, are combined with carbon; the

formula of most of these compounds may be expressed as Cn(H2O)n.

Structurally, however, these compounds are not hydrates of carbon, as theformula would seem to indicate.

They are produced by green plants and by bacteria using the process known

as photosynthesis, in which carbon dioxide is taken from the air by means of

solar energy to yield the carbohydrates as well as all the other chemicals

needed by the organisms to survive and grow.

The carbohydrate group consists principally of sugar, starch, dextrin,

cellulose, and glycogen, substances that constitute an important part of the

human diet and that of many animals.

Carbohydrates are classified in 3 groups namely

Monosaccharides Oligosaccharide polysacchridesThe simplest of them are the simple sugars, or monosaccharides, which

contain either an aldehyde or a ketone group. The most important is glucose.

They are simple sugars which possess a free aldehyde (- CHO) or ketone (CO)group and 2 or more hydroxyl (-OH) groups. Monosacchrides cannot be

hydrolysed into smaller units.

Two monosaccharide molecules joined together by an oxygen atom, with

the elimination of a molecule of water, yield a disaccharide.

Disaccharide have thefollowing formula Cn(H2O) n-1 of which the most

important are sucrose lactose, and maltose. It can be classified into:

Reducing sugar such as Lactose, maltose and celluloceNonreducing sugars such as sucrose, Trehalose.

Free disaccharide is found more often in plants than animals. Sucrose is the

chief constituent of sugarbeet and sugarcane. It can be hydrolysed into

constituent monosaccharide by acid or by an enzyme called invertase.

Polysaccharides have this formula ( C6H10O2)X, these are enormous

molecules made up of one type or several types of monosaccharide units

about 10 in glycogen, for example; 25 in starch; and 100 to 200 in cellulose.

Some common examples are:

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Homopolysacchrides which include starch, glycogen,inulin,cellulose,pectin and chitin.

Heteropolysaccharides which include hyaluronic acid, and chondroitin.Within living organisms, carbohydrates serve both essential structural and

energy-storage functions. In plants, cellulose and hemicellulose are the main

structural elements. In invertebrate animals, the polysaccharide chitin is the

main component of the exoskeletons of arthropods. In vertebrate animals, the

cell coatings of connective tissues contain carbohydrates. Cell membranes are

rich in glycoproteins.

Glycoprotein is a conjugated protein having a carbohydrate component. These

contain oligosaccharide chains attached to polypeptide side.

N-H

I

H-C- CH3

I

C=O

N-H

IH.C-CH3

CH2OH I

I O C=O

HO H I

I I N-H

CH2OH I

I O O-CH- CH

O NH I I

OH I CH C=OI I C=O

OH I

CH3

Glycoprotein serves several roles in the body. Some glycoprotein is released in

the blood, while others carry blood group. Those embedded in the plasma

membrane carry out a variety of functions, which include transportation of

large water soluble molecules such as sugars and certain amino acids.

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Glycoprotein also serve in part to identify the cell as belonging to a unique

organism, enabling the immune system to detect foreign cells, such as invading

bacteria, which carry different glycoproteins. Below is a structure of one of the

glycoprotein.

The above segment is a recurring glycol-tripeptide unit. Each disaccharide unit,composed of- 1,3 linked residuesof galactose and N acetylgalactosamine is

attached to the polypeptide chain by a glucosidic linkage to threonine residue.

Glycolipids are lipids with a carbohydrate attached. Their role is to provide

energy and also serve as markers for cellular.

They are membrane components composed of lipids that are covalently

bonded to monosaccharaides. Can also be said to be fats that have attached

carbohydrate groups called glycan.

Glycolipids are present ubiquitously in cell surface membranes. They have

polar heads and non-polar tails. They may act as receptor site for chemical

signals. With are also involved in sticking the correct cells together in tissues.

Sterols are composed of complicated molecules. Each containing 20 or more

carbon atoms in an interlocking or fused ring structure.

Cellulose is an insoluble substance that is the main constituent of plant

cell walls and of vegetable fibbers such as cotton. It is a polysaccharide. It is

a long chain of linked sugar molecules that gives wood its remarkable

strengthen. Cellulose is normally combined with woody, fatty, or gummy

substances. With some exceptions among insects, true cellulose is not

found in animal tissues.

Microorganisms in the digestive tracts of herbivorous animals break

down the cellulose into products that can then be absorbed. Cellulose is

insoluble in all ordinary solvents and may be readily separated from the

other constituents of plants.

Depending on its concentration, sulfuric acid acts on cellulose to produce

glucose, soluble starch, or amyloid. It is a form of starch used for the then

exposed to the fumes of carbon disulfide, the solution yields films and

threads.

Rayon and cellophane are cellulose regenerated from such solutions.

Cellulose acetates are spun into fine filaments for the manufacture of some

fabrics and are also used for photographic safety film, as a substitute for

glass, for the manufacture of safety glass, and as a molding material.

Cellulose ethers are used in paper sizings, adhesives, soaps, and

synthetic resins.

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CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH

O I o I o I o I o I o I

OI O I O I OI I O I OI O I O I O I OI OCH2OH CH2OH CH2OH CH2OH CH2OH CH2OH

Carbohydrates are important in that:

Glucose is a preferred energy source of man tissue especially the brain. It is

the primary fuel for living cells.

In animals , glucose is the preferred energy source of brain cells and that

have few or no mitochondria such as erythrocytes, cells that have a limited

oxygen supply. Such as those in the eyeball, also use large amounts of glucose

to generate energy dietary source include plant starch and the disaccharide,

maltose and sucrose.

Nitrocellulose is used in the production of motion picture film, cement,

guncotton, celluloid, and similar kinds of plastics. Starch is used in the

preparation of foods for livestock and humans; pectin, a jelling agent; gum

arabic; and agar, used in the production of adhesive materials, sizing materials,

and emulsions. Gum arabic is also used in demulcent medicines.

Agar, a With mixtures of nitric and sulfuric acids, cellulose forms a

series of flammable and explosive compounds known as cellulose nitrates,

or nitrocelluloses.Pyroxylin, also called collodion cotton, is a nitrate used in various

lacquers and plastics; another, collodion, is used in medicine, photography,

and the manufacture of artificial leather and some lacquers. A third nitrate,

guncotton, is a high explosive.

Cellulose is a popular food additive used as astabiliser. Carbohydrate can be

converted into viscose rayon, acetate rayon, and paper products.

Constituent of some laxatives is also used as a thickening agent in food and

as a medium for bacterial culture. Hemicellulose is used to modify paper

during its manufacture. The carbohydrate dextran is a polysaccharide used inmedicine as a blood-plasma-volume expander to counteract acute shock.

Another carbohydrate, heparin sulfate, is a blood anticoagulant.

Chains Plants use starch and animals use glycogen to store energy, when the

energy is needed, the carbohydrates are broken down by enzymes.

Carbohydrates are used in the manufacture of fabrics, photographic film,

plastics, and other products.

Galactose is hexose found mostly commonly as a component of disaccharide

lactose or milk sugar. Galactose is necessary for synthesis a variety of

biomolecules.4


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REFERENCES

1.Biology. Peter Raven and George Johnson by McGraw HillCompanies, (1999), New York NYIOO2O USA.

2.Biochemistry, Trudy Mickee ental, (2003), by McGraw HillCompanies, New York USA

3. chem4kids.com/Biochemistry/Enzymeregulation.

4. BURTON. S.C.(1988): UNDERSTANDING BIOLOGYHARCORT JAVANOVIEL INC.


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