chapter - ii - wordpress.com file · web view3/4/2016 · oxidation red ppt. reduction: when...

CHAPTER 3

CARBOHYDRATES

Introduction occurrence, classification, biochemical functions and physiological importance of carbohydrates.

INTRODUCTION Carbohydrates occur widely in nature. Cellulose of wood and paper,

starches of cereals, roots and tubers, sugars of fruits and milk are examples of

carbohydrates. Animal tissues contain glycogen (animal starch). They are the

major sources of energy and play key role in many biochemical processes

including energy generation.

DEFINITIONChemically all the carbohydrates are aldehyde or ketone derivatives of

polyhydroxy alcohol or their condensation products. They contain C, H and O. The

general (empirical) formula is Cn (H2O)n . However, there are many exception to

this general formula such as deoxy ribose ( C5 H10 O4 ), formaldehyde (HCHO),

acetic acid (C2 H4O2 ) (not carbohydrates).

CLASSIFICATION The carbohydrates are classified in four different groups.

1. Monosaccharides: Those sugars which can't be hydrolysed to smaller

units.

2. Disaccharides: Those sugars which are composed of two mono

saccharides.

3. Oligosaccharides: Upon hydrolysis can yield 3 to 10 monosaccharides.

4. Polysaccharides:Simple polysaccharides and Complex polysaccharides

PROPERTIES :

(A) PHYSICAL PROPERTIES OF MONOSACCHRIDIES:

1. Colourless, odourless, neutral, sweet in taste

2. Soluble in water, insoluble in ether

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3. Optically active

4. Melting point indefinite

5. Heating effect : Melt on heating, on more heating , they are converted

to black carbon.

CARBOHYDRATES ▼

SUGARS NON SUGAR Mono Disaccharide Oligo Simple poly Complex Poly Saccharides (C12H22 O11) Saccharides Saccharide Saccharides Pentose----Hexose Sucrose * Trisaccharides Pentosan Hemicellulose (C5H10O5) (C6H12O6) (Glu+Fru) Pectic substances Maltose Raffinose Gum Arabinose Glucose (Glu+Glu) (Glu+Fru+Gala) # Araban MucilageXylose Galactose Lactose *Tetrasaccharide # Xylan Lignin Ribose Fructose (Glu+Gala) Stachyose # Riban Glycosides (2Gala+Glu+Fru) Hexosans

1.Glucosan (Starch,dextrin, cellulose, Glycogen)2. Fructosan (Inulin)3. Galactan and4. Mannan

(B) CHEMICAL PROPERTIES :1. Oxidation with mild oxidising agent : In presence of weak oxidising

agent such as alkaline CuSO4, Ammonical AgNO3 or bromine water give

acid ( Gluconic acid), Glucosaccharic acid, Tri-hydroxy butyric acid .

R-CHO + CuSO4 ------------ R-COOH Oxidation Red ppt.

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2. Reduction : When monosaccharides are heated in presence of catalyst

with hydrous solution, they give sorbitol or mannitol.

R-CHO + Reducing agent ------------- R-OH

3. Reaction with HCN : Upon reaction with HCN, they produce

corresponding cyanohydrins

4. Reaction with hydrazine : Monosaccharides produce corresponding

hydrazone with phenyl hydrazine

5. Reducing properties : Some monosaccharides are strong reducing

agents and they reduce oxidising agents such as

CuSO4 and they are converted to acid

OTHER PROPERTIES OF MONOSACCHARIDES:

1. ACETYLATION : When reacted with acetyl chloride, give acetyl derivative

2. FERMENTATION : Fermentation take place with the help of enzymes of

microorgs to yield alcohol, acetic acid, putrefaction of

meat, sourness of milk etc

a. FERMENTATION OF SUCROSE FROM MOLASSES:

C12H22O11 + H2O ---------------> C6H12O6 + C6H12O6

Sucrose Invertase Glucose Fructose

from yeast

C6H12O6 ---------------------------------> 2C2H5OH + 2CO2

Zymase from yeast ethanol

b) MALTING OF BARLEY (BEER FERMENTATION): n ( C6H12O5 ) n + H2O -----------------------> n ( C12H22O11 )

Starch from barley Hydrolysis by amylase Maltose

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C12H22O11 + H2O --------> C6H12O6 + C6H12O6

Maltose Maltase Glucose Glucose

C6H12O6 ---------------------------------> 2C2H5OH + 2CO2

Glucose Zymase from yeast ethanol

3. ESTERIFICATION : When monosaccharides react with acid they yield

Esters.

4. METHYLATION : Methylating agents such as CH3I + Ag2O or CH3OH +

HCl, CH3Cl to yield glycosides.

C6H11 O5 OH + CH3OH ------ C6H12O5-OCH3 + H2O

HCl Methyl glycoside

5. OPTICAL ACTIVITY: Many of the soluble carbohydrates are optically

active when present in solution. Compounds which in solution, rotate the plane

of polarised light to the right are called dextro rotatory (clock wise) and to left

side are called leavo rotatory (anti-clock wise).

e.g. 1. Glucose = (+) 52.5 0 (dextro-rotatory)

2. Fructose = (-) 133.5 0 (Leavo-rotatory)

Glucose exists in two isomeric forms α - d-glucose and β -d-glucose.

6. ISOMERISM: It is a phenomenon in which substances have same

molecular formula but different structural formulas are called isomerism.

There are two types of isomerism:a. CHEMICAL ISOMERISM: Where the molecular formula is same but

the atomic groupings are different.e.g. Glucose (aldehyde) and fructose

(keto). Both have molecular formula = C6H12O6 .

Glucose ------------------ Fructose

(Aldehyde) --------------- ( Keto )

-CHO -------------------- C = 0

C6 H12 O6 --------------- C6 H12 O6

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Pyranose structure Furanose structure

(8) Polymerisation and Condensation:

a) Polymerisation: It is a process in which two or more

same monomers are united together to form a complex polymer without

evolution of water, NH3 or other compound. The total number of elements

remains unaltered.

e.g. CO2 + H2O --------------- HCHO + H2O

Photosynthesis Formaldyhyde

6(HCHO) ------------------- C6H12O6 (glucose)

b) Condensation: It is a process in which two or more same or different

compounds are united together to form a complex polymer with or without

evolution of water, NH3 or other compound. The total number of elements may not

remain same because of loss of H2O,NH3 etc.

E.g. Synthesis of Starch: n(C6H12O6) ----------- n (C6H10O5 ) + n (H2O)

Glucose Starch

PENTOSES

There are 5 carbon sugars such as xylose, arabinose and ribose in a

complex polymeric form which has importance is agriculture.

1. D-Xylose : Wood gum (xylan) Wheat bran oat hulls, cotton seed hull.

2. L-Arabinose : Vegetable gum Gum- - It has a industrial importance

arabic, cherry tree gum

3. D- Ribose : Nucleic acid ------- Part of DNA and RNA structure.

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CH2OH| OHH

|OH

H|

OHH|

OH

H|

OH

OH|

H

OH |H

CH2OH | H

OH | |CH2OH

O

HEXOSES

GLUCOSE FRUCTOSE GALACTOSE 1. Aldehyde sugar Keto sugar Aldehyde sugar

2. C6 H12 O6 C6 H12 O6 C6 H12 O6

3. Grape sugar Fruit sugar Brain sugar

4. Structure

(5) Reducing sugar Reducing sugar Reducing sugar

(6) Osazone +Ve Osazone +Ve Osazone +Ve

(7) Can occur freely Occur in fruit and honey Doesn’t occur freely

in nature. Can be

obtained only from

lactose.

GLUCOSE FRUCTOSE GALACTOSE CHO CH2OH CHO

| | | H – C - OH C=O H-C-OH | | | OH – C – H OH-C-H OH-C-H

| | | H – C – OH H-C-OH OH-C-H | | | H – C – OH H-C-OH H-C-OH | | | CH2OH CH2OH CH2OH

Pyranose structure Furanose structure Pyranose structure

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CH2OH| OHH

|OH

H|

OH

H|

OHH|

OH

H|

OH

H|

OH

CH2OH| OH

OH|

H

OH |H

OH|

H OH |H

CH2OH | H

OH | |CH2OH

O

DISACCHARIDES

The disaccharides are group of compounds having two Monosaccharides

linked with a glycosidic bond with elimination of water.

Molecular formula: C12H22O11

C6 H12 O6 + C6 H12 O6 --------- C12 H22 O11 + H2O

Two types of disaccharides: 1. Reducing sugars. 2. Non reducing sugar.

REDUCING SUGARS: Those sugars which have either free aldehyde or keto

group for the reaction.

NON REDUCING SUGAR: Those sugars which do not have either free aldehyde

or keto group for the reaction.

1. SUCROSE: It is also called cane sugar composed of glucose and fructose.

Glucose + Fructose ------- Sucrose + H2O

+ ------

Glucose Fructose

SUCROSE

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CH2OH| OHH

|OH

H|

OH H|

OH

OH |H

CH2OH | H

OH | |CH2OH

O

H|

OH

OH|

H

H

H|

OH

CH2OH| OHOH

|H OH

|H

H|

OH

OH |H

CH2OH | H CH2OH

OO

The linkage in the sucrose involves the C1 of glucose with the C2 of

fructose with a oxygen bond. During the formation of sucrose, the aldehyde group

of glucose and keto group of fructose is lost so the sucrose becomes non-

reducing.

It is hydrolysed to monosaccharide (glucose and fructose with the enzyme

invertase (sucrase). It gives Osazone negative reaction. It does not show

mutarotation and can be crystallised. It is also called invert sugar.

2. MALTOSE : Malt sugar is composed of glucose + glucose with α 1,4

linkage. It is a reducing sugar.

Molecular formula : C12 H22 O11

C6 H12 O6 + C6 H12 O6 --------- C12 H22 O11 + H2O

Glucose Glucose Maltose

It is a product formed from starch during malting process. The hydrolysis of

starch by diastase enzyme yields maltose.

Maltose is hydrolysed to two glucose by maltase enzyme. It is also called

glucose glucoside. Specific rotation is 138.5 0 and forms characteristic osazone.

STRUCTURE :

3. LACTOSE :It is called milk sugar and composed of glucose + galactose

Molecular formula : C12 H22 O11

It is joined with the β 1,4 linkage.

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H

H|

OH

CH2OH| OHH

|OH OH

|H

H|

OHH|

OH

CH2OH| OHH

OH |H

O

It is a reducing sugar giving a characteristic osazone positive reaction.

It does not ferment easily hence ideal constituent of milk. It furnishes

galactose (brain sugar) requirement for brain function. It is hydrolysed by β-

galactosidase (old name lactase) to

glucose + galactose. It is also hydrolysed by emulsion enzyme.

Structure of Lactose :

POLYSACCHARIDES : The polysaccharides are complex carbohydrates, which are polymerised

anhydride of a large but undetermined number of simple sugar.

Empirical formula : ( C6 H10 O5 ) n

PROPERTIES OF POLYSACCHARIDES: High molecular weight, unreactive, form colloidal solution, insoluble in

water. Upon hydrolysis, it yields their constituent monosaccharides,. They are

most important nutrients of foods of plant origin, Their main function is storage of

reserve energy and structure.

STARCH :

It’s a glucose polymer with α 1,4, and α 1,6 linkage (branching). It occurs in

leaves, seeds, fruits and tubers. It is found in leaf chloroplast wherein they are

accumulated in daytime and hydrolysed and trans-located in night. It consists of

amylose and amylopectin. Amylose occurs inside the starch granule while

amylopectin is found in outer layer of starch grain. There are basic differences

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HH|

OH

CH2OH| OHOH

|H OH

|H

H|

OHH|

OH

CH2OH| OHH

OH |H

O

between Amylose and Amylopectin. It is a white amorphous powder, insoluble in

water.

AMYLOSE AMYLOPECTIN

1. Inner layer of starch 1. Outer layer of starch

2. Straight chain 2. Branched

3. Alpha 1, 4, linkage 3. Alpha 1, 6 linkages

4. Mol.wt. 1 K – 50 K 4. mol.wt. 500 K – 1000 K

5. Blue color with I2 5. Purple colour wilth I2

6. Forms gel 6. Doesn’t form gel

7. 20% in starch 7. 80% in starch

PROPERTIES OF STARCH:

a) Action of enzyme:

Starch ------------ Amylodextrin ----------- Arithrodextrin

Diastase

V

Glucose-------- Maltose ------------Achrodextrin

Maltase

b) ACTION OF ACIDS : When boiled with dilute acid, it is hydrolysed to dextrin,

maltose and finally to glucose. When treated with concentrated H2SO4 + Conc.

HNO3, it gives nitro starch which is used as blasting explosive.

c) GEL FORMATION : It is a characteristic used in a dry cell and sizing

operations.

d) It reacts with iodine to give blue colour.

USES : 1. It is used as food. It provides glucose upon hydrolysis to give the energy

from the food of plant origin such as cereals and tubers.

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2. Used in cloth washing

3. Used as explosive

4. Used as indicator in the lab.

5. Used in the preparation of gum

6. Used as electrolyte medium in dry cell.

CELLULOSE :Cellulose is the most abundant organic substance found in nature. It is the

principal constituent of cell walls in higher plants. It occurs in almost pure form

(98%) in cotton fibres and to a lessor extent in flax (80%), jute (60-70%), wood

(40-50%) and cereal straws (30-43%).

It is linear, unbranched homoglycan of 10,000 to 15,000 D-glucose units

joined by b-1®4 linkages. The structure of cellulose can be represented as a

series of glucopyranose rings in the chair conformation. The most stable

conformation for the polymer is the chair turned 180° relative to the adjacent

glucose residues yielding a straight extended chain. Celluose molecules within

the plant cell walls are organized into biological units of structure known as

microfibrils. A microfibril consists of a bundle of cellulose molecules arranged

with its long axis parallel to that of the others. This arrangement permits the

formation of intramolecular hydrogen bonding between the hydroxyl group of C-3

of one glucose residue and the pyranose ring oxygen atom of the next glucose

residue. This hydrogen bond impart a double bond character to the glycosidic

bond and impedes the rotation of adjacent glucose residues around the glycosidic

bond. Within the microfibril, the adjacent cellulose molecules are linked by

intermolecular hydrogen bond between C-6 hydroxyl group of one molecule and

the glycosidic bond oxygen atom of adjacent cellulose molecule.

Wood 40 - 52% Flax, Hemp, Cotton 70 – 90%

Wool and brans 30 - 40%

Hemicellulose : Mixed polysaccharide containing uronic acid, hexose and

pentose units which are soluble in alkali and digested by enzymes. Act as

cementing materials of plant cellulose also utilised in compost making.

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PROPERTIES :

It is a colourless, tasteless, fibrous compound, which is insoluble in water

and organic solvents but soluble in cupric amino.hydroxide prepared in cold HCl

and ZnCl2

Mol. Wt. : 20.000 ----50.000

CHEMICAL PROPERTIES :

1. HYDROLYSIS : It is converted to glucose when either boiled in conc.

H2SO4 for a long time or treated with enzyme cellulase which breaks β 1,4

linkage.

cellulase

Cellulose ------------- Cellobiose (Disaccharide) ------------ Glucose

Glucose+glucose with β 1,4 linkage

2. HYDRATION : When treated with H2O, it gives hydro- cellulose which is a

characteristics of plant sap and important for physiological functions of

plants.

3. NITRATION : When heated with mixture of H2SO4 + HNO3, it is converted

to nitro cellulose.

4. ACTION OF ALKALI : No reaction with dilute alkali but with conc. alkali,

the cellulose occupy the shape of smooth bright fibre which is translucent.

This process is called mercerisation.5. REACTION WITH LIGNIN : When it reacts with lignin, it gives ligno-

cellulose which is a crusting material in the wood bark. The lingo-cellulose

is not easily broken down.

The amylase enzyme cannot act upon cellulose. The animal gut

(Cows, buffalo etc.) and microorgs such as fungi and bacteria, which

secrete cellulase enzyme which breaks the β 1,4 linkage to yield glucose.

Human cannot digest cellulose because we do not have cellulase enzyme.

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IMPORTANCE

1. Cotton and textile industry : Production of wrapping, gunny bags. Ropes

and netting are made from cellulose. The cotton cloth which involve the use

of pure cellulose.

2. Paper manufacture : The papers of all types are prepared either from low

grade or pure Cellulose, which are used as newspaper wrappers or better

quality papers.

3. Explosive : Fully nitrated cellulose (gum coated) in form of compressed

blocks are used as explosives. When gun cotton + nitro-glycerine +

Vaseline are prepared, it produces smokeless explosive which is called

cordite.

4. Used in making of celluloid: Toys, bangles, combs, knife, garlands etc.

5. Used in preparing celluloid paper for motorcar.

6. Used in artificial silk and in bamboo furniture

7. Used in photographic films.

GLYCOGEN

The storage of carbohydrate in animal is in form of glycogen (animal

starch). It is similar to amylopectin in its structure except that the mol.wt is lower

and chains are shorter. The branching is extensive. It is found in liver and muscle.

It dissolves in water to yield opalescent solution. It is easy hydrolysed to glucose.

It is non-reducing and gives red colour with iodine.

OTHER POLYSACCHARIDES : Inulin : Inulin is a non-digestible fructosyl oligosaccharide found naturally in

more than 36000 types of plants. It is a storage polysaccharide found in

onion,garlic,chicory,artichoke,asparagus,banana,wheat and rye.It consists of

mainly,if not exclusively, of b- 2®1 fructosyl-fructose links.A starting glucose

moiety can be present,but is not necessary.Inulin is asoluble fibre that helps

maintain normal bowel function,decreases constipation,lowers cholesrerol and

38

triglycerides.It is used for fat replacement and fibre enrichment in processed

foods.

Chitin : Structural polysaccharides in invertebrates (Insect wings).

pectin substances.

Pectin substances are natural components of plants and their fruits. They

occur in plants in connection with cellulose and such substances are called

protopectin. Protopectin is the binder of cell walls. Especially large amounts of

pectin substances are present in fruit such as: currant, gooseberry, citrus fruits

and apples. Pectin is a preparation obtained in industrial conditions, containing

pectin substances isolated from plant material and soluble in water. Those

preparations are used as food and medicine additives and they have the ability to

make gels in proper conditions. Raw material for our pectin is dried apple

pomace, containing 8-12 % pectin substances, and dried lemon peel, containing

18-25 % pectin substances, from where they are extracted by diluted acid

solution and subsequently precipitated by alcohol, purified, dried and crumbled.

Being the substance of plant origin, it is the best gelling agent for jams and fruit

jellies production. Being the naturally compound of fruit, it makes products

manufactured with its addition retain fully organoleptic characteristics

Pectin classification

Depending on the applied raw material the following pectins can be distinguished:

Apple pectins Citrus-apple pectins

Citrus pectins

Depending on degree of esterification (DE) they are divided into :

High esterificated pectins (DE above 50 %) Low esterificated pectins (DE below 50 %)

AgarAgar is a heteropolysaccharide obtained from red algae.It is composed of

agarose –neutral gelling fraction

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Agaropectin – sulfated non-gelling fraction

It is the most effective gelling agents known andis soluble in hot water.It melts in

the temperature range of 60 –90oC and sets between 32 and 39oC to form gel.

Uses As Solidifying agent, emulsifier

Pharmaceuticals, cosmetics and food

Laxative

Sizing material in tentile industry

Emulsifierin dairy products

Microbial lab.

Seaweed PolysaccharidesStructurally they are highly branched and composed of many different

monosaccharides.

Alginic acid is obtained from seawood principally from brown algae. It is

composed of b1 ®4 linked D-mannuronic acid and ® 4 linked L-guluronic acid –

random or alternating sequence

Uses: 1. Sauces, frozen deserts, fruit pies

2. Tabilize emulsions

3. Soft drinks (gum Arabic)

4. Beer making to stabilize foam (gum arabic0

5. Cosmetics and lotions, paints, ink

6. Adhesive, laxative, foods paper industry (karaya)

7. Food industry-ice creams, salad dressings, pie fillings (locust gum)

8. Ice cream, paper industry, fire hoses, medicines (guar)

9. Salad dressing, cheese (improves sprcading) lower cholesterol, helps

diabetics to control sugar.

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METABOLISM OF CARBOHYDRATES:

Metabolism is comprised of two processes :

a) Anabolism : The process of formation of complex polymeric substances from

simple monomeric units involving the energy is known as

anabolism.

Sun light

CO2 + H2O ------> HCHO -------> C6H12O6 -----> OLIGOSACCHARIDE

photosynthesis

-------------> POLYSACCHARIDE

b) Catabolism: The process of breakdown of complex polymeric substances into

simple monomeric units with release of energy.

Carbohydrate (Polysaccharide) -----> oligosaccharide ------>monosaccharide

Hydrolysis

Glycolysis--------------> Pyruvic acid --------------> CO2 + H2O + ENERGY

TCA cycle

BIOLOGICAL SIGNIFICANCE OF CARBOHYDRATE :

1. Structural component of cells : Cellulose, chitin etc.

2. Major source of energy : Starch, glycogen.

3. Key role in metabolism : Metabolism of amino acids and fatty acids.

4. Specific functions :

Ribose ------------- Nucleic acid and nucleoprotein

Galactose---------- Cerebroside - brain lipid

Lactose ------------ Milk

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