© SSER Ltd.

OPTOM FASLU MUHAMMED

Carbohydrates are compounds of great importance in both the biological andcommercial world

They are used as a source of energy in all organisms and as structural materials inmembranes, cell walls and the exoskeletons of many arthropods

All carbohydrates contain the elements carbon (C), hydrogen (H) and oxygen (O)with the hydrogen and oxygen being present in a 2 : 1 ratio

THE GENERAL FORMULA OF A CARBOHYDRATE IS:

Cx(H2O)yEXAMPLES

The formula for glucose is C6H12O6

The formula for sucrose is C12H22O11

The Nature of Carbohydrates

Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose.

Disaccharides - 2 monosaccharides covalently linked. Oligosaccharides - a few monosaccharides covalently

linked. Polysaccharides - polymers consisting of chains of

monosaccharide or disaccharide units.

I (CH2O)n or H - C - OH

I

Carbohydrates (glycans) have the following basic composition:

THE CLASSIFICATION OF CARBOHYDRATESCarbohydrates are classified as either sugars or polysaccharides

CARBOHYDRATES

SUGARS POLYSACCHARIDES

MONOSACCHARIDES DISACCHARIDES STORAGE STRUCTURALMonosaccharides aresingle sugar units that

include:GLUCOSE

FRUCTOSEGALACTOSE

Disaccharides aredouble sugar units that

include:SUCROSEMALTOSELACTOSE

GLYCOGEN andSTARCH are

storage carbohydrates;

animal cells storeglucose as

glycogen and plant cells store glucose as starch

CELLULOSE and CHITIN are

important structural

carbohydrates;cellulose forms

the fabric of many cells walls and

chitin is a major component of the exoskeletons of

many arthropodsGLUCOSE

MONOSACCHARIDESMonosaccharides are single sugar units that form the building blocks for

the larger carbohydratesThere are many different monosaccharides; they vary according to the number

of carbon atoms that they possess and in the way the atoms are arranged in the molecules

Glucose, the main source of energy for most organisms, is a hexose sugar withsix carbon atoms and the formula C6H12O6

Glucose exists in both straight chain and ring form with rings forming when glucose is dissolved in water

C

C

C

C

C

C

H

H

H

O

O

O

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

1

23

4

5

6

C

CH O H

H

C

H

H

O H

HO H

H

HOOH

H

C C

C

O

CHAIN STRUCTURE RING STRUCTURE

Classification of Monosaccharides

•The monosaccharides are the simplest of the carbohydrates, since they contain only one polyhydroxy aldehyde or ketone unit.

•Monosaccharides are classified according to the number of carbon atoms they contain:

•Thepresence of an aldehyde is indicated by the prefix aldo-and a ketone by the prefix keto-.

Monosaccharide 3C- triose (DHAP) glyceraldehyde4C- tetrose erythrose5C- pentose arabinose 6C- hexose

ALDEHYDE GROUPC

C

C

C

C

C

H

H

H

O

O

O

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

GLUCOSE

This straight chainrepresentation of the glucose

molecule shows how thecarbon atoms are numbered

Glucose, in common withmany other hexose sugars

has an aldehyde groupas part of the structure

The carbon atom that forms part of this aldehydegroup is always carbon 1

The C = O carbonyl grouphas reducing properties such

that all monosaccharidesare reducing sugars

The remainder of the moleculeis a series of bonded carbon

atoms with attached hydrogenatoms and hydroxyl (OH) groups

The carbon atomof the carbonyl groupis referred to as the

ANOMERIC CARBONATOM and, for glucose,

this is carbon 1

1

23

4

5

6

C

CH O H

H

C

H

H

O H

HO H

H

HOOH

H

C C

C

O

GLUCOSEIn solution glucose exists in ring form

Glucose forms asix-membered ring whenthe hydroxyl group (OH)on carbon 5 adds to the

aldehyde group oncarbon 1

Hemiacetal & hemiketal formation

An aldehyde can react with an alcohol to form a hemiacetal.

A ketone can react with an alcohol to form a hemiketal.

O C

H

R

OH

O C

R

R'

OHC

R

R'

O

aldehyde alcohol hemiacetal

ketone alcohol hemiketal

C

H

R

O R'R' OH

"R OH "R

+

+

Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH.Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran.

These representations of the cyclic sugars are called Haworth projections.

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

Fructose forms either a 6-member pyranose ring, by reaction of the C2 keto

group with the OH on C6, or a 5-member furanose ring, by reaction of the C2 keto

group with the OH on C5.

CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

HOH2C

OH

CH2OH

HOH H

H HO

O

1

6

5

4

3

2

6

5

4 3

2

1

D-fructose (linear) -D-fructofuranose

Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, & .

Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:

(OH below the ring) (OH above the ring).

H O

OH

H

OHH

OH

CH2OH

H

-D-glucose

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose

23

4

5

6

1 1

6

5

4

3 2

GLUCOSE

1

CH OH2

HH H

HO OHOH

H OH

H

O

The ring structure of glucose is usually represented inHowarth projection

ISOMERSEach hexose sugar exists in both alpha and beta forms

These ISOMERS can be distinguished by the arrangement of theOH and H groups about the extreme right carbon atom IN the ring

Monosaccharides

Aldoses (e.g., glucose) have an aldehyde group at one end.

Ketoses (e.g., fructose) have a keto group, usually at C2.

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

D vs L Designation

D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde.

The lower representations are Fischer Projections.

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

L-glyceraldehydeD-glyceraldehyde

L-glyceraldehydeD-glyceraldehyde

Sugar Nomenclature

For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.

Most naturally occurring sugars are D isomers.

O H O H C C H – C – OH HO – C – H HO – C – H H – C – OH H – C – OH HO – C – H H – C – OH HO – C – H CH2OH CH2OH

D-glucose L-glucose

D & L sugars are mirror images of one another.

They have the same name, e.g., D-glucose & L-glucose.

Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc.

The number of stereoisomers is 2n, where n is the number of asymmetric centers.

The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).

O H O H C C H – C – OH HO – C – H HO – C – H H – C – OH H – C – OH HO – C – H H – C – OH HO – C – H CH2OH CH2OH

D-glucose L-glucose

Properties of glucose

• Oxidation• Monosaccharides are easily oxidised by the

oxidising agents.• With mild reagents such as Tollen’s reagent Glucose gives Gluconic acid. Pottasium ferricyanide can be reduced to

ferrocyanide .(test for sugars having free carbonyl groups. Hence also called reducing sugars.

• Strong oxidizing agents like Conc nitric acid yields dicarboxylic acid Saccharic acid.

• Reduction• Free CHO & C=O of monosacchrides are

reduced to alcohol by sodium amalgam and water.

• Glucose yields Sorbitol & Mannitol.

• Free CHO & C=O groups of sugar reacts with phenyl hydrazine to form corresponding osazone.

• Glucose – Glucosazone• Reaction with HCN – Cyanohydrin• Esterification- The hydroxyl group of

alcohols in the carbohydrate may be converted to esters by treatment with acetylating agents.(Glucose Penta Acetate)

Formation of Phosphate Esters•Phosphate esters can form at the 6-carbon of

aldohexoses and aldoketoses.

• •Phosphate esters of monosaccharides are found in the sugar-phosphate backbone of DNA and RNA, in ATP, and as intermediates in the metabolism of carbohydrates in the body.

• Methylation• Reacting with methyl alcohol monosaccrides

yields glycosides• Fermentation • Mono sacchrides like glucose & fructose

yields ethanol & Carbon dioxide on hydrolysis

• MUTAROTATIONThe two stereoisomeric forms of glucose, i.e., α-D-glucose and β-D-glucose exist in separate crystalline forms and thus have different melting points and specific rotations. For example α-D-glucose has a m.p. of 419 K with a specific rotation of +112° while  β-D-glucose has a m.p. of 424 K and has a specific rotation of +19°.

• However, when either of these two forms is dissolved in water and allowed to stand, it gets converted into an equilibrium mixture of α-and β-forms through a small amount of the open chain form.

• As a result of this equilibrium, the specific rotation of a freshly prepared solution of α-D-glucose gradually decreases from of +112° to +52.7° and that of β-D-glucose gradually increases from +19° to +52.7°.

•  

• This change in specific rotation of an optically active compound in solution with time, to an equilibrium value, is called mutarotation.

• During mutarotation, the ring opens and then recloses either in the inverted position or in the original position giving a mixture of α-and-β-forms.

• All reducing carbohydrates, i.e., monosaccharides and disacchardies (maltose, lactose etc.) undergo mutarotation in aqueous solution.

Galactose

Ring structure

Fructose

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar.

The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.

O

H

HO

H

HO

H

OHOHH

H

OH

O

H

HO

H

HO

H

HOHH

OH

OH

-D-glucopyranose -D-glucopyranose

1

6

5

4

32

Glycosidic BondsThe anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond:

R-OH + HO-R' R-O-R' + H2OE.g., methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose).

O

H

HO

H

HO

H

OHOHH

H

OH

-D-glucopyranose

O

H

HO

H

HO

H

OCH3

OHHH

OH

methyl- -D-glucopyranose

CH 3-O H+

methanol

H2O

Cellobiose, a product of cellulose breakdown, is the otherwise equivalent anomer (O on C1 points up). The (1 4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other.

H O

O H

H

O HH

O H

CH 2O H

HO H

O H

H

O HH

O H

CH 2O H

H

O

HH

1

23

5

4

6

1

23

4

5

6

m altose

H O

O H

H

O HH

O H

CH 2O H

HO O H

H

H

O HH

O H

CH 2O H

H

H

H

O1

23

4

5

6

1

23

4

5

6

cellobiose

Disaccharides:Maltose, a cleavage product of starch (e.g., amylose), is a disaccharide with an (1 4) glycosidic link between C1 - C4 OH of 2 glucoses. It is the anomer (C1 O points down).

Other disaccharides include: Sucrose, common table sugar, has a glycosidic bond

linking the anomeric hydroxyls of glucose & fructose.

Because the configuration at the anomeric C of glucose is (O points down from ring), the linkage is (12).

The full name of sucrose is -D-glucopyranosyl-(12)--D-fructopyranose.)

Lactose, milk sugar, is composed of galactose & glucose, with (14) linkage from the anomeric OH of galactose. Its full name is -D-galactopyranosyl-(1 4)--D-glucopyranose

Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with (14) linkages.Every other glucose is flipped over, due to linkages. This promotes intra-chain and inter-chain H-bonds and

c e l lu lo s e

H O

OHH

OHH

OH

CH 2 OH

HO

H

OHH

OH

CH 2 OH

HO

H H O

O H

OHH

OH

CH 2 OH

HH O

H

OHH

OH

CH 2 OH

H

H

OHH O

O H

OHH

OH

CH 2 OH

HO

H H H H

1

6

5

4

3

1

2

van der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles - microfibrils. See: Botany online website; website at Georgia Tech.

Schematic of arrangement of cellulose chains in a microfibril.

Multisubunit Cellulose Synthase complexes in the plasma membrane spin out from the cell surface microfibrils consisting of 36 parallel, interacting cellulose chains. These microfibrils are very strong. The role of cellulose is to impart strength and rigidity to plant cell walls, which can withstand high hydrostatic pressure gradients. Osmotic swelling is prevented.Explore and compare structures of amylose & cellulose using Chime.

c e l lu lo s e

H O

OHH

OHH

OH

CH 2 OH

HO

H

OHH

OH

CH 2 OH

HO

H H O

O H

OHH

OH

CH 2 OH

HH O

H

OHH

OH

CH 2 OH

H

H

OHH O

O H

OHH

OH

CH 2 OH

HO

H H H H

1

6

5

4

3

1

2

DISACCHARIDES

Disaccharides are sugars composed of two monosaccharides covalently bondedtogether by a glycosidic linkage

Maltose, also known as malt sugar, is formed from two glucose molecules

Lactose, or milk sugar, is a disaccharide formed when the monosaccharides glucose and galactose bond

Sucrose is common household sugar and is formed when the monosaccharidesglucose and fructose bond

MALTOSE = GLUCOSE + GLUCOSELACTOSE = GLUCOSE + GALACTOSESUCROSE = GLUCOSE + FRUCTOSE

- H2O

THE FORMATION OF MALTOSE

CH OH2

HH

HO

H

O HO H

H O H

H

GLUCOSE

CH OH2

HH

HO

H

O HO H

H O H

H

O

GLUCO SE

condensation reaction

1 4glycosidic bond

Maltose formswhen two alpha

glucose moleculesundergo a

condensationreaction and forma glycosidic bondbetween the two

molecules

CH O H2

HH

O HO H

H OH

H

OCH O H2

HH

HO

H

OO H

H

H

O1

23

4

5

6

1

234

5

6

MALTOSE

MALTOSE IS A DISACCHARIDE FORMED WHEN TWO ALPHAGLUCOSE MOLECULES ARE COVALENTLY BONDED TOGETHER

REDUCING SUGARSAll the monosaccharides and many of the disaccharides are

REDUCING SUGARSBenedict’s test is used to determine the reducing properties of the

different sugars

If a sugar is a reducing sugar then the Cu2+

(Cupric ions)

ions are reduced to Cu+ (Cuprous) which, in the presence of alkaline sodium hydroxide,

form copper oxideCopper oxide is insoluble and precipitatesout of the solution as a brick-red

precipitate

Benedicts solution is a turquoise solutioncontaining copper ions and sodium

hydroxide; the copper ions exist as Cu2+

in this reagent

REDUCING SUGARS

When Benedicts test is performed with the disaccharides maltose andsucrose, the following result is obtained:

Sucrose is anon-reducing sugar

Maltose is a reducing sugar

SUCROSERESULT

MALTOSERESULT

REDUCING SUGARSWhy is sucrose a non-reducing sugar?

CH O H2

HH

OHOH

H OH

H

OCH O H2

HH

HO

H

OO H

H OH

H

O1

23

4

5

6

1

234

5

6

MALTOSE1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

Sugars reduce Benedicts solution when the anomericcarbon atom is made available to reduce the copper

ions in the solutionThe anomeric carbon atom is the carbon of the

carbonyl group present in the straight chain form of the sugar

The anomeric carbon atom for glucoseis carbon 1

C

C

C

C

C

C

H

H

H

O

O

O

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

REDUCING SUGARSWhy is sucrose a non-reducing sugar?

CH O H2

HH

OHOH

H OH

H

OCH O H2

HH

HO

H

OO H

H OH

H

O1

23

4

5

6

1

234

5

6

MALTOSE1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

The anomeric carbon atom for fructoseis carbon 2

C

C

C

C

C

C

H

H

O

O

O

H

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6 Fructose bonds to glucose to formsucrose

Sugars reduce Benedicts solution when the anomericcarbon atom is made available to reduce the copper

ions in the solution

Why is sucrose a non-reducing sugar?

CH O H2

HH

OHOH

H OH

H

OCH O H2

HH

HO

H

OO H

H OH

H

O1

23

4

5

6

1

234

5

6

MALTOSE1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

When maltose is boiled with Benedict’s reagent, theregion of the ring containing the anomeric carbon

atom (carbon 1) may open exposing a carbonylgroup capable of reducing Benedicts reagent – ONLYAN ANOMERIC CARBON ATOM THAT IS NOT

INVOLVED IN THE FORMATION OF THEGLYCOSIDIC BOND MAY BE EXPOSED

This potential anomeric carbon atom is available to reduce

Benedict’s reagent

This potential anomeric carbon atom is unavailable

The one available anomeric carbon atom is sufficientfor this molecule to reduce Benedict’s solution and

thus MALTOSE is a reducing sugar

1

1

2

2

3

3

4

4

5

5

6

6

SUCROSE

C

C

C

C

C

C

H

H

O

O

O

H

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

Sucrose is formed when glucose forms a glycosidic bondwith fructose

glucose

fructose

glycosidicbond

The anomeric carbonatom for fructose

is carbon 2

C

C

C

C

C

C

H

H

H

O

O

O

H

H

H

H

H

O

O

O

H

H

H

H

1

2

3

4

5

6

The anomeric carbonatom for glucose

is carbon 1

As both the anomeric carbon atoms are involved in formingthe glycosidic bond when glucose and fructose join,

there are no potentially free anomeric carbon atoms availableto reduce Benedict’s solution

SUCROSE IS A NON-REDUCING SUGAR

TEST FOR SUCROSE

In order to determine if sucrose is present in a sample or solution then the following procedure is performed;

The sample or solution under consideration is boiled for at least fifteen minutesin hydrochloric acid

Boiling in acid breaks glycosidic bonds – the glycosidic bond is hydrolysed

This procedure is called ACID HYDROLYSIS

The solution is then neutralised by adding drops of alkali while testing with pH paper

Benedict’s test is now performed on the resulting solutionIf a brick-red precipitate forms then sucrose was present in the original solution

Acid hydrolysis breaks the glycosidic bonds in the sucrose moleculesreleasing free glucose and free fructose into the solution

Glucose and fructose are both monosaccharides and therefore reducing sugars

If no precipitate is obtained then sucrose was not present in the original sampleThe need to neutralise the solution following acid hydrolysis is due to the

fact that the Benedict’s test requires an alkaline medium

POLYSACCHARIDESPolysaccharides are large polymers of the monosaccharides

Unlike monosaccharides and disaccharides, polysaccharides are eitherinsoluble or form colloidal suspensions

The principal storage polysaccharides are STARCH AND GLYCOGEN

Starch is a polymer of alpha glucose and is, in fact, a mixture oftwo different polysaccharides – AMYLOSE AND AMYLOPECTIN

STARCH

AMYLOSE – long unbranched chain of glucoseunits

AMYLOPECTIN – highly branched polymerof glucose units

Polysaccharides:Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch.

Glucose storage in polymeric form minimizes osmotic effects.

Amylose is a glucose polymer with (14) linkages.

The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end.

Soluble in water, formed of 300-400 glucose units.

End of the chain have reducing property.

H O

OHH

OHH

OH

CH 2 OH

HO H

H

OHH

OH

CH 2 OH

H

O

HH H O

OH

OHH

OH

CH 2 OH

HH H O

H

OHH

OH

CH 2 OH

H

OH

HH O

OH

OHH

OH

CH 2 OH

H

O

H1

6

5

4

3

1

2

a m y lo s e

Amylopectin is a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) linkages. Branches are generally longer than shown above.The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur.Insoluble in waterPartially digested form is called dextrin.

H O

OHH

OHH

OH

CH2OH

HO H

H

OHH

OH

CH2OH

H

O

HH H O

OH

OHH

OH

CH2

HH H O

H

OHH

OH

CH2OH

H

OH

HH O

OH

OHH

OH

CH2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH2OH

HH H O

H

OHH

OH

CH2OH

HH

O1

OH

3

4

5

2

amylopectin

Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more (16) branches. The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants.

H O

OHH

OHH

OH

CH 2OH

HO H

H

OHH

OH

CH 2OH

H

O

HH H O

OH

OHH

OH

CH 2

HH H O

H

OHH

OH

CH 2OH

H

OH

HH O

OH

OHH

OH

CH 2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH 2OH

HH H O

H

OHH

OH

CH 2OH

HH

O1

OH

3

4

5

2

glycogen

AMYLOSE STRUCTURE

CH OH2

HH

HO

H

OOH

H OH

H

OCH OH2

HH H

OOH

H OH

H

OCH OH2

HH H

OOH

H OH

H

O

G LUC O SE GLUC O SEG LUC O SE

Amylose is formed by a series of condensation reactions that bondalpha glucose molecules together into a long chain

forming many glycosidic bonds

The amylose chain, once formed, coils into a helix

AMYLOSE STRUCTURE

O

O

O

O

O

O

O

O

O

O

O

O

OO

OO

OO

OO

O

O

O

O

O

OO

O

O

O

O

O

O

O

O

THE AMYLOSE HELIX

AMYLOPECTIN STRUCTURE

O

C H O H2

HH

HO

H

OOH

H OH

H

OCH OH2

HH H

OH

H OH

H

O

G LUC O SE GLUC O SE

CH OH2

HH

HO

H

OOH

H OH

H

OCH OH2

HH H

OOH

H OH

H

OCH 2

HH H

OOH

H OH

H

O

G LUC O SE G LUC O SEG LUC O SE

Branch point1 6 glycosidic bond

1 4 chain

6

1

Amylopectin consists of a straight chain of alpha glucose units with branch pointsoccurring at approximately every twelth glucose unit along the straight chain

The branch points form when carbon 6 of a glucose molecule in the straight chainforms a glycosidic bond with carbon 1 of a glucose molecule positioned above

the chain

AMYLOPECTIN STRUCTURE

This highly branched amylopectin molecule is wrapped around the amylose tomake up the final starch molecule

This large insoluble molecule with branch points that allow for easyaccess for enzymes when breaking down the molecule, makes

starch an ideal food storage compound

REACTION BETWEEN STARCH AND IODINE SOLUTIONWhen iodine solution is added to a suspension of starch, the iodinemolecules pack inside the amylose helix to give a blue-black colour

When iodine reacts with the starch in apiece of bread, the bread itself develops

the blue-black colour

N.B. Iodine is virtually insoluble in water – ‘Iodine Solution’ is really iodine dissolved in an aqueous solution of Potassium Iodide (KI)

GLYCOGEN

GLUCOSE IS STORED AS GLYCOGEN INLARGE AMOUNTS IN BOTH THE LIVER

AND SKELETAL MUSCLES

GLYCOGEN

Glycogen is often referred to as animal starchGlycogen has the same overall structure as amylopectin but

there is significantly more branching in this molecule

O

C H O H2

HH

HO

H

OOH

H OH

H

OCH OH2

HH H

OH

H OH

H

O

G LUC O SE G LUC O SE

CH OH2

HH

HO

H

OOH

H OH

H

OCH OH2

HH H

OOH

H OH

H

OCH 2

HH H

OOH

H OH

H

O

G LUC O SE G LUC O SEGLUC O SE

Branch point1 6 glycosidic bond

1 4 chain

6

1

More of thesebranch points form

STRUCTURAL POYSACCHARIDESCellulose is one of the most important structural polysaccharides as it is the

major component of plant cell walls

1 4 glycosidic bonds

OO

CH OH2

HOH

H OH

H

O

GLUCOSE

123

4

5

6

HOH

H OH

H

OCH OH2

GLUCOSE

3 2

1

6

5

4O

CH OH2

HH

HOOH

H OH

H

O

GLUCOSE

123

4

5

6

HOH

H OH

H

OCH OH2

GLUCOSE

3 2

1

6

5

4

Cellulose is a polymer of beta glucose units where each glucose moleculeis inverted with respect to its neighbour

The orientation of the beta glucose units places many hydroxyl (OH) groupson each side of the molecule

Many parallel chains of beta glucose units form and each chain forms hydrogen bonds between the OH groups of adjacent chains

STRUCTURAL POYSACCHARIDES

The bundles of parallel chains forming hydrogen bondswith each other creates a molecule that confers rigidityand strength to the structures of which they form a part

The rigidity and strength of plant cell walls is a consequenceof the incorporation of cellulose into their structure

hydrogen bonds between parallel chains of beta glucose

STRUCTURAL POYSACCHARIDES

Chitin is a polysaccharide forming the exoskeletons of many invertebrates. It is a polymer of N-acetylglucosamine in beta 1 to 4 glycosidic linkage. It is the major element in the exoskeleton of

insects and crustacea where it affords protection and support.

N-Acetylglucosamine

CHITIN

Benedict’s Reagent

Sucrose (table sugar) contains two sugars (fructose and glucose) joined by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict's reagent.. The products of sucrose decomposition are glucose and fructose, both of which can be detected by Benedict's reagent.

• The Benedict’s test is used to differentiate between reducing and nonreducing sugars.

• In the test, Cu2+ is reduced to Cu+ and the reducing sugar is oxidized to a carboxylic acid

• The appearance of copper (I) oxide, which is brick red, confirms the sugar to be a reducing sugar.

• Nonreducing sugars do not react.

• Iodine Test• The iodine test is used to differentiate between starch and

glycogen.• Starch contains amylose, a polymeric chain of glucose residues

bonded together by a-1,4’ linkages. Amylose form an a-helix.• Molecular iodine can intercalate into the a-helical structure. The

resulting complex is bluish black.• The extensive branching of glycogen deters the formation of an

a-helix. The addition of molecular iodine to a solution of glycogen does not give a bluish black color; instead, the color of the solution is reddish purple

• Osazone Formation• Osazones are formed by the reaction of a sugar with

phenylhydrazine.

• An osazone is a solid derivative of a sugar containing two phenylhydrazone moieties. From the observation

of the rate at which the osazone forms and the appearance of the precipitate can differentiate between

• epimeric sugars. • Osazones form at different rates for different sugars:

fructose reacts very rapidly, while glucose takes longer to react. The appearance of the precipitate can also be different.

• The crystalstructure ranges from coarse (for glucose) to very fine (for arabinose).

• Barfoed’s Test• The Barfoed’s test also differentiates between

reducing and nonreducing sugars, but because it is more sensitive than the Benedict’s test,

• it can be used to differentiate between reducing monosaccharides and reducing disaccharides.

• The reduction of Cu2+ to copper (I) oxide occurs more rapidly for monosaccharides than for disaccharides.

Acknowledgements

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