BASICS OF

CARBOHYDRATES

By,Mrs.Bhargavi.GRamanagara.

Introduction:

•They have a general formula – Cn(H2O)n , n>3.

•They contain 3 main elements ; carbon ,hydrogen and oxygen.

Definition: “carbohydrates are polyhydroxy aldehydes or ketones

and their derivatives.”

Role of carbohydrates:

Classification of Carbohydrates:

1) Monosaccharides- cannot be broken down into smaller units by hydrolysis. Sometimes called simple sugars. a) Trioses – glyceraldehydeb) Tetroses- Erythrosec) Pentoses- ribosed) hexoses- glucose

2) Disaccharides- can be broken down (hydrolyzed) into two monosaccharide units. Ex -lactose, maltose etc,

3) Oligosaccharides- can be broken into three to six monosaccharide units. Ex- Raffinose

4) Polysaccharides- composed of 7 or more mono-saccharide units .a) Homo polysaccharides: Ex- Starch, Glycogen etc,.b) Hetero polysaccharides: Ex- hyaluronic acid, heparin etc,

Structure of monosaccharide’s, disaccharides and polysaccharides:

Reducing and non-reducing sugars:

•Reducing sugars: - a sugar which has a free aldehyde group or a free

ketone groupthat can reduce benedict’s reagent.

Ex.-glucose, lactose, maltose etc,

•Non-Reducing sugars: - a sugar which do not have a free aldehyde

group or a free ketone group and thus, cannot reduce benedict’s

reagent.

Ex.- Sucrose.

Mutarotation:

Mutarotation is the change in the optical rotation because of the change

in the equilibrium between α and β anomeric forms of glucose.

Disorders of carbohydrate metabolism:

•Glycogen storage disorders:

Glycogen storage disease (GSD, also glycogenosis) is the result of

defects in  glycogen-synthesis or breakdown related enzymes within 

muscles, liver, and other cell types. It can be genetic or acquired.

Example- type-1disorder (Gierke’s disease)- due to defect of glucose-6-phosphatase enzyme, cori’s disease etc,.

Qualitative tests or identification tests for carbohydrates:

1)Molisch’s test:

-test for carbohydrates.

Few drops Molisch reagent+ 2ml sample+ conc.H2SO4-

along sides of test tube: carbohydrates on dehydration by sulfuric

acid produce furfural which then condenses with the -naphthol

resulting in a red or purple-colored ring at the junction of 2 liquids.

2) Iodine test:

-test for polysaccharides.

2ml sample+iodine solution drop wise-

Iodine solution based on adsorption reacts with starch producing a

blue-black color.

3) Barfoed’s test:

-test for monosaccharides.

2ml sample+5ml reagent+ boil 2min-

In Barfoed’s reagent, cupric acetate in acetic acid, is balanced so that is

can only be reduced by monosaccharides forming brick-red cuprous

oxide precipitate.

4) Seliwanoff’s test:

-test for ketoses.

2ml sample + 5ml reagent+ boil 2min-

When heated with reagent, ketoses are more rapidly dehydrated

than aldoses, a red color appears.

5) Benedict’s test:

-test for reducing sugars.

5ml Benedict's reagent +few drops sample-

 The reducing sugar here reduces the cupric ions in

these test solutions to cuprous ions, which then forms a brick red

copper oxide precipitate.

6) Fehling’s test:

-test for reducing sugars.

5ml Fehling’s reagent+ few drops sample-

The reducing sugar here reduces the cupric ions in

these test solutions to cuprous ions, which then forms a brick red

copper oxide precipitate.

BENEDICT’S TEST:

7) Osazone test:

• microscopic test for crystalline structure of carbohydrates.•Osazones are formed when the sugars react with a compound known as phenylhydrazine hydrochloride at boiling point.•Osazone crystals have a characteristic shape nder the light microscope, ths help in identification of sugar type.

Osazone formation (microscopic view):