CARBOHYDRATES - DEFINITION OF CARBOHYDTRATES - FUNCTIONS OF CARBOHYDRATES - USES OF CARBOHYDRATES - CLASSIFICATION OF CARBOHYDRATES

CARBOHYDRATES – DEFINITION, FUNCTIONS, CLASSIFICATION, ISOMERISM & PROPERTIES

INTRODUCTION TO CARBOHYDRATES

Carbohydrates are the polyhydroxy Aldehydes or Ketones, are major macronutrient and the primary sources of energy.

Consists of Carbon, Hydrogen, and Oxygen.

The general formula for carbohydrates is Cn(H2O)n.

For e.g. – C6(H2O)6 =  C6H12O6

                          (n=6)        (Glucose)

The sugars which contain Aldehydic group are called as Aldoses & the sugars which contain Ketonic group is called as Ketoses.

FUNCTIONS OF CARBOHYDRATES

Carbohydrates are the main source of energy.

1 gm carbohydrates provides 3.9 Kcal

Carbohydrates are stored in our body in the form of Glycogen.

Excess of Carbohydrates converts into fats and stored in Adipose tissues.

Glycoprotein & Glycolipids are the components of cell membrane and receptors.

They are the components of Nucleic acids (DNA & RNA).

Carbohydrates are the structural bases of many organisms. For e.g. – Cellulose in plants & exoskeleton of insects.

The recommended dietary allowance for carbohydrates is 400gm/ day.



CLASSIFICATION OF CARBOHYDRATES

Carbohydrates have been broadly classified into three groups –

1.) Monosaccharides – They are the simple sugars consists of 3 – 7 carbon atoms.

NO. OF CARBON ATOMSALDOSESKETOSES
TRIOSE (3-C)GlyceraldehydeDihydroxyacetone
TETROSE (4-C)ErythroseErythrulose
PENTOSE (5-C)RiboseRibulose
HEXOSE (6-C)GlucoseFructose
HEPTOSE (7-C)GlucoheptoseSedoheptose

2.) Oligosaccharides – These are the compound sugars which consist of the 2–10 monosaccharides. If an oligosaccharide consists of two monosaccharides, it is called as disaccharides & the one which consists of three monosaccharides, called Trisaccharides & so on….

DISACCHARIDESMOLECULES PRESENT
MaltoseGlucose + Glucose
LactoseGlucose + Galactose
SucroseGlucose + Fructose

TRISACCHARIDESMOLECULES PRESENT
RaffinoseGlucose + Fructose + Galactose

TETRASACCHARIDESMOLECULES PRESENT
StachyoseGlucose + Fructose + Galactose + Galactose

3.) Polysaccharides – These are the compound sugars which consist of more than 10 molecules of monosaccharides. If polysaccharides consist of the same type of monosaccharides, it is called as Homopolysaccharides & if the polysaccharide consists of different monosaccharides they are called as Heteropolysaccharides.

HOMO -
POLYSACCHARIDES
HETRO -
POLYSACCHARIDES
StarchHyaluronic acid
GlycogenChondratin sulfate
CelluloseDermatan sulfate
Chitin

ISOMERISM IN CARBOHYDRATES

Isomers are the compounds having same molecular formula but different structural formula and this phenomenon are known as isomerism.

Isomers are of two types –

  • Structural isomers – The isomers having same molecular formula but different structure are called as structural isomers.
  • Stereoisomers – The compounds having same molecular formula and structure but differs only in the spatial configuration.

⇒ Enantiomers –  They are non-superimposed mirror images of each other. D- and L- isomers are mirror images of each other. When the hydroxyl group is present on the right side the sugar is D- isomer and if it is on the left side it is L- isomer. For e.g. D-Glyceraldehyde and L- Glyceraldehyde.

ENANTIOMERS

⇒ Diastereomers –  The stereoisomers having the different configuration at two or more stereocenters are not the mirror images of each other they are called as diastereomers.

DIASTERIOMERS

⇒ Optical activity –  The substances which rotate the plane polarised light either towards right or towards left are said to be optically active substances and the property is known as optical activity. Those compounds which rotate light towards the right are called as Dextro-rotatory and the compounds which rotate plane polarised light towards left are known as Laevo-rotatory.



MONOSACCHARIDE STRUCTURES – FISCHER PROJECTION FORMULA or OPEN CHAIN STRUCTURES

1.) ALDOSES

 I.) 3-C COMPOUND

D- GLYCERALDEHYDE

II.) 4-C COMPOUND

D- ERYTHROSED- THREOSE

III.) 5-C COMPOUNDS

D - LYXOSED - RIBOSE
D - XYLOSED- ARABINOSE

IV.) 6-C COMPOUND

D - ALLOSED - ALTROSE
D - GALACTOSED - GLUCOSE
D - MANNOSE

 

CHEMICAL PROPERTIES OF CARBOHYDRATES

I.) Tautomerization or Enolisation

Tautomerization or Enolization is also known as Lobry De Bruyn Alberta’ van ekenstein transformation.

It is a type of isomerization in which glucose Fructose and mannose are interconvertible in weak alkaline solutions such as sodium hydroxide solution at low temperature.

II.) Reducing property of carbohydrates

 Carbohydrates maybe reducing or nonreducing depending on reducing property of Carbohydrates because of free aldehyde or Ketone group of anomeric carbon.

Metal hydroxides like Copper Hydroxide oxidized the free aldehyde or Ketone group of reducing sugar.

III.) Oxidation reaction

The Terminal aldehyde group order terminal alcohol may be oxidized in the presence of the oxidizing agent.  For example oxidation of aldehyde group results in the formation of gluconic acid and the oxidation of terminal hydroxyl group results in the formation of glucuronic acid.

IV.) Reduction reaction

The aldehyde or carbonyl group of monosaccharides are reduced to their corresponding alcohols by treating them with reducing agents like sodium amalgam.

V.) Dehydration reaction

when carbohydrates treated with concentrated sulphuric acid they undergo dehydration with elimination of 3 water molecules hexoses gives hydroxymethylfurfural while pentoses give furfurals on dehydration

The farfural  can be condensed with phenolic compounds like Alpha naphthol to form color product ( purple ring)  this is the chemical basis of molisch test.

VI.) Osazone (crystal) formation

 When monosaccharides react with phenylhydrazine then it results in the formation of osazones.

Glucose, Fructose, and mannose give the needle-shaped osazones.

Maltose forms sunflower type crystals and Lactose forms cotton balls shaped osazones.

VII.) Formation of esters

The alcoholic group of monosaccharides maybe esterified esterification of Carbohydrates with phosphoric acid is a common reaction in carbohydrates metabolism, for example, Glucose-6-phosphate & Glucose-1-phosphate is formed by esterification of glucose.

VIII.) Mutarotation

In aqueous solution, the α- & β- anomers of monosaccharides convert into each other by a process called muta-rotation.

IX.) Glycoside formation

 They are formed when hydroxyl group of an anomeric carbon of Carbohydrates react with the hydroxyl group of another carbohydrate, the Bond so formed  Alpha 1→4 or Beta 1→4 is known as glycosidic Bond. 



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