Carbohydrates

Dr. Khalid Hussain

Introduction• Chemically: Polyhydroxy aldehydes and ketones• Have hydrogen and oxygen in a ratio of water

(2:1)Ribose (C5H10O5) and Glucose (C6H12O6)

• General formula Cn(H2O)n.Hence, also called hydrates of carbon

• Substances which are converted into polyhydroxy aldehydes and ketones after hydrolysis

Importance of carbohydrates• Provide major source of energy (4.1C)• Serve also as stored forms of energy as

glycogen in liver and muscles• Protein sparing action• Reduce breakdown of fatty acids and

preventing ketosis• Involved in biological recognition processes of

immunoglobulin

Contn’d

• Flavoring and Sweetening agents• Dietary fiber (Cellulose)• Important components of brain cells as

neuraminic acids, cerebrosides (Glycolipids) and gangliosides

• Important component of nucleic acids as pentose sugars - ribose and deoxyribose

Contn’d• Function as physiological anticoagulant

(Heparin)• Plasma expanders (Dextran)• Parenteral nutrition• Binders in tablets• Disintegrators in tablets• Analytical techniques: Agar agar and starch• Diagnostic markers

Classification

• Have been classified into 4 main groups–Monosaccharides –Disaccharides–Oligosaccharides–Polysaccharides

Monosaccharides• Carbohydrates which can’t further be broken down

into simpler units by acid hydrolysis• Classified according to the number of carbon atoms

present in them– For example trioses, tetroses and hexoses

• Usually have 3-9 carbon atoms• Further sub-classified into aldoses and ketoses• Exact name is based on

– number of carbons atoms – presence of aldehyde or ketone groups – i.e aldotriose or ketotriose

Aldotrioses/ ketotrioses

C3H6O3 Glyceraldehyde and dihydroxy acetone

Aldotetroses/ ketotetroses

C4H8O4 Erythrose, Erythrulose

Aldopentoses/ ketopentoses

C5H10O5 Ribose , Ribulose

Aldohexoses/ ketohesoses

C6H12O6 Glucose, Fructose

Erythrose Ribose Ribulose

• Monosaccharides also include derivatives of above mentioned monosaccharides e.g. amino sugars, sugar alcohols, sugar acids and esters of sulfuric acid and phosphoric acid

Disaccharides• Biose• Formed when two monosaccharides undergo

a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only

Sucrose

Oligosaccharides

• Greek word oligos, meaning "a few", and sacchar which means "sugar“

• Saccharide polymers containing a small number (typically three to ten) of component sugars- Monosaccharides

For example Raffinose

Polysaccharides• Polymeric carbohydrate structures, formed

by repeating units (either mono- or di-saccharides) joined together by glycoside bond

• Such structures are often linear, but may contain various degrees of branching

• Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit

• When all the monosaccharides in a polysaccharide are of same type the polysaccharide is called a homopolysaccharide: starch and glycogen, but when more than one type of monosaccharides are present, they are called heteropolysaccharides: pectin, lignin

• Polysaccharides have a general formula of (C6H10O5)n

• Not sweet, hence called non-sugars

H O

OH

H

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

H

1

6

5

4

3

1

2

a m y lo s e

c e l lu lo s e

H O

OH

H

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

Upon acid hydrolysis produce monosaccharides

Significance• Key role in maintaining energy balance in the

biosphere• Building blocks and source of energy• Used as excipients in pharmaceuticals

Dusting Powders, disintegrant, mucilage

• Bio-markers • As Medicine –heparin, glucose, dextran etc.• Chromatography and electrophoresis

Oxidation of glucose• Mild oxidizing agents such as bromine water convert

glucose to gluconic acid, which is single step reaction• This reaction is used to differentiate sugars having –

CHO (reducing) group or those yielding such group

Example is Fehling's solution test• Fehling's solution is composed of equal parts

of two solutions:1-Fehling's solution A

69.28 grams copper sulfate pentahydrate dissolved in 1 litre of distilled water

2-Fehling's Solution B346 grams potassium sodium tartrate and 120 grams sodium hydroxide in 1 litre of distilled water

• Under alkaline conditions fructose is converted to glucose and mannose

Fructose also give positive test

Tollens' reagent test "silver mirror"• Tollens' reagent: when silver nitrate solution is

treated with sodium hydroxide(NaOH) till it forms precipitate and then NH4OH is added till the precipitate is dissolved- Ag(NH3)2]+

• The diammine silver complex is an oxidizing agent, which is itself reduced to silver metal