Introduction

• Carbohydrates are sugars and starches, which provide energy for humans and animals, and cellulose which make up many plant structures.

• There are three types of carbohydrates:1. Simple, or monosaccharides:• Found in fruits and milk products are more easily digested

by the body. • Also often found in processed refined foods such as white

sugar and white bread. • A monosaccharide is a triose, tetrose, pentose or hexose,

cannot be hydrolyzed to simpler carbohydrates

• Glucose: "blood sugar"• Galactose: a sugar in milk• Fructose: a sugar found in honey.

2. Disaccharides :

• monosaccharides covalently linked can be hydrolyzed into two monosaccharide units.

• Sucrose: common table sugar = glucose + fructose

• Lactose: major sugar in milk = glucose + galactose

• Maltose: product of starch digestion = glucose + glucose .

Note: Oligosaccharides: a few monosaccharides covalently linked

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3. Complex, or polysaccharides: which take longer for the body to digest, are most commonly found in vegetables (cellulose), whole grain, breads, rice and legumes.

A. Starches are polymers of glucose: Two types are found:

• amylose consists of linear, unbranched chains of several hundred glucose residues (units). The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms.

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• amylopectin differs from amylose in being highly branched. a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand.

B. Glycogen• Animals store excess glucose by polymerizing it to

form glycogen. The structure of glycogen is similar to the amylopectin, although the branches in glycogen tend to be shorter and more frequent.

• Glycogen is broken back down into glucose when energy is needed (a process called glycogenolysis).

C. Cellulose

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Structure of Monosaccharides

Aldoses ketoses

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D vs L Designation

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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

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Reducing and Non-reducing sugars • Sugars exist in solution as an equilibrium mixture of

open-chain and closed-ring (or cyclic) structures.• Sugars that can be oxidized by mild oxidizing agents

are called reducing sugars because the oxidizing agent is reduced in the reaction.

• A non-reducing sugar is not oxidized by mild oxidizing agents.

• All common monosaccharides are reducing sugars.• The disaccharides maltose and lactose are reducing

sugars.• The disaccharide sucrose is a non-reducing sugar.• Common oxidizing agents used to test for the

presence of a reducing sugar are: Benedict's solution, Fehling's solution.

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Ex(1). Benedict's Test• Benedict's test determines whether a monosaccharide

or disaccharide is a reducing sugar.• To give a positive test, the carbohydrate must contain

a hemiacetal which will hydrolyse in aqueous solution to the aldehyde form.

• Benedict's reagent is an alkaline solution containing cupric ions, which oxidize the aldehyde to a carboxylic acid.

• In turn, the cupric ions are reduced to cuprous oxide, which forms a red precipitate. This solution has been used in clinical laboratories for testing urine.

RCHO  +  2Cu2+  +  4OH- ----->   RCOOH  +  Cu2O  +  2H2O

Hemiacetal & hemiketal formation

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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

+

+

Procedure

1. Place 1 ml of carbohydrates solutions in test tube.

2. To each tube, add 1 ml of Benedict's reagent.

3. Heat the tubes in a boiling water bath for 3-5 minutes.

4. Remove the tubes from water bath and record the results.

Result: In the presence of a reducing sugar a precipitate which may be red , yellow or green will form.

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Ex(2). Barfoed's Test

• Barfoed's test is similar to Benedict's test, but determines if a carbohydrate is a monosaccharide or a disaccharide.

• Barfoed's reagent reacts with monosaccharides to produce cuprous oxide at a faster rate than disaccharides do:

RCHO  +  2Cu2+  +  2H2O ----->   RCOOH  +  Cu2O  +  4H+

Procedure

1. Place 1ml of the following 1% carbohydrate solutions in test tubes: glucose, fructose, sucrose, lactose, and maltose.

2. To each tube, add 1 ml of Barfoed's reagent.

3. Heat in a boiling water bath for 10 minutes.

4. Note and record your observations.

Result: A red precipitate will form if the test is positive.

Ex(3). Bial's (Orcinol) Test for Pentoses

• This test is used to distinguish pentoses from hexoses.• Bial's reagent contains {orcinol (5-methylresorcinol) in

concentrated HCl with a small amount of FeCl3 catalyst}. • The test reagent dehydrates pentoses to form furfural.• Furfural further reacts with orcinol and the iron ion present in the

test reagent to produce a bluish product.

• Not: Be extremely careful when heating the test tubes. Bial's reagent contains concentrated hydrochloric acid. Do not point the tube toward yourself or any of your fellow lab workers.

Procedure1. Add about 2 ml of 1% carbohydrates solutions.2. Add 3 ml of Bial's reagent to each tube and mix

well.3. Carefully heat each tube (with some agitation)

directly over the burner flame. 4. Hold the tube at a diagonal and heat along the

sides of the tube rather than at the bottom to prevent eruption of the liquid from the tube.

5. Move the tube diagonally in and out of the flame, until the mixture just begins to boil.

6. Stop heating when the mixture begins to boil.

Result• A bluish color indicates a positive result.

• Prolonged heating of some hexoses yields hydroxymethyl furfural which also reacts with orcinol to give colored complexes.

Two negative tests (left, middle) and a positive test (right)

Ex(4). Seliwanoff's (Resorcinol) Test For Ketoses

• This test can differentiate between ketoses and aldoses.• Seliwanoff's reagent (resorcinol dissolved in 6 M HCl),• The test reagent dehydrates ketohexoses to form 5-

hydroxymethylfurfural.• 5-hydroxymethylfurfural further reacts with resorcinol

present in the test reagent to produce a red product within two minutes.

• Aldohexoses react to form the same product, but do so more slowly.

• Ketohexoses (such as fructose) and disaccharides containing a ketohexose (such as sucrose) form a cherry-red condensation product.

• Other sugars may produce yellow to faint pink colors.

Procedure1. Add about 3 ml of Seliwanoff's reagent to each labeled test

tube. 2. Add 1 drop of the respective sugar solution to the

appropriate test tubes and mix well. 3. Place all the test tubes in the boiling water bath at the same

time and heat for 3 min after the water begins to boil again. 4. Record your observations. 5. A positive result is indicated by the formation of A red

color with or without the separation of a brown-red precipitate.

a negative test (left) and a positive test (right)