1.

2 Carbohydrates
Learning outcomes:
At the end of the lesson, students should be able to:
a) State the classes of carbohydrates such as
monosaccharide, disaccharide and polysaccharide
b) Illustrate the formation & breakdown of maltose
c) Compare the structures & functions of starch,
glycogen and cellulose
 Carbohydrates

Disaccharides

Monosaccharides Polysaccharides

Starch
Aldose
Glycogen
Ketose
Cellulose
3
 1.2 Carbohydrates
• Organic molecule containing the element
carbon, hydrogen and oxygen in a ratio of
1:2:1
• The empirical formula (CH2O)n ;
** n = number of carbon
 1.2 Carbohydrates
Divided into 3 main classes
1
3
Monossacharides Polysaccharides
(simple sugar; 1 sugar (many sugar molecules)
molecule)

2
Disaccharides
(2 sugar molecules)
 1.2.1 Monosaccharides
• Greek words, monos = simple; sacchar = sugar
• The basic unit of carbohydrate
(the simplest sugar molecule)
 CHAPTER 1: MOLCULES 0F LIFE 1.2: Carbohydrates

Monosaccharides

CARBOHYDRATE

MONOSACCHARIDES DISACCHARIDES POLYSACCHARIDES

Triose Pentose Hexose

(3C) (5C) (6C)

Glyceraldehyde
-Ribose -Glucose
-Deoxyribose -Galactose
-Fructose

1.2a) State the classes of carbohydrate such as monosaccharides,

LEARNING OUTCOMES:
disaccharides & polysaccharides.
 Types of Monosaccharides
Classification of monosaccharides are based on:
1. Size of the carbon skeleton
(number of carbon atoms that they contains)
2. The location of the carbonyl group
( aldehyde or ketone functional group)
 1. Size of the carbon skeleton
Depend on number of
carbon atom.

(3C) (4C) (5C) (6C)

 2. Location of the carbonyl group
• Monossacharides are either aldoses or ketoses
• If the location of carbonyl group is at the end
of the carbon skeleton: aldose
• If the location of carbonyl group is in the
middle of the carbon skeleton : ketose
• Example:
1. Glucose – aldose sugar (aldehyde)
2. Fructose – ketose sugar(ketone)
 Monosaccharides: Different between aldoses
and ketoses
Aldoses (e.g., glucose) have an Ketoses (e.g., fructose) have
aldehyde group at one end. a ketone group, usually at
C2.
H O
C CH2OH

H C OH C O

HO C H HO C H

H C OH H C OH

H C OH H C OH

CH2OH CH2OH

glucose
D-glucose Dfructose
-fructose
Structure of Monosaccharides
• Have the same chemical formula (C6H12O6) but
with different structures
• Known as isomers.
Structure of Monosaccharides

16
 Monosaccharides as reducing sugar
• Aldoses and ketoses are reducing sugars
• What are the properties of reducing sugars?
When a reducing sugar is heated with alkaline
solution of copper (II) sulphate (Benedict’s
solution) the aldehyde or ketone groups
reduces the Cu2+ .
Figure: Free aldehyde group in glucose molecule
18
 Hexose (6C)
• Example of hexose is:
1. Glucose : the immediate source of energy for
cellular respiration
2. Galactose : sugar found in milk and yogurt
3. Fructose : sugar found in honey and some fruits

• Have identical formula but different structural

formulas : structural isomer.
 Linear and ring forms of glucose
H O

1 C 6 CH
2OH 6 CH
2OH

2
CH2OH
H C OH 5C
O H 5C O 6
3 H H H O
H H 5 H
HO C H
H H H
4C 1C 4C 1C
4
OH OH 4
OH
1

H C OH H H
O HO 3 2 OH
OH OH
5 3 C 2C 3C 2C OH
H C OH
H OH
6 H OH H OH
H C OH
Ring chain

Straight chain
22
 Isomers of glucose
• Glucose can exist in two possible ring forms known as:

Alpha (α) form Beta (β) form

• The hydroxyl group (-OH) • The hydroxyl group (-OH)

on carbon atom 1 can on carbon atom 1 can
project below the ring project above the ring
Isomers of glucose

24
α-glucose
β-glucose
 1.2.2 Disaccharides
• Disaccharides are formed when two
monosaccharides joined together

There are THREE COMMON disaccharides:

1. Maltose : α- glucose + α- glucose

2. Sucrose : α- glucose + β- fructose
3. Lactose : β – galactose + β- glucose
 Formation of Disaccharides

• In general, two monosaccharides are joined

together by a condensation process.
• In condensation process:
• Water is removed
• The bond formed between two
monosaccharides are called glycosidic
bond.
 Formation
formation
of Maltose
• Maltose is formed from two molecules of α-
glucose by condensation process.
• One molecule of water is removed.
condensation

+ H20

α- 1,4 glycosidic bond

α- glucose α- glucose maltose
Figure: Condensation reaction in the synthesis of maltose
 Structure of Maltose
6 6

5
5

4 1 4 1

2 3 2
3

α- 1,4 glycosidic bond

 Hydrolysis of Maltose
• α- 1,4 glycosidic bond can also be broken
down to release separate monomer units.

• This is called hydrolysis process because water

is needed to split up the bigger molecule.
 Hydrolysis of Maltose
• When maltose is hydrolyzed, 2 molecule of
α- glucose is formed.

Hydrolysis
+ H2O

CH2OH CH2OH

O O
H H H H
H H
Maltose +
OH H OH H
HO OH HO OH

H OH
H OH
α-glucose
α-glucose
 1.2.3 Polysaccharides
• Polysaccharide are polymers that formed
from condensation of many monosaccharides
• The chains of monosaccharide molecules are
linked together by glycosidic bond.
• The chains may be branched or unbranched.
 Function of polysaccharides
 Energy source - starch & glycogen
 Basic component of structure - cellulose &
hemicelluloses
 Protection & immunization - heparin in
mammals blood prevent/dissolve blood clotting

 Heparin - is a complex polysaccharides

composed of repeating disaccharide. Produce
in basophil especially in the lung and liver
 Polysaccharides : Starch

• A polymer of α-glucose

Figure: Starch is link by α-1,4 glycosidic bond

 1. Polysaccharides : Starch

Two types of starch:

1. Amylose
Simple and unbranched
2. Amylopectin
Complex and branched
 Amylose
• Composed of about 200-1500 of α-glucose molecules
linked together in long, unbranched chain.
• Each linkage occurs between the carbon number 1 of
one α-glucose molecule and the carbon number 4 of
another α-glucose molecule.
• Bond form between the molecule is α - 1,4 glycosidic
bond

CH2OH 6CH OH CH2OH CH2OH CH2OH

2
O 5 O H O H O H H O H
H H H H H
H H H H H
OH H 1 4 OH H 1 OH H OH H OH H
O O O O OH
OH 2
3
H OH H OH H OH H OH H OH
α - 1,4 glycosidic bond amylose α - 1,4 glycosidic bond
 Amylopectin
• A branched polymer of 2000 to 200000 α-glucose
molecules.
• The linear chains of α-glucose units are held
together by α-1,4 glycosidic bond.
• Branches occur at intervals of approximately 25 to
30 where α-1,6 glycosidic bond occurs.
Amylopectin

α-1,6 glycosidic
bond

α-1,4 glycosidic bond

 2. Polysaccharides : Glycogen

 Glycogen is made up of short & highly

branched chains of α– glucose
 Found in muscle and liver cells
 Characteristics:
1) Not sweet in taste
2) insoluble in water
3) cannot be crystallize
4) compact molecule
 2. Polysaccharides : Glycogen

CH 2OH CH 2OH
H O O
glycogen
H H H
H H
OH H OH H 1
O
OH
O α-1,6 glycosidic
H OH H OH
bond
CH 2OH CH 2OH 6 CH 2 CH 2OH CH 2OH
H O H H O H H 5 O H H O H H O H
H H H H H
OH H OH H 4 OH H 1 4 OH H OH H
O O O O OH
OH
3 2
H OH H OH H OH H OH H OH

α-1,4 glycosidic bond

 3. Polysaccharides : Cellulose
• A polymer of ß- glucose
• Cellulose is a main component that build cell
walls in a plant.
• It has ß-1,4 glycosidic linkage which are linked
together by hydrogen bonds to form a rigid
structure

β- 1,4 glycosidic bond

Hydrogen
bond
Different between structure of
amylose and cellulose
Amylose
• Monomer : α– glucose
• Type of Bond:
α - 1,4 glycosidic bond
• Function : Storage in plant

Cellulose
• Monomer: ß- glucose
• Type of bond :
ß - 1,4 glycosidic bond
• Function: main component
of cell wall