CHEMISTRY TEXTBOOK

Use your brain power

• Is galactose an aldohexose or
  a ketohexose?


• Which carbon in galactose has different
  configuration compared to glucose?


• Draw Haworth formulae of
  ∝-D-galactose and b-D-galactose.


• Which disaccharides among sucrose,
  maltose and lactose is/are expected to
  give positive Fehling test?


• What are the expected products of
  hydrolysis of lactose?

c. Lactose : Lactose (C 12 H 22 O 11 ) is a
disaccharide present in milk. It is formed
from two monosaccharide units, namely
D-galactose and D-glucose. The glycosidic
linkage is formed between C-1 of b-D-
galactose and C-4 of glucose. Therefore the
linkage in lactose is called b-1,4-glycosidic
linkage. The hemiacetal group at C-1 of the
glucose unit is not involved in glycosidic
linkage but is free. Hence lactose is a
reducing sugar. Figure 14.10 shows Haworth
formula of lactose.

14.2.10 Polysaccharides : Polysaccharides
are formed by linking large number of
monosaccharide units by glycosidic linkages.

O O

O

b-D-glucose

HO

OH OH

OH OH

OH

HOCH 2 HOCH 2

H H

H H

H H

H H

H

H

6 6

5 5

4 4

3 2 3 2

1 1

b- 1, 4- glycosidic linkage

b- D - galactose

Fig. 14.10 : Haworth formula of lactose

O O O O

-O O O O O -

OH OH OH OH

OH OH OH OH

CH 2 OH CH 2 OH CH 2 OH CH 2 OH

H H H H

H H H H

H H H H H H H H H H H H

6 6 6 6

5 5 5 5

4 4 4 4

3 2 3 2 3 2 3 2

1 1 1 1

α- 1, 4 - glycosidic linkages

Fig. 14.11 : Amylose

O

O

O

O O

O O

O O O -

• O
  
  
  • O
  
  
  
  

OH

OH

OH

OH OH

OH

OH

OH

OH OH

CH 2 OH

CH 2 OH

CH 2 OH

CH 2 CH 2 OH

H

H

H

H H

H

H

H

H H

H

H H

H

H

H

H

H

H H

H

H

H

H H

6

6

6

6 6

5

5

5

5 5

4

4

4

4 4

3

3

3

3 3

2

2

2

2 2

1

1

1

1

α- 1, 6 - glycosidic linkage (branch)

α- 1, 4 - glycosidic linkage

Fig. 14.12 : Amylopectin

Starch, cellulose and glycogen are the most

common natural polysaccharides. Starch is

storage carbohydrate of plants and important

nutrient for humans and other animals.

Cellulose is the main constituent of cell wall

of plant and bacterial cells. It is also main

constituent of wood and cotton. Glycogen

constitutes storage carbohydrate of animals

and is present in liver, muscles and brain. It

is also found in yeast and fungi.