NCERT Textbook: Biomolecules | Biotechnology for Class 11 - NEET PDF Download

 Page 1


Biomolecules
3.1 Carbohydrates
3.2 Fatty Acids and 
Lipids
3.3 Amino Acids 
3.4 Protein Structure
3.5 Nucleic Acids
In the previous chapter you have learnt about the cell and 
its organelles. Each organelle has distinct structure and 
therefore performs different function. For example, cell 
membrane is made up of lipids and proteins, cell wall is 
made up of carbohydrates, chromosomes are predominantly  
made up of proteins and DNA, and ribosomes are made up 
of proteins and RNA. These ingredients of cellular organelles 
are also called macromolecules or biomolecules. There are 
four major types of biomolecules— carbohydrates, proteins, 
lipids, and nucleic acids. Apart from being structural entities 
of the cell, these biomolecules play important functions in 
cellular processes. In this chapter you will study the structure 
and functions of these biomolecules.
3.1 Carbohydrates Carbohydrates are one of the most abundant classes of 
biomolecules in nature and found widely distributed in all life 
forms. Chemically, they are aldehyde and ketone derivatives 
of the polyhydric alcohols. Major role of carbohydrates 
in living organisms is to function as a primary source 
of energy. These molecules also serve as energy stores, 
Chapter 3
Chapter 3 Carbohydrade Final 30.018.2018.indd   50 09/01/2025   15:22:35
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Page 2


Biomolecules
3.1 Carbohydrates
3.2 Fatty Acids and 
Lipids
3.3 Amino Acids 
3.4 Protein Structure
3.5 Nucleic Acids
In the previous chapter you have learnt about the cell and 
its organelles. Each organelle has distinct structure and 
therefore performs different function. For example, cell 
membrane is made up of lipids and proteins, cell wall is 
made up of carbohydrates, chromosomes are predominantly  
made up of proteins and DNA, and ribosomes are made up 
of proteins and RNA. These ingredients of cellular organelles 
are also called macromolecules or biomolecules. There are 
four major types of biomolecules— carbohydrates, proteins, 
lipids, and nucleic acids. Apart from being structural entities 
of the cell, these biomolecules play important functions in 
cellular processes. In this chapter you will study the structure 
and functions of these biomolecules.
3.1 Carbohydrates Carbohydrates are one of the most abundant classes of 
biomolecules in nature and found widely distributed in all life 
forms. Chemically, they are aldehyde and ketone derivatives 
of the polyhydric alcohols. Major role of carbohydrates 
in living organisms is to function as a primary source 
of energy. These molecules also serve as energy stores, 
Chapter 3
Chapter 3 Carbohydrade Final 30.018.2018.indd   50 09/01/2025   15:22:35
Reprint 2025-26
Biomolecules 51
metabolic intermediates, and one of the major components 
of bacterial and plant cell wall. Also, these are part of DNA 
and RNA, which you will study later in this chapter. The 
cell walls of bacteria and plants are made up of polymers 
of carbohydrates. Carbohydrates also act as informational 
materials and linked to surfaces of proteins and lipids to 
act in cell–cell interaction, and in the interaction between 
cells with other elements in the cellular environment where 
they play a role.
(A)	 Classi??cation 	 of	carbohydrates	
Carbohydrates are found in various forms ranging from 
simple sugars to complex polymers of more than one unit, and 
accordingly these have been classi??ed. These are commonly 
classi??ed into three categories namely, monosaccharides, 
oligosaccharides, and polysaccharides. 
1.	Monosaccharides
Monosaccharides are simple sugars which cannot be further 
hydrolysed into simpler forms. These monosaccharides 
contain free aldehyde (-CHO) and ketone (>C=O) groups, 
with two or more hydroxyl (-OH) groups with a general 
formula of C
n
(H
2
O)
n
. Based on the number of carbon atoms 
and functional groups, monosaccharides are classi??ed as  
given in Table 3.1. 
Table 	3.1:	Classi??cation 	 of	monosaccharides
S. No.
Class 	of	monosaccharides	
based	on	number	of	 carbon	
atoms
Class 	of	monosaccharides	 based 	 on	functional	 groups
Aldoses	 Ketoses
1. Trioses (C
3
H
6
O
3
) Glyceraldehyde (an aldotriose)
Dihydroxyacetone 
(a ketotriose) 
2. Tetroses (C
4
H
8
O
4
) Erythrose Erythrulose 
3. Pentoses (C
5
H
10
O
5
) Ribose Ribulose 
4. Hexoses (C
6
H
12
O
6
) Glucose Fructose 
2.	Oligosaccharides
Conventionally, oligosaccharides are carbohydrates 
having two to ten units of monosaccharides joined 
together by glycosidic bond. Some commonly occurring 
oligosaccharides are maltose, lactose, sucrose, etc., which  
are disaccharides (having two monosaccharides).
Chapter 3 Carbohydrade Final 30.018.2018.indd   51 09/01/2025   15:22:36
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Page 3


Biomolecules
3.1 Carbohydrates
3.2 Fatty Acids and 
Lipids
3.3 Amino Acids 
3.4 Protein Structure
3.5 Nucleic Acids
In the previous chapter you have learnt about the cell and 
its organelles. Each organelle has distinct structure and 
therefore performs different function. For example, cell 
membrane is made up of lipids and proteins, cell wall is 
made up of carbohydrates, chromosomes are predominantly  
made up of proteins and DNA, and ribosomes are made up 
of proteins and RNA. These ingredients of cellular organelles 
are also called macromolecules or biomolecules. There are 
four major types of biomolecules— carbohydrates, proteins, 
lipids, and nucleic acids. Apart from being structural entities 
of the cell, these biomolecules play important functions in 
cellular processes. In this chapter you will study the structure 
and functions of these biomolecules.
3.1 Carbohydrates Carbohydrates are one of the most abundant classes of 
biomolecules in nature and found widely distributed in all life 
forms. Chemically, they are aldehyde and ketone derivatives 
of the polyhydric alcohols. Major role of carbohydrates 
in living organisms is to function as a primary source 
of energy. These molecules also serve as energy stores, 
Chapter 3
Chapter 3 Carbohydrade Final 30.018.2018.indd   50 09/01/2025   15:22:35
Reprint 2025-26
Biomolecules 51
metabolic intermediates, and one of the major components 
of bacterial and plant cell wall. Also, these are part of DNA 
and RNA, which you will study later in this chapter. The 
cell walls of bacteria and plants are made up of polymers 
of carbohydrates. Carbohydrates also act as informational 
materials and linked to surfaces of proteins and lipids to 
act in cell–cell interaction, and in the interaction between 
cells with other elements in the cellular environment where 
they play a role.
(A)	 Classi??cation 	 of	carbohydrates	
Carbohydrates are found in various forms ranging from 
simple sugars to complex polymers of more than one unit, and 
accordingly these have been classi??ed. These are commonly 
classi??ed into three categories namely, monosaccharides, 
oligosaccharides, and polysaccharides. 
1.	Monosaccharides
Monosaccharides are simple sugars which cannot be further 
hydrolysed into simpler forms. These monosaccharides 
contain free aldehyde (-CHO) and ketone (>C=O) groups, 
with two or more hydroxyl (-OH) groups with a general 
formula of C
n
(H
2
O)
n
. Based on the number of carbon atoms 
and functional groups, monosaccharides are classi??ed as  
given in Table 3.1. 
Table 	3.1:	Classi??cation 	 of	monosaccharides
S. No.
Class 	of	monosaccharides	
based	on	number	of	 carbon	
atoms
Class 	of	monosaccharides	 based 	 on	functional	 groups
Aldoses	 Ketoses
1. Trioses (C
3
H
6
O
3
) Glyceraldehyde (an aldotriose)
Dihydroxyacetone 
(a ketotriose) 
2. Tetroses (C
4
H
8
O
4
) Erythrose Erythrulose 
3. Pentoses (C
5
H
10
O
5
) Ribose Ribulose 
4. Hexoses (C
6
H
12
O
6
) Glucose Fructose 
2.	Oligosaccharides
Conventionally, oligosaccharides are carbohydrates 
having two to ten units of monosaccharides joined 
together by glycosidic bond. Some commonly occurring 
oligosaccharides are maltose, lactose, sucrose, etc., which  
are disaccharides (having two monosaccharides).
Chapter 3 Carbohydrade Final 30.018.2018.indd   51 09/01/2025   15:22:36
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Biotechnology 52
3.	Polysaccharides	
Polysaccharides are polymers of more than ten 
monosaccharide units joined together by glycosidic linkages. 
These are classi??ed in a number of ways depending upon 
the type of repeating monosaccharide unit (homo- and 
hetero-polysaccharides); in the degree of branching, and 
in the type of glycosidic linkage between the monomeric 
units. Examples of some common polysaccharides are 
starch, glycogen, cellulose, and chitin.
Carbohydrates can be conjugated to proteins and 
lipids to form glycoconjugates. There are three types 
of glycoconjugates; glycoproteins, 	 proteoglycans	 and 
glycolipids. If the protein component is predominant in 
the association of carbohydrate and protein, it is called 
glycoprotein. If the association contains major amount of 
carbohydrate than protein, then it is called proteoglycan. 
When the carbohydrate conjugates with lipids, it is called 
glycolipid.
(B)	 Structure 	 and 	 properties	 of	 carbohydrates
(a)	Monosaccharides
Structure of some common monosaccharides are given in 
(Fig 3.1). Monosaccharide such as glucose exists both as 
Fig. 3.1: Structure of some monosaccharides
CHO
OH C H
H C HO
CH OH
2
H C HO
OH C H
D- Galactose (Hexose) D-Glucose (Hexose)
CHO
OH C H
H C HO
CH OH
2
OH C H
OH C H
CHO
OH C H
OH C H
CH OH
2
OH C H
D- Ribose (Pentose) D-Fructose (Hexose)
O C
CH OH
2
CH OH
2
OH C H
OH C H
OH C H
D- Glyceraldehyde (Triose)
CHO
OH C H
CH OH
2
CHO
OH C H
OH C H
CH OH
2
D-Erythrose (Tetroses)
OH C H
H C HO
CH OH
2
OH C H
D-Xylose (Pentose)
CHO
Chapter 3 Carbohydrade Final 30.018.2018.indd   52 09/01/2025   15:22:36
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Page 4


Biomolecules
3.1 Carbohydrates
3.2 Fatty Acids and 
Lipids
3.3 Amino Acids 
3.4 Protein Structure
3.5 Nucleic Acids
In the previous chapter you have learnt about the cell and 
its organelles. Each organelle has distinct structure and 
therefore performs different function. For example, cell 
membrane is made up of lipids and proteins, cell wall is 
made up of carbohydrates, chromosomes are predominantly  
made up of proteins and DNA, and ribosomes are made up 
of proteins and RNA. These ingredients of cellular organelles 
are also called macromolecules or biomolecules. There are 
four major types of biomolecules— carbohydrates, proteins, 
lipids, and nucleic acids. Apart from being structural entities 
of the cell, these biomolecules play important functions in 
cellular processes. In this chapter you will study the structure 
and functions of these biomolecules.
3.1 Carbohydrates Carbohydrates are one of the most abundant classes of 
biomolecules in nature and found widely distributed in all life 
forms. Chemically, they are aldehyde and ketone derivatives 
of the polyhydric alcohols. Major role of carbohydrates 
in living organisms is to function as a primary source 
of energy. These molecules also serve as energy stores, 
Chapter 3
Chapter 3 Carbohydrade Final 30.018.2018.indd   50 09/01/2025   15:22:35
Reprint 2025-26
Biomolecules 51
metabolic intermediates, and one of the major components 
of bacterial and plant cell wall. Also, these are part of DNA 
and RNA, which you will study later in this chapter. The 
cell walls of bacteria and plants are made up of polymers 
of carbohydrates. Carbohydrates also act as informational 
materials and linked to surfaces of proteins and lipids to 
act in cell–cell interaction, and in the interaction between 
cells with other elements in the cellular environment where 
they play a role.
(A)	 Classi??cation 	 of	carbohydrates	
Carbohydrates are found in various forms ranging from 
simple sugars to complex polymers of more than one unit, and 
accordingly these have been classi??ed. These are commonly 
classi??ed into three categories namely, monosaccharides, 
oligosaccharides, and polysaccharides. 
1.	Monosaccharides
Monosaccharides are simple sugars which cannot be further 
hydrolysed into simpler forms. These monosaccharides 
contain free aldehyde (-CHO) and ketone (>C=O) groups, 
with two or more hydroxyl (-OH) groups with a general 
formula of C
n
(H
2
O)
n
. Based on the number of carbon atoms 
and functional groups, monosaccharides are classi??ed as  
given in Table 3.1. 
Table 	3.1:	Classi??cation 	 of	monosaccharides
S. No.
Class 	of	monosaccharides	
based	on	number	of	 carbon	
atoms
Class 	of	monosaccharides	 based 	 on	functional	 groups
Aldoses	 Ketoses
1. Trioses (C
3
H
6
O
3
) Glyceraldehyde (an aldotriose)
Dihydroxyacetone 
(a ketotriose) 
2. Tetroses (C
4
H
8
O
4
) Erythrose Erythrulose 
3. Pentoses (C
5
H
10
O
5
) Ribose Ribulose 
4. Hexoses (C
6
H
12
O
6
) Glucose Fructose 
2.	Oligosaccharides
Conventionally, oligosaccharides are carbohydrates 
having two to ten units of monosaccharides joined 
together by glycosidic bond. Some commonly occurring 
oligosaccharides are maltose, lactose, sucrose, etc., which  
are disaccharides (having two monosaccharides).
Chapter 3 Carbohydrade Final 30.018.2018.indd   51 09/01/2025   15:22:36
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Biotechnology 52
3.	Polysaccharides	
Polysaccharides are polymers of more than ten 
monosaccharide units joined together by glycosidic linkages. 
These are classi??ed in a number of ways depending upon 
the type of repeating monosaccharide unit (homo- and 
hetero-polysaccharides); in the degree of branching, and 
in the type of glycosidic linkage between the monomeric 
units. Examples of some common polysaccharides are 
starch, glycogen, cellulose, and chitin.
Carbohydrates can be conjugated to proteins and 
lipids to form glycoconjugates. There are three types 
of glycoconjugates; glycoproteins, 	 proteoglycans	 and 
glycolipids. If the protein component is predominant in 
the association of carbohydrate and protein, it is called 
glycoprotein. If the association contains major amount of 
carbohydrate than protein, then it is called proteoglycan. 
When the carbohydrate conjugates with lipids, it is called 
glycolipid.
(B)	 Structure 	 and 	 properties	 of	 carbohydrates
(a)	Monosaccharides
Structure of some common monosaccharides are given in 
(Fig 3.1). Monosaccharide such as glucose exists both as 
Fig. 3.1: Structure of some monosaccharides
CHO
OH C H
H C HO
CH OH
2
H C HO
OH C H
D- Galactose (Hexose) D-Glucose (Hexose)
CHO
OH C H
H C HO
CH OH
2
OH C H
OH C H
CHO
OH C H
OH C H
CH OH
2
OH C H
D- Ribose (Pentose) D-Fructose (Hexose)
O C
CH OH
2
CH OH
2
OH C H
OH C H
OH C H
D- Glyceraldehyde (Triose)
CHO
OH C H
CH OH
2
CHO
OH C H
OH C H
CH OH
2
D-Erythrose (Tetroses)
OH C H
H C HO
CH OH
2
OH C H
D-Xylose (Pentose)
CHO
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Biomolecules 53
straight chain structure and cyclic structure (Fig. 3.2). 
Cyclic structures are the result of hemiacetal formation by 
intramolecular reaction between carbonyl group and a 
hydroxyl group. 
All monosaccharides except dihydroxy acetone contain 
one or more asymmetric (chiral) carbon (carbon atoms 
bound by four different groups), thus, are optically active 
isomers (enantiomers). A molecule with n chiral centres 
can have 2
n
 stereoisomers. Thus, glyceraldehyde with one 
chiral centre has 2
1
=2 and glucose with four chiral centres 
has 2
4
=16 stereoisomers.
The orientation of the –OH group that is most 
distant from the carbonyl carbon determines 
whether the sugar belongs to D or L sugars. When 
this –OH group is on the right side of the carbon atom 
bearing it then the sugar is D-isomer, and when it 
is on the left, the sugar is L isomers (Fig. 3.3). Most 
of the sugars present in the biological system are 
D sugars.
Isomeric forms of monosaccharide that differ 
only in their con??guration about the hemiacetal 
(formed due to reaction between alcoholic and 
aldehyde groups of a monosaccharide) or hemiketal 
(formed due to reaction between alcoholic and keto 
groups of a monosaccharide) carbon atom are called 
anomers. The carbonyl carbon atom is called the anomeric 
carbon. In the ?? - anomer, the -OH group of the anomeric 
carbon is on the opposite of the sugar ring from CH
2
OH 
group at the chiral centre that designates the D and L 
con??guration (C-5 in case of glucose). The other anomer is 
Fig. 3.2: Structure of glucose: (a) straight chain and (b) cyclic form
CHO
HO H
CH OH
2
C
C
C
C
OH
OH
OH
H
H
H
2
3
4
5
6
CH
H
OH
2
H H
H
H
OH
OH
OH
O
OH
1
2
3
4
5
6
(a) (b)
1
Fig. 3.3: L and D forms of glucose
CHO
HO H
CH OH
2
C
C
C
C
OH
OH
H
H
H
HO
L-Glucose D-Glucose
CHO
HO H
CH OH
2
C
C
C
C
OH
OH
H
H
H
OH
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Page 5


Biomolecules
3.1 Carbohydrates
3.2 Fatty Acids and 
Lipids
3.3 Amino Acids 
3.4 Protein Structure
3.5 Nucleic Acids
In the previous chapter you have learnt about the cell and 
its organelles. Each organelle has distinct structure and 
therefore performs different function. For example, cell 
membrane is made up of lipids and proteins, cell wall is 
made up of carbohydrates, chromosomes are predominantly  
made up of proteins and DNA, and ribosomes are made up 
of proteins and RNA. These ingredients of cellular organelles 
are also called macromolecules or biomolecules. There are 
four major types of biomolecules— carbohydrates, proteins, 
lipids, and nucleic acids. Apart from being structural entities 
of the cell, these biomolecules play important functions in 
cellular processes. In this chapter you will study the structure 
and functions of these biomolecules.
3.1 Carbohydrates Carbohydrates are one of the most abundant classes of 
biomolecules in nature and found widely distributed in all life 
forms. Chemically, they are aldehyde and ketone derivatives 
of the polyhydric alcohols. Major role of carbohydrates 
in living organisms is to function as a primary source 
of energy. These molecules also serve as energy stores, 
Chapter 3
Chapter 3 Carbohydrade Final 30.018.2018.indd   50 09/01/2025   15:22:35
Reprint 2025-26
Biomolecules 51
metabolic intermediates, and one of the major components 
of bacterial and plant cell wall. Also, these are part of DNA 
and RNA, which you will study later in this chapter. The 
cell walls of bacteria and plants are made up of polymers 
of carbohydrates. Carbohydrates also act as informational 
materials and linked to surfaces of proteins and lipids to 
act in cell–cell interaction, and in the interaction between 
cells with other elements in the cellular environment where 
they play a role.
(A)	 Classi??cation 	 of	carbohydrates	
Carbohydrates are found in various forms ranging from 
simple sugars to complex polymers of more than one unit, and 
accordingly these have been classi??ed. These are commonly 
classi??ed into three categories namely, monosaccharides, 
oligosaccharides, and polysaccharides. 
1.	Monosaccharides
Monosaccharides are simple sugars which cannot be further 
hydrolysed into simpler forms. These monosaccharides 
contain free aldehyde (-CHO) and ketone (>C=O) groups, 
with two or more hydroxyl (-OH) groups with a general 
formula of C
n
(H
2
O)
n
. Based on the number of carbon atoms 
and functional groups, monosaccharides are classi??ed as  
given in Table 3.1. 
Table 	3.1:	Classi??cation 	 of	monosaccharides
S. No.
Class 	of	monosaccharides	
based	on	number	of	 carbon	
atoms
Class 	of	monosaccharides	 based 	 on	functional	 groups
Aldoses	 Ketoses
1. Trioses (C
3
H
6
O
3
) Glyceraldehyde (an aldotriose)
Dihydroxyacetone 
(a ketotriose) 
2. Tetroses (C
4
H
8
O
4
) Erythrose Erythrulose 
3. Pentoses (C
5
H
10
O
5
) Ribose Ribulose 
4. Hexoses (C
6
H
12
O
6
) Glucose Fructose 
2.	Oligosaccharides
Conventionally, oligosaccharides are carbohydrates 
having two to ten units of monosaccharides joined 
together by glycosidic bond. Some commonly occurring 
oligosaccharides are maltose, lactose, sucrose, etc., which  
are disaccharides (having two monosaccharides).
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Biotechnology 52
3.	Polysaccharides	
Polysaccharides are polymers of more than ten 
monosaccharide units joined together by glycosidic linkages. 
These are classi??ed in a number of ways depending upon 
the type of repeating monosaccharide unit (homo- and 
hetero-polysaccharides); in the degree of branching, and 
in the type of glycosidic linkage between the monomeric 
units. Examples of some common polysaccharides are 
starch, glycogen, cellulose, and chitin.
Carbohydrates can be conjugated to proteins and 
lipids to form glycoconjugates. There are three types 
of glycoconjugates; glycoproteins, 	 proteoglycans	 and 
glycolipids. If the protein component is predominant in 
the association of carbohydrate and protein, it is called 
glycoprotein. If the association contains major amount of 
carbohydrate than protein, then it is called proteoglycan. 
When the carbohydrate conjugates with lipids, it is called 
glycolipid.
(B)	 Structure 	 and 	 properties	 of	 carbohydrates
(a)	Monosaccharides
Structure of some common monosaccharides are given in 
(Fig 3.1). Monosaccharide such as glucose exists both as 
Fig. 3.1: Structure of some monosaccharides
CHO
OH C H
H C HO
CH OH
2
H C HO
OH C H
D- Galactose (Hexose) D-Glucose (Hexose)
CHO
OH C H
H C HO
CH OH
2
OH C H
OH C H
CHO
OH C H
OH C H
CH OH
2
OH C H
D- Ribose (Pentose) D-Fructose (Hexose)
O C
CH OH
2
CH OH
2
OH C H
OH C H
OH C H
D- Glyceraldehyde (Triose)
CHO
OH C H
CH OH
2
CHO
OH C H
OH C H
CH OH
2
D-Erythrose (Tetroses)
OH C H
H C HO
CH OH
2
OH C H
D-Xylose (Pentose)
CHO
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Biomolecules 53
straight chain structure and cyclic structure (Fig. 3.2). 
Cyclic structures are the result of hemiacetal formation by 
intramolecular reaction between carbonyl group and a 
hydroxyl group. 
All monosaccharides except dihydroxy acetone contain 
one or more asymmetric (chiral) carbon (carbon atoms 
bound by four different groups), thus, are optically active 
isomers (enantiomers). A molecule with n chiral centres 
can have 2
n
 stereoisomers. Thus, glyceraldehyde with one 
chiral centre has 2
1
=2 and glucose with four chiral centres 
has 2
4
=16 stereoisomers.
The orientation of the –OH group that is most 
distant from the carbonyl carbon determines 
whether the sugar belongs to D or L sugars. When 
this –OH group is on the right side of the carbon atom 
bearing it then the sugar is D-isomer, and when it 
is on the left, the sugar is L isomers (Fig. 3.3). Most 
of the sugars present in the biological system are 
D sugars.
Isomeric forms of monosaccharide that differ 
only in their con??guration about the hemiacetal 
(formed due to reaction between alcoholic and 
aldehyde groups of a monosaccharide) or hemiketal 
(formed due to reaction between alcoholic and keto 
groups of a monosaccharide) carbon atom are called 
anomers. The carbonyl carbon atom is called the anomeric 
carbon. In the ?? - anomer, the -OH group of the anomeric 
carbon is on the opposite of the sugar ring from CH
2
OH 
group at the chiral centre that designates the D and L 
con??guration (C-5 in case of glucose). The other anomer is 
Fig. 3.2: Structure of glucose: (a) straight chain and (b) cyclic form
CHO
HO H
CH OH
2
C
C
C
C
OH
OH
OH
H
H
H
2
3
4
5
6
CH
H
OH
2
H H
H
H
OH
OH
OH
O
OH
1
2
3
4
5
6
(a) (b)
1
Fig. 3.3: L and D forms of glucose
CHO
HO H
CH OH
2
C
C
C
C
OH
OH
H
H
H
HO
L-Glucose D-Glucose
CHO
HO H
CH OH
2
C
C
C
C
OH
OH
H
H
H
OH
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Biotechnology 54
known as ß-anomer. The interconversion of a and ß anomers 
in aqueous solution is called mutarotation, in which one ring 
form opens brie??y into the linear form, then closes again to 
produce ß anomers (Fig 3.4). 
Isomers having different con??guration of –OH only at 
one carbon atom are known as epimers. The most 
important epimers of glucose are mannose (epimers at 
C-2) and galactose (epimers at C-4) as shown in Fig. 3.5.
(b) 	Oligosaccharides 	
Oligosaccharides consist of two to ten monosaccharides 
joined by glycosidic linkage. For example, disaccharide 
maltose contains two D-glucose residues joined by a  
glycosidic linkage, which is a covalent bond formed by 
joining of –OH group of one monosaccharide with the 
anomeric carbon of the other sugar unit. Lactose is made 
up of D-galactose and D-glucose residues (Fig. 3.6 and 3.7).
Disaccharides can be hydrolysed to yield their 
constituent monosaccharides by boiling with dilute acid. 
For example, hydrolysis of sucrose yields a mixture 
of glucose and fructose. Other oligosaccharides are 
Fig. 3.4: Two cyclic forms of glucose 
-D-glucopyranose -D-glucopyranose
CH OH
2
H
H
H
H
OH
OH
OH
O
OH
H
anomeric
carbon
CH OH
2
H
H
OH
H
H
OH
OH
OH
O
H
anomeric
carbon
CH OH
2
H
H
H
H
OH
OH
OH
O
OH
H
CH OH
2
H
H
H
H
OH
OH
OH
O
OH
H
CH OH
2
H
H
H
OH
OH
H
OH
O
OH
H
-D-galactose  -D-glucose  -D-mannose 
Fig. 3.5: The epimers of glucose
Chapter 3 Carbohydrade Final 30.018.2018.indd   54 09/01/2025   15:22:36
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