UNIT 10.

BIOMOLECULES
The molecules present in living system like carbohydrates, proteins, nucleic acids, lipids,
vitamins etc. which are essential for the growth and maintenance of our body are called
Biomolecules.

Carbohydrates

These are the hydrates of carbon and most of them have a general formula C x(H2O)y.
They can be defined as polyhydroxy aldehydes or ketones or the compounds which
produce such units on hydrolysis.

Based on their behaviour on hydrolysis:

Based on this, carbohydrates are classified into three types:

1) Monosaccharides: These are carbohydrates which cannot be hydrolysed into simpler

units of polyhydroxyaldehydes or ketones. E.g. glucose, fructose, ribose, galactose etc.

2) Oligosaccharides: These are carbohydrates which give two to ten monosaccharide

units on hydrolysis. They are further classified as disaccharides, trisaccharides,
tetrasaccharides etc. e.g. Sucrose, maltose, lactose etc. Sucrose on hydrolysis gives one
molecule each of glucose and fructose, maltose gives two molecules of glucose while
lactose gives one molecule each of glucose and galactose.

3) Polysaccharides: These are carbohydrates which give a large number of

monosaccharide units on hydrolysis. E.g. starch, cellulose, glycogen etc.

II) Based on their reducing character:

Based on this, carbohydrates are of two types – reducing sugar and non-reducing sugar.

Carbohydrates which contain free aldehydic or ketonic groups are called reducing
sugars, while those which do not contain free aldehydic or ketonic group are called non-
reducing sugars.

All monosaccharides are reducing sugars.

Disaccharides like maltose and lactose are reducing while sucrose is non-reducing

. III) Based on the functional group and no. of carbon atoms:

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A monosaccharide containing an aldehyde group is known as aldose, while a
monosaccharide containing a ketonic group is known as ketose

Monosaccharides containing 3 carbon atoms are called triose, 4 carbon atoms are called
tetrose etc.

Q1. Give the methods used for the preparation of glucose?

1. From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or H2SO4 in alcoholic
solution, glucose and fructose are obtained in equal amounts
C12H22O11 + H2O ⎯⎯⎯→ C6H12O6 + C6H12O6
2. From starch: Commercially glucose is obtained by hydrolysis of starch by boiling it
with dilute H2SO4 at 393 K under pressure.
(C6H10O5)n + nH2O ⎯⎯⎯⎯⎯⎯⎯→ nC6H12O6

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Q2.

Write a note on the structure of glucose?

Glucose is an aldohexose and is also known as dextrose.
Its molecular formula is C6H12O6.
Experiments suggest that
i) all the six carbon atoms are linked in a straight chain
ii) there is a free aldehydic group and 5 hydroxyl groups and
iii) one of the alcoholic group is primary. Based on the above informations,
Fischer proposed an open chain structure for glucose as follows:

But this open chain structure cannot explain the following observations:
1. Glucose does not react with 2,4-Dinitrophenyl hydrazine, Schiff’s reagent
and with NaHSO3.
2. The pentaacetate of glucose does not react with hydroxylamine
indicating the absence of free —CHO group.
3. The existence of two different crystalline forms of glucose (α and β form)

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 In order to explain the above, it was proposed that one of the –OH groups
may add to the –CHO group and form a cyclic hemi-acetal structure. The –OH
at C5 is involved in ring formation. (1,5 – oxide ring).

Thus the two cyclic forms exist in equilibrium with the open chain structure.
The two cyclic hemi-acetal forms of glucose differ only in the configuration at
first carbon (anomeric carbon). So they are called anomers. They are stereo
isomers which differ only in the configuration at the first carbon.
The Pyranose structure of Glucose:
Thus the two cyclic forms exist in equilibrium with the open chain structure.
The two cyclic hemi-acetal forms of glucose differ only in the configuration at
first carbon (anomeric carbon). So they are called anomers. They are stereo
isomers which differ only in the configuration at the first carbon.
The six membered cyclic structure of glucose is called Pyranose structure. The
anomeric forms of glucose can be represented as follows

3. Fructose
 Fructose is an important ketohexose also with molecular formula C 6H12O6.

Structure

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 Importance of Carbohydrates
a. As storage molecules as starch in plants and glycogen in animals.
b. Cell wall of bacteria and plants is made up of cellulose.
c. Used as raw materials for industries like textiles, paper, lacquers and breweries.
d. Carbohydrate in the form of wood is used for making furniture etc.

Q1.Give the methods used for the preparation of glucose?

Q2. Write a note on the structure of glucose?

Q3.Hydrolysis of cane sugar is also called inversion of cane sugar. Why?

Q4. What is mean by glycosidic linkage?

During the formation of a disaccharide or polysaccharide, the

monosaccharides are joined together through oxide linkage by losing water
molecules. Such a linkage (C-O-C) between monosaccharide units through
oxygen atom is called glycosidic linkage.
Give the monosaccharides, reducing character and glycosidic linkage of the
following?

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 PROTEINS
Amino acids

These are compounds containing amino (–NH2) and carboxyl (–COOH) groups.
Depending upon the relative position of amino group with respect to carboxyl group, the
amino acids can be classified as α, β, γ, δ and so on. The simplest amino acid is glycine
(H2N-CH2-COOH). Except glycine, all other naturally occurring αamino acids are optically
active, since the α-carbon atom is asymmetric. Amino acids are generally represented by
a three letter symbol. (e.g. ‘Gly’ for glycine, ‘Ala’ for alanine etc).

Amino acids are classified as acidic, basic or neutral depending upon the relative
number of amino and carboxyl groups in their molecule. Amino acids having equal
number of amino and carboxyl groups is neutral; those containing more number of
amino groups are basic and those containing more number of carboxyl groups are acidic.
For e.g. glycine, alanine, valine etc. are neutral, arginine, lysine etc. are basic and
glutamic acid, aspartic acid etc. are acidic.

The amino acids which can be synthesised in the body are known as non-essential
amino acids. While which cannot be synthesised in the body and must be obtained
through diet, are known as essential amino acids.

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In aqueous solution, the carboxyl group can lose a proton and amino group can accept
a proton, giving rise to a dipolar ion known as zwitter ion. This is neutral but contains
both positive and negative charges. In zwitter ionic form, amino acids show
amphoteric behaviour as they react both with acids and bases.

What are peptides and polypeptides?

A peptide is formed by the combination of α-amino acid molecules. Chemically peptide

linkage is an amide formed between –COOH group and -NH2 group. When two
molecules of amino acids combine, the amino group of one molecule reacts with –COOH
group of another molecule by losing one water molecule to form a CO-NH linkage,
commonly called peptide linkage.

The peptide formed between two amino acid molecules is called a dipeptide

H2N-CH2-COOH + H2N-CH2-COOH → H2N-CH2-CO-NH-CH2-COOH

Glycylalanine (Gly-Ala)

The peptide formed by the combination of 3 amino acid molecules is called a tripeptide.
When the number of amino acid molecules is more than 10, the product is called a
polypeptide.

A polypeptide with more than 100 amino acid residues and molecular mass greater than
10,000u is called a protein.

Explain the different types of proteins?

Based on the shape of molecules, proteins are classified into 2 types:

a) Fibrous proteins: They have fibre – like structure. Here the linear polypeptide chains
are held together by H-bond and disulphide bond. They are generally insoluble in water.

E.g. Keratin (present in hair, wool, silk etc.) and myosin (present in muscles).

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b) Globular proteins: Here the chains of polypeptides coil around to give a spherical
shape. These are usually soluble in water.

Insulin and albumins are the common examples of globular proteins.

structure of proteins

There are four types of structure for a protein.

They are primary, secondary, tertiary and quaternary structure.

1. Primary structure: It gives the sequence of amino acid molecules in a polypeptide

chain of protein. Any change in the primary structure creates a different protein.

2. Secondary structure: The secondary structure of protein refers to the shape in which
a long polypeptide chain can exist. There are two different types of secondary structures
- α-helix and βpleated sheet structure. These structures arise due to the regular folding
of the backbone of the polypeptide chain due to hydrogen bonding between >CO and –
NH– groups of the peptide bond.

3. Tertiary structure: The tertiary structure represents overall folding of the polypeptide
chains. i.e., further folding of the secondary structure. It gives rise to two major
molecular shapes - fibrous and globular.

4. Quaternary structure: Some of the proteins contain two or more polypeptide chains
called sub-units. The spatial arrangement of these sub-units is known as quaternary
structure.

What is meant by denaturation of protein?

When a protein is subjected to physical change (like change in temperature) or chemical

change (like change in pH), it loses the biological activities. This process is called
denaturation of protein. During denaturation, secondary and tertiary structures are
destroyed, while primary structure remains unaffected.

e.g. coagulation egg white on boiling, curding of milk etc.

Nucleic Acids
Nucleic acids are chain like polymers of thousands of nucleotide units, hence they are
also called polynucleotides.

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Nucleoside
A unit formed by the attachment of a base to 1′ position of sugar is known as
nucleoside.

Nucleotide
A nucleotide consists of three subunits: a nitrogen containing heterocyclic aromatic
compound which is called base, a pentose sugar and a molecule of phosphoric acid.

Structure of Nucleic Acid

Thus a nucleic acid chain is represented as shown below.

Types of Nucleic Acids

The nucleic acids are of two types:

(i) Deoxyribonucleic acid (DNA)

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(ii) Ribonucleic acid (RNA)

• DNA is mainly localized in the nucleus, within the chromosome. While small amount
is present in cytoplasm. RNA is also present in the nucleus as well as cytoplasm.

• DNA is mainly used in protein synthesis involving RNA and also a major source of
genetic information.

• DNA contains doxyribose while RNA contains ribose.

Structure of DNA
• DNA has a double helix structure in which the two strands are antiparellel and are
held together by hydrogen bonds.

• In DNA molecule, adenine (A) pairs up with thymine (T) via two hydrogen bonds and
guanine (G) pairs up with cytosine (C) via three hydrogen bonds. Therefore CG base pair
has more stability than AT base pair.

Structure of RNA
It is usually a single strand of ribonucleotides and take up right handed helical
conformation.

Structural difference between DNA and RNA

There are mainly two structural differences Between DNA and RNA

• Sugar part: DNA contains deoxyribose while RNA contain riboses.

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• Base part: In DNA, four bases have been found. They are adenine (A), guanine (G),
cytosine (C) and thymine (T). The first three of these bases are found in RNA but
thymine (T) is replaced by uracil (U) in place of Thymine.

Importance of DNA
• DNA is the store house of the hereditary information of the organism.

• DNA is involed in protein synthesis.

QUESTION AND ANSWERS

1.Write the main structural difference between DNA and RNA. Of the two bases, thymine
and uracil, which one is present in DNA?
Answer:

2.The two strands of DNA are not identical but are complementary.” Explain.
Answer:
DNA consists of two strands of polynucleotides coiled around each other in the form of a
double helix. The nucleotides making up each strand of DNA are connected by
phosphodiester bonds. This forms the backbone of each DNA strand from which the
bases extend. The bases of one strand of DNA are paired with bases on the other strand
by means of hydrogen bonding. This hydrogen bonding is very specific as the structures
of bases permit only one mode of pairing. Adenine pairs only thymine through two
hydrogen bonds and guanine pairs with cytosine through three hydrogen bonds. The two
strands of DNA are said to be complementary to each other in sense that the sequence
of bases in one strand automatically determines that of the other. These strands are not
identical but complementary.

3.State what the following are and how they differ from each other:
(i) a nucleotide and
(ii) a nucleoside.
Answer:
(i) Nucleotides: They are the monomers of nucleic acids. They are made up of a
heterocyclic base containing nitrogen, a five carbon sugar and a phosphate group, e.g.
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adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine
triphosphate (ATP). They contain P—O — P bonds which are high energy phosphate
bonds and due to these bonds, nucleotides are energy carriers.
or
Sugar + Phosphate + Heterocyclic base.
(ii) Nucleosides: A base joined to a sugar molecule is called nucleoside, e.g. adenosine
contains adenine and ribose, guanosine contains ribose and guanine, cytidine contains
ribose and cytosine.
or
Sugar + Heterocyclic base.

4.Name the bases present in RNA. Which one of these is not present in DNA?
Answer:
Uracil, Cytocine, Guanine and Adenine are present in RNA.
Uracil is not present in DNA.

5.What are three types of RNA molecules which perform different functions?
Answer:
(i) r-RNA (ribosomal RNA)
(ii) m-RNA (messenger RNA)
(iii)t-RNA (transfer RNA)

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