© Nav R.

Phulara 1
Molecules of Life

CHAPTER: MOLECULES OF LIFE

CARBOHYDRATES
Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones or any organic
compounds that produce these compounds on hydrolysis. Carbohydrates containing
‘aldehyde’ group are called aldose while those containing ‘keto’ group are called ketose.
Glucose contains aldehyde group so it is called aldose whereas fructose contains keto group
so it is called ketose.

Classification of Carbohydrates
1) On the basis of hydrolysis behaviour-
Carbohydrates can be classified into three categories on the basis of product obtained by
hydrolysis.
a) Monosaccharides: Those carbohydrates which cannot be further hydrolysed into
simpler form are monosaccharides. For eg. Glucose (C6H12O6), fructose (C6H12O6),
arabinose (C5H10O5) etc.
b) Oligosaccharides: Those carbohydrates which on hydrolysis give rise to two to ten
monosaccharides are called oligosaccharides. A carbohydrate that can be
hydrolyzed to two monosaccharide molecules is called a diasaccharides. For eg.
Sucrose (C12H22O11), maltose (C12H22O11) etc.

c) Polysaccharides: Those carbohydrates which on hydrolysis give more than ten

monosaccharides are called polysaccharides. For eg. cellulose, starch etc.
2) On the basis of test and solubility-
a) Sugar
They have sweet taste and dissolve in water. Most of monosaccharides and
diasaccharides are sugar. Fructose is sweetest among all carbohydrates.
b) Non-sugar
They are generally tasteless and insoluble in water. All polysaccharides are non-
sugar. For eg. cellulose, starch etc.
3) On the basis of reducing behaviour-
a) Reducing sugar
Carbohydrates which can reduce Tollen’s reagent and Fehling’s solution are
called reducing sugar. All monosaccharides and most diasaccharides are reducing
sugar.
b) Non-reducing sugar
They cannot reduce Tollen’s reagent and Fehling’s solution. Polysaccharides are
non-reducing sugar.
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Molecules of Life

Structure of Glucose and Fructose

1) Glucose
i) Open chain structure

Fig. Open chain structure of glucose(aldohexose)

Open chain structure of glucose cannot explain some of the properties of glucose, such as
a) It doesn’t reacts with NaHSO3 though it contains aldehyde group.
b) D(+) glucose exists in two crystalline forms. One with melting point 146 °C (α-D(+)-
Glucose) and another with melting point 150 °C (β-D(+)-Glucose).

To account these facts, cyclic structure was proposed.

ii) Cyclic structure

Haworth projection formula

Fischer projection formula

2) Fructose

Fig. Open chain structure of fructose Fig. Cyclic structure of fructose

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Functions of Carbohydrates
1) They are major source of energy in living beings.
2) They help to support plant structure. (Cellulose is chief structural component in plant
cells)
3) Wood, cotton, etc. are polymeric carbohydrates which are used for different
purpose.

PROTEINS
Proteins are biomolecules which are formed by polymerization of amino acids. They are
structural components of skin, muscle, hair, etc., and as enzymes and antibodies.

Amino acids: Amino acids are organic compounds in which amino group (-NH2) is attached
to the α-carbon of carboxylic group.

General formula:

Where ‘R’ is the side chain having

different substituents.

Examples-

Classification of amino acids

1) Essential amino acids
Those amino acids which are not synthesized in our body and fulfilled through the
food are called essential amino acids. Examples- valine, lysine etc.
2) Non-essential amino acids
Those amino acids which are synthesized in our body and not essential to supply
from food are called non-essential amino acids. Examples- glycine, cysteine etc.
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Molecules of Life

Zwitter ion (Dipolar ion)

Amino acid contains both acidic group (-COOH) as well as basis group (-NH2). In aqueous
solution –COOH loses a proton and –NH2 accepts a proton forming dipolar ion called Zwitter
ion.

Therefore amino acid can reacts with acids as well as with bases (amphoteric character). It
exists as a positive ion in acidic medium and negative ion in basic medium as follows:

When solution of an amino acid is placed in an electric field, in acidic medium it migrates
toward the cathode and in basic medium it migrates towards the anode of electric field. The
pH of solution in which particular amino acid doesn’t migrate under the influence of an
electric field is called isoelectric point. For example, the isoelectric point of glycine is 6.1.

Peptide Linkage
Acidic carboxyl group (-COOH) of an amino acid reacts with the basic amino group (-NH2) of
other amino acid to give –NCHO- group, which is called peptide linkage or peptide bond. The
compound formed by condensation reaction between two amino acids is called dipeptide.
Depending upon the number of amino acid residue per molecule, the peptide is called
dipeptide, tripeptide and so on.
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Molecules of Life

Classification of Proteins
Proteins can be classified into two categories on the basis of arrangement of polypeptide on
it.
1) Fibrous protein
Molecules of fibrous proteins are long thread like and tend to lie side by side to form
fibers. The intermolecular force of attraction between different polypeptide chains is
strong. So they are insoluble in water.
eg. -Keratin in skin
-Myosin in muscle
2) Globular protein
Molecules of globular proteins are folded into compact unit of spherical structure.
There is a presence of weak hydrogen bonding between polypeptide chains. So they
are soluble in water.
eg. -Albumin in egg
-Casein in milk

Denaturation of Protein
The process which brings out changes in physical and biological properties of protein
without change in chemical properties due to external factors such as heat, chemicals etc. is
called denaturation of protein. In this process, secondary and tertiary structures of protein
change but no change in primary structure.

Examples, -Coagulation of egg white by heating

- Preparation of cheese from milk by heating with an acid

Functions of Proteins
1) Proteins are major structural materials of animal tissues.
2) Some proteins act as food source for embryo and newly developed organism.
3) Hormones act as chemical regulator.
4) Enzymes (proteins) catalyze large number of reactions.

Enzymes
Enzymes are complex organic compounds which are produced by living plants and animals.
They are protein molecules which act as catalyst to speed up organic reactions in living cells.
Each enzyme is produced in a particular living cell to catalyze a reaction occurring in the cell.
Some common examples of the biochemical reactions catalyzed by enzymes are.

1) Invertase: it is used to breakdown sugar.

2) Zymase: it is used to breakdown glucose and fructose.
3) Urease: it is used to breakdown urea.
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Characteristics of enzyme
1) All enzymes are proteins of high molecular weight.
2) Extremely small amounts of enzymes are able to bring about measurable changes.
3) It accelerates biochemical reactions.
4) Enzyme does not alter amount or nature of the product.
5) Enzymes do not affect the amount of energy released or absorbed during the
reaction.
6) Enzymes retain their identity at the end of the reaction, as in the beginning.
7) Enzymes are highly specific.

NUCLEIC ACIDS
Nucleic acids are non-protein, nitrogenous substances which are polymers of
nucleotides; hence they are also called polynucleotides. Nucleic acids are genetic
materials of the cells which are responsible for transmission of heredity character and
also carry out biosynthesis of proteins.

Components of Nucleic Acids

1) Pentose sugar
The pentose sugars ribose and deoxyribose are part of the nucleotides that make up
the crucial nucleic acids like DNA and RNA.

2) Nitrogeneous bases
A nitrogenous base, or nitrogen-containing base, is an organic molecule with a
nitrogen atom that has the chemical properties of a base. The main biological
function of a nitrogenous base is to bond nucleic acids together. There are two types
of nitrogenous bases, pyrimidine and purine.

Nucleic acid Purine Pyrimidine

DNA Adenine, Guanine Cytosine, Thymine
RNA Adenine, Guanine Cytosine, Uracil

3) Phosphate group
Nucleotides can have one, two, or three phosphate groups.
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Nucleotides are the building blocks of

nucleic acids; they are composed of
three sub unit molecules: a
nitrogenous base (also known as
nucleobase), a five-carbon sugar
(ribose or deoxyribose), and at least
one phosphate group. A nucleoside is
a nitrogenous base and a 5-carbon
sugar.

Double Helix Structure of DNA

Watson and Crick reported the double helix structure of DNA. According to this model, DNA
contains two right handed helical polynucleotide chains that run in opposite directions and
coiled around common axis.

Two chains are held together by hydrogen bonding between bases. The purine base of one
chain is always paired with pyrimidine base of other. Cytosine is always bonded to guanine
by three H-bonds and thymine is always bonded to adenine by two H-bonds.

Double helix is 20 Å in diameter and each

complete turn is 34 Å along the axis.

Fig. Double helix structure of DNA

Differences between DNA and RNA

DNA RNA
It is a double stranded and helical It is a single stranded and non-helical
The pentose sugar is deoxyribose The pentose sugar is ribose
Nitrogen bases are Adenine(A), Guanine(G), Nitrogen bases are Adenine(A), Guanine(G),
Thymine(T) and Cytosine(C) Uracil(U) and Cytosine(C)
DNA can undergo replication It doesn’t replicate
It transmits heredity character It helps in protein synthesis
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Molecules of Life

Biological Functions of Nucleic Acids

1) The nucleic acid ‘DNA’ carries genetic information from parent to offspring.
2) DNA controls all biological activities of cell.
3) The nucleic acid ‘RNA’ plays major role in protein synthesis.
4) DNA synthesizes RNA by the process of transcription.

LIPIDS
Water insoluble compounds like fats and oils and their derivatives occurring in the living
organisms are called lipids. Lipids are the group of biomolecules which are greasy, insoluble
in water and soluble in organic solvents like ether, chloroform etc.

On the basis of molecular structure, lipids are divided into three types.
1) Simple lipids (Triglycerides)
2) Phospholipids
3) Waxes

1) Simple lipids

Those lipids which on hydrolysis give one or more carboxylic acid and an alcohol are called
simple lipids. Fats and oils (Triglycerides) are simple lipids. Chemically, a fat or an oil
molecule is an ester of glycerol with fatty acids, i.e. Triglyceride.

Fat or oil

Fatty Acids: They are unbranched long chain monocarboxylic acid containing high number
of carbon atoms. They are involved in the formation of fat and oil. That’s why they are called
fatty acids. There are two types of fatty acid-

1) Saturated fatty acids; eg. palmitic acid (CH3-(CH2)14-COOH)

2) Unsaturated fatty acids; eg. oleic acid (CH3-(CH2)7-CH=CH-(CH2)7COOH)

Hydrogenation of Oil
When hydrogen gas is passed through oil in presence of Ni or Pd catalyst, it gets converted
into fat. This process is called hydrogenation. This is due to the conversion of unsaturated
form to saturated form.

Oil + H2 Ni, Fat

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Hydrolysis of Fats (Saponification)

Alkaline hydrolysis of fats or oils with sodium or potassium hydroxide solution gives glycerol
and sodium or potassium salt of fatty acid. These salts are called soap and the process is
called saponification. Hydrolysis process can also be carried out by dilute acids.

3) Phospholipids
Phospholipids are triglycerides in which two hydroxyl groups of glycerol are
esterified with fatty acids and third hydroxyl group is esterified with derivative of
phosphoric acid.

4) Waxes
Waxes are esters of higher fatty acids with long chain monohydric alcohols.
Example-
C15H31-CO-OC30H61 Bee wax(Myricyl Palmitate)

Functions of Lipids
1) Lipids are energy source for living beings.
2) Fat acts as heat insulator and shock absorber for living beings.
3) Phospholipid acts as major component of cell membrane.
4) Wax can be used to make different types of cosmetics and candles.

The End!