Bio molecules

DISACCHARIDES

• The two monosaccharides are joined together by an oxide linkage formed by the loss of a
water molecule.
• Such a linkage between two monosaccharide units through oxygen atom is called glycosidic
linkage.
• In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups
are bonded, these are non-reducing sugars, e.g., sucrose.

• On the other hand, sugars in which these functional groups are free, are called reducing

sugars, for example, maltose and lactose.

 SUCROSE

• One of the common disaccharides is sucrose which on hydrolysis gives equimolar mixture of D-(+)-
glucose and D-(-) fructose.

• These two monosaccharides are held together by a glycosidic linkage between C1 of α-D-glucose and
C2 of β-D-fructose.
• The reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a
non reducing sugar.
• Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory
fructose.
• Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose (+ 52.5°),
the mixture is laevorotatory.
• Hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo
(–) and the product is named as invert sugar.
 MALTOSE
• Maltose is composed of two α-D-glucose units in which C1 of one glucose (I) is linked to C4 of

another glucose unit (II).

• The free aldehyde group can be produced at C1 of second glucose in solution and it shows reducing

properties so it is a reducing sugar.

 LACTOSE
• It is more commonly known as milk sugar since this disaccharide is found in milk. It is composed of β-

D-galactose and β-D-glucose.

• The linkage is between C1 of galactose and C4 of glucose. Free aldehyde group may be produced at C-

1 of glucose unit, hence it is also a reducing sugar.

 POLYSACCHARIDES
• Polysaccharides contain a large number of monosaccharide units joined together by glycosidic

linkages.

• They mainly act as the food storage or structural materials.

STARCH

• Starch is the main storage polysaccharide of plants. It is the most important dietary source for human

beings. High content of starch is found in cereals, roots, tubers and some vegetables.

• It is a polymer of α-glucose and consists of two components— Amylose and Amylopectin.

• Amylose is water soluble component which constitutes about 15-20% of starch. Chemically amylose is

a long unbranched chain with 200-1000 α-D-(+)-glucose units held by C1-C4 glycosidic linkage.
• Amylopectin is insoluble in water and constitutes
about 80-85% of starch.
• It is a branched chain polymer of α- D-glucose
units in which chain is formed by C1-C4
glycosidic linkage whereas branching occurs by
C1-C6 glycosidic linkage.
 CELLULOSE

• Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom.
It is a predominant constituent of cell wall of plant cells.
• Cellulose is a straight chain polysaccharide composed only of β-D-glucose units which are joined by
glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.
 Glycogen
• The carbohydrates are stored in animal body as glycogen.

• It is also known as animal starch because its structure is similar to amylopectin and is rather more

highly branched.

• It is present in liver, muscles and brain. When the body needs glucose, enzymes break the glycogen

down to glucose. Glycogen is also found in yeast and fungi.

Importance of Carbohydrates
• Carbohydrates are essential for life in both plants and animals.
• These are major portion of our food. Honey is instant source of energy . Glucose is used as a food for
patients and children.
• Carbohydrates are used as storage molecules as starch in plants and glycogen in animals.
 Enzymes
• Enzymes are biocatalysts.

• Almost all the enzymes are globular proteins. Enzymes are very specific for a particular reaction and

for a particular substrate.

Mechanism of Enzyme Action

• Enzymes are needed only in small quantities for the progress of a reaction.

• Similar to the action of chemical catalysts, enzymes are said to reduce the magnitude of activation

energy.

• For example, activation energy for acid hydrolysis of sucrose is 6.22 kJ mol–1, while the activation

energy is only 2.15 kJ mol–1 when hydrolysed by the enzyme, sucrase

 VITAMINS
• The organic compounds required in the diet in small amounts to perform specific biological
functions for normal maintenance of optimum growth and health of the organism.
• Vitamins are designated by alphabets A, B, C, D, etc. Some of them are further named as sub-groups
e.g. B1, B2 , B6 , B12, etc.
CLASSIFICATION OF VITAMINS
Vitamins are classified into two groups depending upon their solubility in water or fat.
(i) Fat soluble vitamins: Vitamins which are soluble in fat and oils but insoluble in water are kept in
this group. These are vitamins A, D, E and K. They are stored in liver and adipose (fat storing) tissues.
ii) Water soluble vitamins: B group vitamins and vitamin C are soluble in water so they are grouped
together. Water soluble vitamins must be supplied regularly in diet because they are readily excreted in
urine and cannot be stored (except vitamin B12) in our body.
 NUCLEIC ACIDS

• The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made

up of proteins and another type of biomolecules called nucleic acids.

• These are mainly of two types, the deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since

nucleic acids are long chain polymers of nucleotides, so they are also called polynucleotides.

Chemical Composition of Nucleic Acids

• Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen

containing heterocyclic compounds (called bases).

• In DNA molecules, the sugar moiety is β-D-2-deoxyribose whereas in RNA molecule, it is β-D-ribose.
• DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T).
• RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U).
 Structure of Nucleic Acids
• A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside. In nucleosides,
the sugar carbons are numbered as 1′, 2′, 3′, etc. in order to distinguish these from the bases.
• When nucleoside is linked to phosphoric acid at 5′-position of sugar moiety, we get a nucleotide.
• Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the
pentose sugar.
A simplified version of nucleic acid chain is

• Information regarding the sequence of nucleotides in the chain of a nucleic acid is called its primary
structure.
• Nucleic acids have a secondary structure also DNA has double strand helix structure
• Two nucleic acid chains are wound about each other and held together by hydrogen bonds between
pairs of bases.
• The two strands are complementary to each other because the hydrogen bonds are formed between
specific pairs of bases.
• Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine.
• In secondary structure of RNA single stranded
helics is present which sometimes foldsback on itself.
RNA molecules are of three types and they
perform different functions.
• They are named as
messenger RNA (m-RNA), ribosomal RNA (r-RNA)
and transfer RNA (t-RNA).
 BIOLOGICAL FUNCTIONS OF NUCLEIC ACIDS

• DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information. DNA
is exclusively responsible for maintaining the identity of different species of organisms over millions of
years.
• A DNA molecule is capable of self duplication during cell division and identical DNA strands are
transferred to daughter cells.
• Another important function of nucleic acids is the protein synthesis in the cell. Actually, the proteins are
synthesised by various RNA molecules in the cell but the message for the synthesis of a particular
protein is present in DNA.
 HORMONES
• Hormones are biomolecules which are produced in the ductless (endocrine) glands and are carried to
different parts of the body by the blood stream where they control various metabolic processes. These
are required in minute quantites and unlike fats and carbohydrates these are not stored in the body but
are continuously produced.
• Hormones have several functions in the body. They help to maintain the balance of biological activities
in the body.
• The role of insulin in keeping the blood glucose level within the narrow limit.
• hormone glucagon tends to increase the glucose level in the blood. The two hormones together regulate
the glucose level in the blood.