Biology_Today_-_October_2016

Functions of Proteins

(i) Protective structure Keratin forms protective structures of animals like nails, hoofs, hair, feathers.

(ii) Defence proteins Immunoglobulins recognise and neutralise foreign proteins, toxins and other pathogens.

(iii) Structural proteins Take part in the formation of colloidal complex of cell organelle, cell membrane. Supporting

structure: elastin of ligaments, collagen of tendons.

(iv) Contractile proteins Contractile system of muscles is made up of actin and myosin.

(v) Enzymes Catalyse chemical reactions that occur in living world, e.g., pepsin, trypsin, flavoprotein, etc.

(vi) Transport proteins Haemoglobin of RBCs transport oxygen from lungs to different parts of the body. Myoglobin of

muscles stores oxygen.

(vii) Storage proteins Casein is milk protein, ovalbumin is protein of egg white, ferritin is iron storing protein in animal

tissue.

(viii) Blood clotting proteins Fibrinogen and thrombin prevent blood loss from injured vessels by causing clotting of blood.

NUCLEOTIDES

Nucleotides are compounds of carbon, hydrogen, oxygen, nitrogen and phosphorus. They form about 2% of the cell content. They
are the unit of nucleic acids.

Composition of Nucleotides and Nucleosides

Nucleotides are composed of a nitrogenous base, a pentose sugar and a phosphate. The molecule consisting of nitrogenous base
and sugar is termed nucleoside. Thus, nucleotides are nucleosides + phosphate.

Pentose Sugar

The sugar molecule in a nucleotide may

be ribose (C 5 H 10 O 5 ) or deoxyribose

(C 5 H 10 O 4 ). Both sugars are in furanose

or pentagon shaped state with four

carbon and one oxygen atom. The fifth

carbon along with H 2 OH occurs outside

the ring.

Deoxyribose sugar has one oxygen less at

2 ′C compared to ribose i.e., deoxygenated

thus named so.

Phosphate Group

Phosphoric acid

(H 3 PO 4 ) is biologically

called phosphate

group because it

dissociates under physiological conditions,

releasing hydrogen ions. A nucleotide may

have 1, 2 or 3 phosphate groups. But in

nucleic acid composition only nucleotides

with one phosphate group are involved.

Nitrogenous Bases

The nitrogenous bases found in nucleotides are aromatic heterocyclic (having more than one type of compounds in ring) compounds. They

are of two types – purines and pyrimidines.

A pyrimidine molecule is smaller and has one 6-membered ring, with nitrogen at 1 and 3 positions. A purine molecule is larger and has two

rings, a 6-membered ring joined to a 5-membered ring. These rings are joined at 4 and 5 positions. The 5 membered ring carries nitrogen at 7

and 9 positions while the 6 membered ring carries nitrogen at 1 and 3 positions.

Building Blocks of Nucleic Acids

Purines: Pyrimidines:

O

O O

OH+

H P H