The basic structural and functional unit of cellular organization is the cell. Within a selective and relative semi permeable membrane, it contains a complete set of different kinds of units necessary to permit its own growth and reproduction from simple nutrients. All organisms, more complex than viruses, consist of cells, yet they consist of a strand of nucleic acid, either DNA or RNA, surrounded by a protective protein coat (the capsid). The word cell is derived from the Latin word cellula, which means small compartment. Hooke published his findings in his famous work, Micrographia. Actually, he only observed cell walls because cork cells are dead and without cytoplasmic contents. A.G. Loewy and P. Siekevitz have defined cell as ―A unit of biological activity delimited by a semi permeable membrane and capable of self reproduction in a medium free of other living organisms‖. John Paul has defined the cell as ―The simplest integrated organization in living systems, capable of independent survival‖.