A cell is a structural and functional unit of all living organisms. It is microscopic and capable of independent existence. All living things are made up of cells.
The outward differences among the various biological forms may bewilder us. But underlying these differences is a powerful uniformity.
That is all biological systems are composed of same types of molecules and they all employ similar principles of organization at the cellular level. We shall see for example, that all living organisms employ the same genetic code and a similar machinery for protein synthesis.
Organisms contain organs, organs composed of tissues, tissues are made up of cells; and cells are formed of organelles and organelles are made up of molecules. However, in all living organisms, the cell is the functional unit.
All of biology revolves around the activity of the cell. Loewy and Siekevitz defined cell as a unit of an organism delimited by a plasma membrane in animal cells and cell wall and plasma membrane in plant cells. Thus cell forms the basic unit of life.