Cell is the basic and structural unit of a living organisms. In the lower level organisms, their bodies can be composed of single cell or multi-cells. However, in the case of the higher level animals the body is composed of trillions of cells. At the cellular level, every cell has the cellular machinery which carries out all functions of the body like cells produce energy. All systems of the body occur at cellular level like respiration, digestion, excretion etc occur at the cellular level. Therefore, cells always help the organisms to stay alive.