When a small wall is built, a number of bricks are arranged end to end. Similarly cells are arranged variously to build the bodies of living beings. In fact, every organism begins life as a single cell which is the fertilized egg. Cells divide to give more cells. Cells form tissues. Tissues make organs.
The invention of microscope helped in the discovery of the cells. Robert Hooke discovered the cell in 1665. He observed a thin piece of cork under his simple microscope and saw many compartments arranged like the honey comb. He named these compartments "cells".
Soon a cell theory was formulated by two German biologists, M J Schleiden (1838) and T Schwann (1839). The cell theory states that:
Cell is the structural and functional unit of all living beings and bodies of all organisms are composed of cells.
All new cells arise by division of pre-existing cells.
The functions of an organism are an outcome of the combined activities and the interactions of the cells that make the organism.
A cell may be defined as the structural and functional unit of living organisms which is capable of independent existence.