All living organisms are made up of cells having cytosol, nucleus, organelles, and covering membranes. Functions of living organisms are the total of the activities of their cells.
A cell can survive independently, but organelles cannot do so. A new cell arises from a pre-existing cell. A cell is a unit of structure, function, and heredity. Life exists in cells. The growth of an organism is due to an increase in size and number of cells.
New cell arises from pre-existing cells “Omnis cellula-e cellula.” It is called cell lineage theory. This concept was given by Rudolf Virchow (1855). Rudolf Virchow gave the final shape to the cell theory.