Cell theory: A cell is the smallest structural and functional unit of any living organism. The concept that the cell is the basic unit of life is called the cell theory. At first, German Scientist, M.J.Schleiden and Theodor Schwann established cell theory. The cell theory was further expanded by Virchow, by expressing that all cells arise from pre-existing cells. 

Thus the modern version of the cell theory summarized as follows: 

1. All living things, whether plants or animals are composed of cells and their products. 

2. A cell is the smallest unit of structure and function, which arise from preexisting cells and their continuity is maintained through the genetic material. 

3. All cells are basically alike, found after chemical analysis and studying the functions of different components. 

Following are the concepts of the modern cell theory: 

1. The body of each living being is composed of one or more cells and cell products. 

2. The cell is the fundamental unit of all biochemical reactions. 

3. A cell is also a hereditary unit because it contains genetic material in it. 

4. New cells are formed from the division of pre-existing cells. 

5. All organisms start life from a single cell. 

6. All organisms function through the activities of cells.