The concept that the cell is the basic unit of life is known as the Cell Theory. It is one of the most fundamental generalizations of biology and ranks with Charles Darwin’s theory of organic evolution and T.H. Morgan’s theory of the gene of modern biology.
Cell theory, like the most basic scientific theories, developed as a result of the research and thought of several workers extending over a period of 175 years. However, the credit for formulating the cell theory is normally given to Matthias Schleiden and Theodor Schwann who clearly outlined the following two features of the theory:
- All organisms are composed of one or more cells.
- The cell is the structural unit of life.
But, this theory was not able to explain how new cells were formed. In 1958, Rudolf Virchow extended the theory and suggested an additional feature of the cell theory:
- Cells can arise only from a preexisting cell.
Exceptions to the Cell Theory:
Many organisms are there which do not fit in the definition of cell theory.
- In Protozoans, the living substance is enclosed in a wall and there is hardly any organization like that of a cell.
- Bacteria and Blue-Green Algae do not have an organized nucleus. Their genetic material is not enclosed by a nuclear envelope and lies directly in the cytoplasm.
- Rhizomes have hype composed of a multinucleate tissue.
- Viruses are also cited as an exception to the cell theory as they do not easily fit into the definition of a cell. They completely lack internal organization of which characteristic feature of a typical cell.
In recent years, a large number of sub-cellular structures have been discovered and studied in detail. Consequently, it may appear a cell is no longer a basic unit of life because life may exist without cells also. Moreover, the details of the ultrastructure of the cell are now known, showing that it is a very complex structure, the cell theory still remains a very useful concept.