In 1838, a German botanist Mathias Jacob Schleiden (1804—1881) put forth the idea that cells were the units of a structure in the plants. In 1839, his coworker, a German zoologist, Theodor Schwann (1810—1882) applied Schleiden’s thesis to the animals.
“Both of them, thus, postulated that the cell is the basic unit of structure and function in all life. This simple, basic and formal biological generalization is known as cell theory or cell doctrine”.
In fact, both Schleiden and Schwann are incorrectly credited for the formulation of the cell theory; they merely made the generalizations which were based on the works of their predecesors such as Oken (1805), Mirbel (1807), Lamarck (1809), Dutrochet (1824), Turpin (1826), etc.
However, Schleiden was the first to describe the nucleoli and to appreciate the fact that each cell leads a double life—one independent, pertaining to its own development, and another as an integral part of a multicellular plant.
Schwann studied both plant and animal tissues and his work with the connective tissues such as bone and cartilage led him to modify the evolving cell theory to include the idea that living things are composed of both cells and the products or secretions of the cells. Schwann also introduced the term metabolism to describe the activities of the cells.
In the coming years, the cell theory was to be extended and refined further. K. Nageli (1817—1891) showed in 1846 that plant cells arise from the division of pre-existing cells. In 1855, a German pathologist Rudolf Virchow (1821—1902) confirmed the Nageli’s principle of the cellular basis of life’s continuity.
He stated in Latin that the cells arise only from the pre-existing cells (viz., his actual aphorism was “omnis cellula e cellula” —every cell from a cell). Virchow, thus, established the significance of cell division in the reproduction of organisms.
In 1858, Virchow published his classical textbook Cellular Pathology and in it he correctly asserted that as functional units of life, the cells were the primary sites of disease and cancer. Later, in 1865, Louis Pasteur (1822—1895) in France gave experimental evidence to support Virchow’s extension of the cell theory.
The modern version of cell theory states that
- All living organisms (animals, plants and microbes) are made up of one or more cells and cell products.
- All metabolic reactions in unicellular and multicellular organisms take place in cells.
- Cells originate only from other cells, i.e., no cell can originate spontaneously or de novo, but comes into being only by division and duplication of already existing cells.
- The smallest clearly defined unit of life is the cell.
The cell theory had its wide biological applications. With the progress of biochemistry, it was shown that there were fundamental similarities in the chemical composition and metabolic activities of all cells. Kolliker applied the cell theory to embryology—after it was demonstrated that the organisms developed from the fusion of two cells—the spermatozoon and the ovum.
However, in recent years, a large number of sub-cellular structures such as ribosomes, lysosomes, mitochondria, chloroplasts, etc., have been discovered and studied in detail. Consequently, it may appear that a cell is no longer a basic unit of life, because life may exist without cells also. Even then, the cell theory remains a useful concept.
Exception to cell theory.
Cell theory does not have universal application, i.e., there are certain living organisms which do not have true cells. All kinds of true cells share the following three basic characteristics:
- A set of genes which constitute the blueprints for regulating cellular activities and making new cells.
- A limiting plasma membrane permits a controlled exchange of matter and energy with the external world.
- Metabolic machinery for sustaining life activities such as growth, reproduction, and repair of parts. Viruses do not easily fit in these parameters of a true cell.
Thus, they lack a plasma membrane and metabolic machinery for energy production and for the synthesis of proteins. However, like any other cellular organism, viruses have
- a definite genetically determined macromolecular organization;
- a genetic or hereditary material in the form of either DNA or RNA;
- a capacity of auto-reproduction; and
- a capacity of mutation in their genetic substance.
In consequence, viruses can only reproduce inside the host cells which may belong to animals, plants or bacteria. They use their own genetic programme for reproduction but rely on the raw materials (i.e., amino acids, nucleotides) and biosynthetic machinery of the host cells ( i.e., ribosomes, tRNA, enzymes) for their multiplication.
Thus, a virus may be defined as an infectious, subcellular and ultramicroscopic particle representing an obligate cellular parasite and a potential pathogen whose reproduction (replication) in the host cell and transmission by infection cause characteristic reaction in the host cells. Outside the host cells, viruses are just like non-living inert particles and like the salt or sugar, they can be purified, crystallized, and placed into jars on a shelf for years.
Due to this fact, viruses have been variously described such as “naked genes that had somehow acquired the ability to move from one cell to another (Alberts et al., 1989), or as “cellular forms that have degenerated through parasitism”, or as “primitive organisms that have not reached a cellular state.”
There are certain other organisms such as the protozoan Paramecium, the fungus Rhizopus and the alga Vaucheria which do not fit into the purview of the cell theory. All of these organisms have bodies containing an undivided mass of protoplasm which lacks a cell-like organization and has more than one nucleus. They tend to raise the question that whether a cell is a basic unit of structure in them.