- The cell was discovered as the smallest unit of life in 1665 by a British scientist named Robert Hooke. He took a thin slice of cork under his microscope, and he was surprised by looking at those tiny units called cells.
- Soon after Robert Hooke, Anton van Leeuwenhoek made a microscope where he observed human cells and bacteria.
- In 1838, a German botanist Mathias Jacob Schleiden (1804—1881) state 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.
- But the actual theory of cell was based on the works of 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, about its development, and another as an integral part of a multicellular plant
- Schwann also introduced the term metabolism to describe the activities of the cells.
- The cell theory was to be extended further by 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 Nageli’s principle of the cellular basis of life’.He stated in Latin that the cells arise only from the pre-existing cells (“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
- Later, in 1865, Louis Pasteur (1822—1895) in France provided support to Virchow’s extension of the cell theory.
- The modern version of cell theory states that
(1) All living organisms (animals, plants and microbes) are made up of one or more cells and cell products.
(2) All metabolic reactions in unicellular and multicellular organisms take place in cells.
(3) Cells originate only from preexisting cell, i.e., no cell can originate spontaneously or de novo, but it can lead to division and duplication of already existing cells.
(4) The smallest defined unit of life is the cell.
- 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.
Exception to cell theory
- Cell theory does not have universal application, i.e., some living organisms do not have true cells. All kinds of true cells share the following three basic characteristics:
- A set of genes which regulate the cellular activities and form new cells.
- The cell should carry life activities such as growth, reproduction and repair of parts.
- Viruses mostly do not fall in the category of a true cell.
- Thus, they lack a plasma membrane and metabolic machinery for energy production and the synthesis of proteins.
- However, like any other cellular organism, viruses have genetic or hereditary material in the form of either DNA or RNA.
- But viruses need host cells to reproduce which may belong to animals, plants or bacteria.