Cell is one of the important aspects of the bodies of living organisms. It is the basic structural and functional unit of living organisms. It consists of microscopic mass of protoplasm surrounded by a cell membrane. It is the smallest unit of organisms that can carry on all the processes of life. In a unicellular organism, the single cell performs all metabolic functions such as respiration, growth, reproduction, digestion, excretion etc. In multicellular organisms all the vital activities are the result of combined action of these cells. Hence, cell is considered as basic unit of life.
Discovery of the Cell: In 1665, a biologist Robert Hooke (1635-1703) cut a thin slick of cork and observed it under his primitive microscope. He observed many small, hollow, honey-comb like compartments. For each compartment, he gave the name cell. The cell is derived from a Latin word cellula meaning small room.
In 1675, a Dutch scientist, Antoni Van Leeuwenhock (1632-1723) observed and described unicellular bodies like bacteria, protozoa, sperms and red blood cells. He also noticed that the cells were not just hollow spaces but contained some organization within them.
The Cell Theory
In 1838, a German botanist, Matthias Schleiden (1804-1881) concluded that all plants were cmposed of cells. A year later (1839) the German Aoologist Theodore Schwann (1810-1882) came to the same conclusion about animals. Schwann recognized that both plant and animal cells had a nucleus and a clear substance around it. He defined the ‘cell’ as a membrane enclosed, nucleus-containing structure. He proposed a hypotheses: the bodies of animals and plants are composed of cells and the products of cells.
The combined views of Schleiden and Schwann led to the formulation of the Cell Theory. However, this theory was not able to explain how new cells are formed. It was Rudolf Virchow (1855) who explained that cells divide and that all new cells must come from pre-existing cells. Therefore, some changes have been made in the old theory. The Cell Theory is also known as the Cell Doctrine or Cell Principle. It states:
(i) All living organisms are made up of cells and their products.
(ii) All cells arise from pre-existing cells.
(iii) Cell is the structural and functional unit of all living organisms.
(iv) All cells are basically alike in chemical composition and metabolic activities.
(v) Life passes from one generation to other generation in the form of a living cell.
(vi) Cells are hereditary units.
(vii) The function of an organism is the outcome of the activities and interactions of the constituent cells.
Exceptions to Cell Theory
Cell theory is one of the most fundamental and universally applied theories except in few cases. They are:
(i) Virus is a living organism but it lacks the cellular organization.
(ii) Bacteria, blue-green algae are prokaryotes and hence are without true cellular structures.