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| Cell Theory and Cell Principle |
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| Two German biologists M.J.Schleiden (1838) and Theodore Schwann (1839) proposed the cell theory (or cell doctrine), which unified the ideas prevailing at that period on the organisation in plants and animals. |
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| The cell theory states that |
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 All living organisms are composed of minute structures called cells, which represent to smallest entities that can be called as 'living'. |
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| A cell is composed of protoplasm containing a nucleus and bound by a cell membrane and in many cases also by a cell wall. |
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 The functioning of an organism is the result of the activities and interactions of the constituent cells. |
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| Although the cell theory was found to be substantially correct, subsequent observations made by scientists on various plants and animals showed that the theory could not be universally applied. Several exceptions were discovered with reference to some of the generalisations made in cell theory. Moreover, the cell theory made no mention about how new cells are formed. In 1858, a German biologist Rudolph Virchow found that all living cells arise from pre-existing cells ('omniscellula e cellula'). In 1866 Ernst Haeckel suggested that nucleus might store and transmit the hereditary information. |
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| The improvement made in the field of microscopes and techniques for the study of cells, enabled scientists to collect enormous information on the structural and functional organisation of cells. As a result of these developments the cell theory had to be modified. |
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| The modified form of cell theory is called cell principle. It applies almost completely and universally to living organisms. The cell principle is also called cell doctrine. Following are the statements of cell principle. |
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 Life exists only in cells. |
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 Living organisms may be composed of a single free cell or composed of cells or multinucleate mass of protoplasm. |
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 A cell is a small mass of cytoplasm, usually containing a nucleus or nuclear material and bound by a cell membrane. |
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 The cell, besides being a unit of structure, is also a unit of function, reproduction and heredity. |
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 Cells always arise from pre-existing cells by a process of division. |
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 Cells maintain a steady state to remain alive. |
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 A cell, though an integral part of an organism, can grow, divide and die independently of the surrounding cells. |
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 Genetic information is stored and expressed within the cells. |
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 Life passes from one generation to another in the form of cells. |
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 Cells may sometime assume forms, which no longer show cellular characteristics. |
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 Cells may die and still remain functional in the body of an organism. |
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 Structure and function of a cell is controlled by its genetic material - DNA. |
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 Each cell type uses only a part of the coded genetic information, which is necessary for its specialised structure and function. |
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| The cell principle is applicable to all forms of life. It incorporates nearly all |
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| the modern findings on the cells. |
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