The cell theory

In 1838, a German botanist, Matthias Jacob Schleiden, and in 1839, a German zoologist, Theodore Schwann, recognized the cell with nuclei as the fundamental unit of all luvung beings - animals and plants. Thus they were the first to give coherence to the biological thought that organism are cellular and if there organisation is to be studied one should focus all attention to the cell. The concept that the cell is basic unit of life is known as Cell theory.

Some 20 year late, Rudolf  Virchow, a german physicsn, in 1858, extended the cell theory and suggested that all living cells arise from pre-existing living cell (Omnis cellula-e-cellula).
This theory later supported by Louis Paster experimentally.

The cell theory, as stands now, states that :

1) The cell is the unit of structure of all living beings.

2) The cell is unit of function of all living beings.

3) All cells arise from previously existing cells.

4) All cells are basically alike in chemical composition and metabolic activities.

5) The cell are totipotent and can form the whole organism under specific set of condition.

The cell theory raised a major question in biology. Is an organism a collection of independent cells that coexist and cooperate with one another or it is a group of independent cells whose function are dedicated by the activities of the cells that constitute them. An organism thus cannot demonstrate activities that are absent in its cells. In recent years, a large number of sub-cellular structures have been discovered, which demonstrate that the cells is no longer a basic unit of life, because life may exist without the cell. The cell theory, as most generalisations, also has certain exceptions.

Exceptions to cell theory. Some important ones are as follows:

1) Viruses are acellular organisms as they do not have cellular organisation.

2) In organisms like Rhizopus and Vaucheria, the cells are coenocytic.

3) In mature sieve tube elements and RBC, the nucleus is lacking.

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