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Cell Discovery and Cell Theory

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Cell- Basic unit of life - definition

The cell is the basic unit of life (smallest unit of a living being). 
  • It is the structural unit of life because several cells make the structure of an entire organism (e.g. animal body is made of different cells; brain-brain/nerve cells, muscle- muscle cells)
  • It is also the functional unit of life because several cells are responsible for proper functioning of an organism as a whole.

Significance of cell theory - definition

The cell theory proposed by Schleiden and Schwann is important because
  • It provides guidelines (rules) to define life i.e. an organism is believed to have life only if they follow the the three concepts of cell theory
  • The theory gave a fundamental (basic) generalization of biology that cell is the smallest unit of all the living organisms (whether  unicellular or multicellular).
  • The theory also proposed the origin of new cells (cells come pre-existing cells)

Exceptions of cell theory - definition

There are certain organisms and cells which do not agree with the cell theory put forward by Schleiden and Schwann, these are considered to be exceptions and include; 
  • Viruses are obligate parasites which do not have a cellular machinery but live and multiply in other cells. (No cell structure, but has life)
  • Viroids (virus without protein coat), prions (infections protein particles) also behave like viruses and are exceptions to the cell theory
  • RBCs and sieve tube cells lack nuclei so, cannot divide and form new cells (new cells are not formed from pre-existing cells)
  • Some protozoans and thallophytes (e.g. Acetabularia) have a body but are said to be acellular (body is not made of multiple cells)
  • In some organisms, the body is not differentiated into cells though it may have numerous nuclei (coenocytes, e.g., Rhizopus)

Contributors to the study of cell - definition

The cell is the basic structural and functional unit of life.
  • Anton Von Leeuwenhoek first saw and described a live cell. 
  • Robert Hooke first coined the term Cell after observing the compartments in the thin-sliced cork.
  • Matthias Schleiden, a German botanist reported that all plants are composed of different kinds of cells which form the tissues of the plant.
  • Theodore Schwann (1839), a British Zoologist proposed that; Cells have a thin layer (plasma membrane), Cell wall is unique to the plant cells and the bodies of animals and plants are composed of cells and products of cells. 
  • Schleiden and Schwann together formulated the cell theory but failed to explain as to how new cells were formed.
  • Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula).

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Cell as Structural and Functional Unit

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