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| Cell as a Self Contained Unit |
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| The cell whether in a unicellular organism or a multicellular organism is capable of exhibiting some of its functions independently. Thus, the cell shows functional autonomy. |
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| In unicellular organisms the cell leads a totally independent life. It does not depend on any other cell for any of its activity, except in cases of sexual reproduction. There is division of labour only at the organelle level since different organelles perform specific functions. The activities of various cell structures collectively contribute to the welfare of the organism. |
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| The different types of cells in the body of a multicellular organism are specialised for different functions. Yet, we see an autonomy in the cells in carrying out some of the fundamental biological processes such as respiration, utilisation of food energy, synthesis of macromolecules, elimination of waste products, cell reproduction and so on. Thus, each cell carries out its own metabolic process and maintains a steady state as long as it survives. |
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| Life originated on earth as single cells. However, thereafter there was the formation of multicellular forms of life. A look at this situation naturally rises a question- what was the need for multicellularity? |
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| A cell is an open system, constantly exchanging matter and energy. A specific area of cell membrane can serve the cell contents of a particular volume. Increase in volume increases the requirements of a cell and hence needs a greater membrane area. If we imagine the cell to be spherical in shape, the surface area of a sphere increases as square of its radius while the volume increases as cube of its radius. Thus, as the cell grows its surface area becomes inadequate for moving the required amount of substances in and out of the cell. The cell, when it divides, forms two cells each having a favourable surface area to volume ratio. This situation explains why in the course of time living organisms have become multicellular and have attained larger size. |
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| fig. 12.3 - Relation of Surface area to Volume |
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| Multicellularity provides division of labour through specialisation. It provides specific advantages, such as |
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 It avoids duplication of work. |
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 It ensures simultaneous and uninterrupted performance of the life activities. |
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 It increases the chances of survival. |
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 It increases the efficiency of cells. |
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 It increases the life span of the cells. |
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| Generally, in the body of multicellular animals, we can recognize three types of cells: |
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 Undifferentiated cells |
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 Differentiated cells and |
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 Dedifferentiated cells |
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| Undifferentiated Cells |
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| Undifferentiated cells or stem cells are unspecialised cells, which give rise to new cells by mitotic divisions. These go to form new tissues or in the maintenance of existing tissues. Examples for these are the Malpighian layer in the epidermis of skin, the germinal epithelium found in the gonads, the stem cells in the bone marrow, the meristematic tissue in plants etc. |
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| Differentiated Cells |
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| Differentiated cells are specialised cells, which carry on specific functions. They have a specific form, structure and function, which normally does not change. Differentiation increases the functional efficiency through division of labour. |
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| Dedifferentiated Cells |
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| Dedifferentiated cells are specialised cells, which can revert back to an embryonic state. Dedifferentiation is seen in dicot plants, particularly at the time of secondary growth. It is also seen in the process of regeneration that involves the ability of an animal to develop the lost parts of its body. Such a phenomenon is seen in coelenterates and echinoderms. |
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| The capacity of cells to undergo dedifferentiation indicates that cells retain their complete genetic information. |
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| Totipotency is the capacity of a living cell to differentiate into any type of cell forming a new organism or regenerating a lost part of the body. A cell exhibits totipotency because it contains the entire genetic information in its nucleus. This information is expressed when it becomes necessary. |
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| A German biologist Haberiandt suggested the concept of totipotency in 1902 on the assumption that each cell of an organism being derived from the mitotic divisions of a zygote must be able to produce the entire organism. In the 1950s Steward and his co-workers succeeded in growing carrot plants from isolated phloem cells. |
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| Totipotency is known to be present in plant cells. This capacity is comparatively limited in animal cells. Totipotency has today formed the basis for tissue culture. |
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| fig. 12.4 - Steward and co-worker's experiment showing totipotency of cells in carrot |
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