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In this article we will discuss about the composite structure of a cell with the help of a suitable diagram.
The living substance of plants and animals is described by the general term, protoplasm, which is bounded by a delicate membrane and contains various microscopic and submicroscopic structures. The smallest unit of protoplasm, capable of carrying out independent existence, is the cell (Fig. 1.1).
The word cell (L. cella=a storeroom, a chamber) was first introduced in the biology by Robert Hooke (1635-1703). The cell is the structural and functional unit of the living matter and is capable of carrying on the processes of life independently. The tissue which form the body consist entirely of cell and of extracellular materials elaborated by cells.
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Furthermore, growth, reproduction and continued responsiveness to stimuli are the characteristics of cells and not of their parts. In unicellular organism like Amoeba, a single cell can perform all these physiological functions, but in multicellular organism different cells have got various activities and accordingly these structures have been changed according to its functions.
These functional differentiations are inevitable in a multicellular organism and for these reason different kinds of cells become dependent upon one another. These functional differentiations of the cells in multicellular organism are mostly dependent upon the specific properties of the protoplasm with which a particular cell is constituted. In the unicellular organism, the protoplasm of the single cell has got the basic properties like irritability, conductivity, contractility, absorption, assimilation, excretion, secretion, growth and reproduction.
Thus in the unicellular organism, a single cell is capable of multiple functions. But in the multicellular organism, all these properties of protoplasm have divided and delegated to specific cells and their protoplasms as well. So for the particular functions of a cell, certain physical equipment capable of expressing their functions must be present in the protoplasm of those cells.
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The protoplasm which bears the specific physical equipment of a particular cell unit can be divided into two major organizations:
i. Nucleus, composed of nucleoplasm (karyoplasms), which controls the activities of a cell.
ii. Cytoplasm (Gr. Kytos-hollow (cell) + plasma = thing formed) surrounding the protoplasmic nucleus, which carries out such activities.
In 1833 Brown recognized a conspicuous spherical body, nucleus, in the interior of plant cell and subsequently it was also found in all animal cells. The nucleus contains chromosomes which harbour the genetic materials and typically one or more nucleoli, which are concerned with the synthesis of proteins.
Nucleus and cytoplasm contain a number of components of characteristic form and staining properties. These components belong to organelles (organoids) and inclusions (paraplasm). The organelles are small internal organs of the cells, which are concerned as organised units of living substance possessing important specific functions in cell metabolism. The nature and number of organelles determine the volume and functions of the cytoplasm.
The inclusions are lifeless (non-protoplasmic) accumulations of metabolites, e.g., protein, lipoids (fatty, phospholipid and steroidal compounds) and carbohydrate, crystals, pigments, secreting droplets, etc. in addition, the nucleus and cytoplasm contain an apparently amorphous protoplasm which serves as a ground or matrix where organelles lie.
At some places, the cell membrane is incomplete and the protoplasm of the adjoining cells runs together, such as in liver, umbilical cord (Wharton’s jelly), etc. Such a mass of undifferentiated protoplasm with scattered nuclei is called syncytium or plasmodium.
