In this article we will discuss about the functions, size and shape of a cell.
Functions of Cell:
The description of different cellular components may lead to this erroneous idea that each component probably carries its activity independently. It is not so. All the cell organelles work together and their work is absolutely dependent upon each other. The working of the entire cell is the cumulative expression of the working of its different components.
Within a multicellular body there are millions of cells, which differ in their shapes and functions. Whatever may be the duty of an individual cell, they work in a common plan. The working of a cell involves the production of new substances in one hand and breakdown of some substances on the other.
In each tiny mass of protoplasm both these functions are carried out effectively and smoothly through a number of chemical reactions. The speed at which these chemical reactions occur is really amazing.
All these activities involve the working of different biological catalysts called enzymes. The most interesting features of these chemical reactions are—(1) existence of a “feedback” mechanism and (2) slow production of energy.
Protein is the most important fabric of the cell. Therefore, the synthesis of protein is the most essential function of cell. The DNA present in the chromosome acts as template or cast. The available raw materials attach in specific sequence of this cast and with the help of an enzyme RNA-polymerase they are linked into a polynucleotide chain.
This results in the formation of a particular type of RNA, called messenger RNA, which carries the information from DNA. Messenger RNA separates from DNA and passes into the cytoplasm.
Within the cytoplasm at the sites of the ribosomes a second kind of RNA (ribosomal RNA) holds the messenger RNA in extended condition. From the cytoplasm, a third kind of RNA (transfer RNA) or soluble RNA collects amino acids and delivers them to the proper position on the messenger RNA.
One kind of transfer RNA can collect only one type of amino acid. Therefore, the number of transfer RNA corresponds to the number of available amino acids. Thus, different transfer RNAs deliver their amino acids on the specific sites along the entire length of the messenger RNA which remains stretched on the ribosomes. These amino acids unite in a sequence to form a particular protein.
During the process of breakdown, carbohydrates, fats and proteins are converted into pyruvic acid. Of course, the cell can use these components only when they are in the form of glucose, fatty acids, glycerols and amino acids respectively. The pyruvic acid enters into the mitochondria and a part of it is further converted into energy-rich molecules called Adenosine triphosphate (ATP).
These ATP molecules are used in different parts of the cell, e.g., in the membrane, in the cytoplasm, etc., to perform different functions. When energy is required this stored ATP is broken down into ADP or Adenosine diphosphate and energy is released. The ADP may be further broken down into Adenosine monophosphate (AMP) and more energy is released.
When synthesis is more than the breakdown, new substances are stored within the body. This results into growth. The phase of growth is not unlimited, after certain period of growth the cell duplicates. Thus reproduction (a process of self-duplication) is also operative at the cellular level, where multiplication takes place by division.
A question may arise that when all the cells are made up of protoplasm and when all of them are working in the same plan, what is the reason of diversity among themselves? The answer is that the working of a cell depends upon the interaction of two systems—message from the DNA and environment. The different parts of the cell including enzymes are made up of proteins.
The control of the synthesis of proteins depends upon the message from DNA. In different species this DNA code (particular arrangement of nucleotides which, in its turn, specify particular amino acid sequence in the protein chain) varies, which results into the variation of forms.
The same code alters slightly within the cells of same individual, resulting into the formation of diverse types of cells. The second important system is the environment which supplies the precursors from which fabrics are built up according to the instruction.
Number, Size and Shape of Cell:
Number of cells in an organism varies. In some organisms, the body is made up of many cells (multicellular) while in others it is made up of single cell (unicellular). In a human body it is calculated that there are 1014 cells on an average.
Number of cells increases up to certain period of life and such increase ceases afterwards. The cells in a multicellular body are of two types, germ cell and somatic cell. The cells which are only responsible for reproduction are called germ cells and the remaining cells are called somatic cells.
Similarly, shape and size of the cells also vary extensively (Fig. 4.16). Some multicellular forms (Rotifers) may be smaller in size than many unicellular forms, i.e., Amoeba, Spirostomun, etc. Whereas some single cells like the eggs of birds and reptiles are quite large. A nerve cell may attain a length of several meters. Red blood cells are only 5-8 µ in diameter.
Within an organism the cells rarely remain in round state. They are flattened (epithelial cells), may be spindle-shaped (muscle cells) or spider-shaped (nerve cell), etc. The size and shape of a cell are related to its function and are governed by four factors—(1) surface-volume ratio, (2) nucleocytoplasmic ratio, (3) rate of cellular activity, (4) cell associations.
(1) Surface-volume Ratio:
The cell membrane separates the inner content from the outer environment. But this separation does not mean isolation. A number of substances pass in and out through the membrane. These substances are essential for the activities of the cell.
The total surface area of a cell is just sufficient for its inner content. Any increase in the surface area produces manifold increase of the cell volume which upsets the balance. Thus the particular shape and size which a cell attains are determined by the ratio of surface area and inner volume.
(2) Nucleo-cytoplasmic Ratio:
The cell functions by the co-ordinated activities of its different parts. Most important is the co-ordination between nucleus and cytoplasm. Nucleus produces certain substances which come into the cytoplasm and control its activities. The cytoplasmic area of a cell is just enough which a nucleus can control.
If the cytoplasmic area is much extended, it will not be possible for the nucleus to control its activity. For this reason a cell never attains size where nucleo-cytoplasmic ratio is more than one-seventh to one-twelfth. In certain cases, this difficulty is overcome by the presence of more than one nucleus in a large cell.
(3) Rate of Cellular Activity:
Though all the cells of a living body are equally important, yet some cells are metabolically more active than the others. The more active cells are usually smaller and in them, the surface- volume ratio and nucleo-cytoplasmic ratio are in such a level that these cells can work more actively than the larger cells.
(4) Cell Association:
The cell-to-cell attachment is important in multicellular forms which render some amount of rigidity. The degree of attachment plays an important role in determining the shape of the cell and their functional property.
It is evident that all the four factors stated above are interrelated and of these the surface-volume ratio and nucleo-cytoplasmic relations play the most important role in determining the size, shape and also characteristics of the cell.
But everything depends upon the functions which are to be carried out. In many cells the surface area is increased by various sorts of folding. Such increase has reduced the chance of the alteration of inner volume.