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Unicellular and Multicellular Organisms

Most cells are tiny and their volume ranges from 1 to 1000 mm3. The eggs of some birds are enormous. A unicellular organism has to perform a large number of functions for its survival such as absorption of nutrients, exchange of gases with environment and metabolism. To carry out these functions, the cell has to be sufficiently large to accommodate a large number of organelles in it. Secondly, it has to increase its surface area. As a cell increases in volume, its surface area also increases, but not to the same extent. The biological significance of this phenomenon lies in the fact that the volume determines the amount of chemical activity of cells per unit time whereas the surface area determines the amount of absorption and the amount of release of waste products by the cells. As the living cell grows, its rate of waste production and its need for intake of substances from outside increases at a rate faster than that of surface area. Small size of the cells of large organisms compensates the disproportionate increase in the volume and surface area of cells.  That is why, by and large, the cells are tiny; cells are small in volume to maintain a large surface area-to volume ratio.  Moreover, to maintain surface area-to-volume ratio in a balanced state, some cells have acquired additional structures in the form of projections such as microvilli. These increase the absorptive surface area. 



Opposed to a single-celled organism, a multicellular organism is not an aggregate of single cells.  Rather, the cells undergo differentiation so as to share various functions which otherwise are independently performed by a unicellular organism. For example, some cells may secrete materials, which move outside the body for protective functions. Other cells may secrete substances, which hold the cells together. Still others may be differentiated to carry out absorption, photosynthesis, translocation of solutes, transmission of nerve impulse, and some cells get highly specialised to carry out reproduction. Even some dead cells have a role to play. For example, in animals the function of outer coat of dead cells forming a part of skin is to protect the inner living cells. In plants, conduction of water is performed by xylem vessels and tracheids, which are dead cells. Thus, a multicellular organism with its various types of cells is considerably more efficient than a single-celled organism. There may be other benefits that a multicellular organism is bestowed with or may have certain constraints in comparison to a unicellular organism. Some of the benefits and constraints of multicellular organisms are listed below: 
  1. There is a unique co-ordination among cells of multicellular organisms like pumping of blood by heart muscle, and transmission of information (nerve impulse) through nerve cells. 
  2. In these capacities the cells have a dual existence as individual and as a part of the community (tissues). 
  3. Even if some cells die in a multicellular organism, the living cells can multiply and replace the lost cells, which is a clear cut benefit over unicellular organisms. For example, the cells of human skin, blood cells, etc. 
  4. The cells of multicellular organisms differentiate to various levels achieving a high degree of specialisation, even though they are derived from a single zygotic cell and carry the same genetic material. 
  5. Many of the differentiated cells lose one or the other basic activity, which is either suspended temporarily or irreversibly lost. For example, liver cells, muscle cells or epidermal cells retain mitotic activity after differentiation.  The nerve cells and the red blood corpuscles (RBC's) lose mitotic activity after differentiation. RBCs do not even show DNA duplication or RNA synthesis.
  6. Differentiation bestows tremendous benefits on the multicellular organisms. These are;
    1. increased survival
    2. increased specialisation
    3. ensured uninterrupted life activity, and
    4. a proper balance between the cell surface and cell volume for receiving external stimuli, exchange of materials, transport, secretion, etc. 
In general, the cells in an organism can be grouped under three major categories on the basis of the levels of differentiation; 
  1. Undifferentiated Cells: These cells are capable of undergoing division and development, for example, the stem cells (animals) and meristematic cells (plants). 
  2. Differentiated Cells: These are post mitotic cells which have undergone specialisation or / and exhibit the division of labour. Therefore, these cells acquire a distinct character and perform a definite function. For example, RBC carry out transportation of oxygen and carbon dioxide, the muscle cells perform kinetic functions or movement, and mesophyll cells carry out photosynthesis.   
  3. Dedifferentiated Cells: Some differentiated cells are capable of reverting back to the undifferentiated meristematic state, when required. These cells are important for wound healing, regeneration, and secondary growth. The process by which they lose their specialization is referred to as dedifferentiation. 
As you would have realised by now, cells of all organisms, whether unicellular or multicellular, have a close similarity in structure, molecular organisation and the various activities performed by them. This strongly suggests unity of life.

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