There is a great amount of variability in cells with respect to their shape and size. They range in size (diameter) from 0.2 micron (u) to about 5.0u in bacteria to 15-2 centimetres in an ostrich egg. The largest cell therefore, is about 75,000 times bigger than the smallest bacterial cell. The bacteriophages or viruses are still smaller in size, but they are not considered to be cellular in their organisation. On the other hand, there exists no relationship between size of cells and body size of an organism. In unicellular organisms, the cell carries out all life processes with the help of cell organelles.
The cell has to be sufficiently large to accommodate the numerous organelles and its efficiency greatly depends upon its surface-volume ratio. An increase in cell volume is accompanied by a much smaller expansion in its surface area. Because, the volume increases as cube of radius, while surface area increases as square of radius. Due to disproportionate increase in cell volume and surface area, adequate area for exchange of material for the increased volume may not be available.
It is for this reason that metabolically active cells, tend to be smaller in size. To overcome this problem, nature has made the cells cylinderical or numerous extension of the cell membrane are formed. Therefore, it has been emphasised that the cell size remains constant in course of evolution, while other characters may change. This is described under the law of constant volume.
The multicellular organisms are made of many cells. They develop from a single-celled zygote by repeated divisions. So a multicellular organism is not simply an aggregate of cells but the cells are differentiated to share varied functions performed by a unicellular organism. In this way, some cells synthesise extracellular materials to hold the cells together, others transmit nerve impulses, while others are specialised to carry out reproduction. Even the dead cells in skin of animals and cork in plants provide protection to the underlying living cells. Dead xylem vessels conduct water in higher plants. So the variety of cells found in multicellular organisms are considerably more efficient than a single-celled one.
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In multicellular organisms, it is essential for a number of cells to work together to perform a particular function. For example, the mechanical strength in trees is the coordinated effort of collenchyma and sclerenchyma tissues. Thus, the cell has a dual existence. Simultaneously, it is present both in an individual capacity and also as a part of the community.
In comparison to unicellular organisms the multicellulars have a greater capacity for survival. Because, death of few cells may not affect the general activity of the organism. A large number of dead cells are regularly replaced by the multiplication of the surviving cells. This phenomenon is not possible in unicellular organisms since any damage may kill the organism as a whole.