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Cell Theory, Size and Shape of Cell
The “cell theory” was formulated by two biologists, M.J. Schleiden (1838), and T. Schwann (1839).
According to them, all the plants and animals are composed of cells and that the cell is the basic unit of life. The cell theory was further expanded by Virchow (1858) by suggesting that all cells arise from pre-existing cells.
1.0Cell theory, states that
(i) All living organisms are composed of cells and products of cells.
(ii) All cells arise from pre-existing cells.
2.0Size of Cell
Normal size of human cell - 20 µm to 30 µm in diameter. (1 µm =1×10–6m)
(i) Largest isolated single cell in animals is an Ostrich egg.
(ii) Longest cell in animals is Nerve cell.
(iii) Smallest cell is PPLO-Pleuropneumonia Like Organism [Mycoplasma], 0.3 μm in length.
Bacteria measure 3-5 µm, while viruses are still smaller and do not have a cellular structure.
The longest living cells in the human's body are nerve cells (or neurons) which can live for an entire lifetime.
Human red blood cells - 7 μm in diameter.
The size of an organism is dependent upon the number of cells not on the size of the cell.
3.0Shape of cell
Shape of cell mainly depends upon the specific function it performs.
(i) Elongated & branched - Nerve cell
(ii) Discoidal/Saucer - RBC
(iii) Spindle - Muscle cell
(iv) Spherical - Eggs
(v) Branched - Pigment cell of the skin
(vi) Slipper shaped - Paramecium
(vii) Cuboidal - Germ cells of gonads
(viii) Polygonal - Liver cells
Unicellular organisms vs multicellular Organisms
The organisms which are made up of single cells are called unicellular organisms. e.g. Bacteria, Amoeba, Paramecium, Chlamydomonas.
The single cell is able to perform all life processes, like obtaining food, respiration, excretion, growth and reproduction.
The organisms which are made up of a number of cells are called multicellular organisms. e.g. some fungi, plants and animals.
All the cells of multicellular organisms have similar basic structure and undertake similar basic functions.
Generally, all cells are found to have the same organelles, irrespective of their function and location in an organism. A cell is able to live and perform all its functions because of these organelles.