The credit of formulation of cell theory, the basic aspects of which hold well today, goes to the German Botanist Mathias Jacob Schneider and Theodore Schwann, a German Zoologist.
This theory established that the cell is the basic unit of life and can be considered as the cell doctrine as it is essentially a fact-based statement. Schleiden in 1838 stated that all plants “are aggregates of fully individualized, independent, separate beings, namely the cells themselves”.
In 1839, Theodor Schwann reported that all animal tissues also consist of individual cells and animal cells lack a cell wall, instead they are covered by a membrane. Both of them compared their findings and jointly proposed the cell theory in 1839 in a paper titled “Microscopic investigations on the similarity of structure and growth in animals and plants”.
However, they did not know the mode of origin of new cells and believed that new cells developed spontaneously. Rudolf Virchow (1858) observed that new cells develop from preexisting cells – Omnis cellula e cellula. Cell theory was modified accordingly.
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Fundamental features of Cell Theory
1. All organisms, plants and animals are composed of one or more cells and their products.
2. Each cell is made of a mass of viscous substance called protoplasm containing a nucleus.
3. New cells arise from pre-existing ones.
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4. All cells are basically similar in their physiology and chemistry.
5. Activities of an organism is the sum total of the activities of its constituent cells.
Exceptions to Cell Theory
Cell theory cannot be considered as a universal generalization. It has certain exceptions
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1. Viruses are nucleoproteins, lacking internal organization and outside the host, they are as good as dead.
2. Protozoa like Paramecium has unicellular, differentiated body with a mouth/ gullet, contractile vacuole and many cilia. Similarly thallophytes like Acetabularia have a unicellular but differentiated structure like foot, stalk and cap. Cell theory does not apply here as they seem to have abandoned cell as a mechanical and structural unit and are regarded as acellular.
3. In some organisms like Vaucheria and Rhizopus the body is not differentiated into cells rather they are multinucleate (coenocytes). It has been regarded that in such coenocytes organisms, each nucleus along with protoplasm surrounding it is a wall- less cell and the organism itself is a group of protoplasmic unit.
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This laid Andre Lwoff in 1962 to propose another theory. According to this theory the entire organism is regarded as the aggregation of unity of plan, unity of function and unity of composition.
4. Bacteria and blue green algae (cyan bacteria) have no true nucleus instead they have incipient nuclei.
5. In the surface cells of animal skins and cork of plants, protoplasm is replaced by non living materials.
6. RBC of animal’s blood and sieve tubes of plants continue to live without nucleus.
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Modern Concept of Cell Theory
1. All living organisms, plants or animals, are composed of one or more cells or their products.
2. Cells are structural and functional units of life.
3. New cells arise from pre-existing ones.
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4. All cells are basically similar in their chemistry and physiology.
5. Growth of an organism depends on the growth and multiplication of its constituent cells.
6. The function of an organism is the sum total of the function and interaction of its cells.
7. Cell is a self-contained unit.
8. In an organism the hereditary or genetic information are stored in its cells.
Structurally a cell is defined as “a mass of protoplasm bounded-by a membrane and having at its centre a spherical body, the nucleus.” But the existence of coenocytes and living cells without nucleus has given rise to a physiological definition of cell.
A.G.Loewy and P.Siekevietz in 1969 defined cell as “the smallest organized unit of any living form which is capable of prolonged independent existence and replacement of its own substance in a suitable environment.” Besides the cell theory, there are other theories too.
The protoplasmic theory by Schultz, 1861 states that the basic unit of organism is protoplasm not cell. Organism theory by Sachs, 1874 states that the body of an organism is made up of continuous mass of living matters incompletely divided into compartments called cells.
Cell size: Generally cell size ranges from 0.1 to 20 micro meters. The smallest cell so far found is of PPLO (pleuropneumonia like organism) or Mycoplasma gallisepticum where the cell is 0.1 micro meters in diameter.
The largest cell is the egg of Ostrich which is six inches in diameter with shell and three inches in diameter without shell. Acetabularia, the unicellular green alga is about ten centimeter in length.
In alga like Caulerpa the length of a cell can be up to one meter. In animals nerve fibers can be as long as 90cm to one meter. In general, metabohcally active cells are smaller and the cells of a particular tissue have almost the same volume.
Cell number: Number of cell varies greatly among the organisms; a single cell in unicellular organism to infinite number of cells in multi cellular organisms.
A man of 8oKg can have about 60 thousand billion cells. In colonial forms each type has a fixed number of cells. For example, in green alga Pandorina each colony has 8, 16 or 32 cells.
Cell shape: The cell maybe spherical, polygonal, discoid, cuboidal, columnar or spindle like. Generally individual cells are spherical, but depending on function and organization the shape of the cells varies. Sometimes a particular cell may undergo shape change as is seen in the case of Amoeba and Leucocytes.