The cells are structural and functional units of life. Cellular level of organization is the most important and fundamental level. In 1665, an English scientist Robert Hooke, while examining thin sections of cork under his simple microscope, observed many hexagonal chambers, like a honeycomb, which had no living matter. He named these compartments cells. After this basic finding, many workers like Leeuwenhoek, Grew, Malpighi, Lamarck, Dutrochet and others studied various plant and animal materials and observed similar structures pr cells. Robert Brown (1831) observed a dense spherical body within the cell and named it areola. Later on it was renamed as nucleus. Dujardin observed a jelly like living substance in the cell which was named protoplasm by Purkinje in 1839.
Between 1838-39 Schleiden, a German botanist and Schwann, a Gernman zoologist, proved that both plants and animals are cellular in character and founded the cell theory. They postulated that the cell is the basic unit of structure in all life. Virchow in 1855 proposed the idea that all cells arise from pre-existing cells. No cell can originate spontaneously but comes into being only by division of already existing cells. After the studies of plant and animal cells by De Bary and Schultze, the protoplasm theory was established in 1861. According to this theory, the cells or units of plants and animals are tiny masses of protoplasm, each containing a nucleus. In 1884, Stgrasburger recognized that the nucleus is related to inheritance of characters.
After the discovery of different technique and electron microscopes, our knowledge about the cell and its organelles has developed tremendously.
Prokaryotic Cells and Eukaryotic Cells
Cells are of two types on the basis of structural differences.
1. Prokaryotic Cells
Cells which do not have organized nucleus are called prokaryotic cells. For example, in blue-green algae and bacteria, there is no definite nucleus. Nuclear membrane is absent and DNA is not associated with histone proteins. In prokaryotic cells, ribosomes are smaller and cell organelles are not surrounded by double membranes.
2. Eukaryotic Cells
Cells which have organized nucleus are known as eukaryotic cells. In these cells DNA is associated with histone proteins and organelles are double membrane such as chloroplasts, mitochondria, endoplasmic reticulum, etc. All plant and animal cells (except bacteria and blue green algae) are examples of eukaryotic cells.
1. Take a young leaf from the steam of the Elodea plant. Place it on a glass slide having a drop of water. Observed it under the low magnification of microscope. Do you observe some movement inside? Movements of green dots reflect streaming movement of the protoplasm. Draw a diagram afar observing it under high magnification
2. With the help of a sterilized tooth-pick, scrape the inner surface of your cheek. Take a clean glass slide and put a drop of water on it. Transfer the scrapings on to the slide. Add a drop of methylene blue over the slide. Put a cover slip over it, observe under microscope and draw a diagram.
Cells may be very small (microscopic) or very large (microscopic). The cell size of very small spherical bacteria varies between 0.1 to 0.5 micron ( a micron is one thousandth part of a millimeter) in diameter. The egg of an ostrich (a bird) represents a largest cell are having dimensions of 170 mm * 135 mm. These egg cells are filled with yolk. Some nerve cells of human beings have a process of about one meter. Acetabularia, a single-celled alga measures about 10 cm in height. Fibre cells of some plants like Manila hemp may be more than 100 cm in length. The size is influenced by the functions of the cell which it has to perform.
The cell volume remains almost constant for a particular type. For example, kidney or liver cells in a horse, bull and mouse are almost of the same size. The size of the organisms is dependent upon the number of cells and not on the size of the cell. Thus, the cells of an elephant are not necessarily larger than those of tiny animals like mouse. Due to the larger number of cells present in this body, the elephant becomes larger.
Different cells vary considerably in their shapes. The variation is mainly dependent upon the functions of the cells. It can be polygonal, spherical, elliptical, spindle-shaped, cuboidal, plate-like or irregular. Some cells can change their shape like, white blood cells (leucocytes), while most of the plant cells and animal cells have almost fixed shapes. ‘Surface tension, mechanical action between adjoining cells and location are the other factors besides function which have a bearing on the shape of the cells. Because of long processes, nerve cells are able to conduct impulses from one part of the body to another. As plant cells have rigid cell wall, there is no great variation in their shapes.
In any organism the number of cells varies from time to time. Unicellular organisms are single-celled while a large number of cells are present in multicellular organisms. It is estimated that a human body weighing about 60 kg may have as many as 60 x 10 x 15 cells. Large animals and plants have more cells than smaller animals and plants.