Figure 3.1 Animal Cell

The study of cell is impossible without the microscope. The first simple microscope was prepared by Anton Van Leewenhoek (1632-1723) who studied the structure of bacteria, protozoa, spermatozoa, red blood cells etc. The word ‘cell’ was first coined by Robert Hooke in 1665 to designate the empty honey-comb like structures viewed in a thin section of bottle cork which he examined. He was impressed by the microscopic compartments in the cork as they reminded him of rooms in a monastery which are known as cells. He therefore referred to the units as cells. In 1838, the German botanist Matthios Schleiden proposed that all the plants are made up of plant cells. Then in 1839, his colleague, the anatomist Theodore Schwann studied and concluded that all animals are also composed of animal cells. Schwann and Schleiden studied a wide variety of plant and animal tissues and proposed the "cell theory" in 1839. It stated that "all organisms are composed of cells." But still the real nature of a cell was in doubt. Cell theory was again rewritten by Rudolf Virchow in 1858.

In his theory he said that all living things are made up of cells and that all cells arise from pre-existing cells. It was German biologist Schulze who found in 1861 that the cells are not empty as were seen by Hooke but contain a ‘stuff’ of life called protoplasm.

During the 1950s scientists developed the concept that all organisms may be classified as prokaryotes or eukaryotes. For example, in prokaryotic cells, there is no nucleus; eukaryotic cells have a nucleus. Another important difference between prokaryotes and eukaryotes is that the prokaryotic cell does not have any intracellular components. Bacteria and blue- green algae come under the prokaryotic group, and protozoa, fungi, animals, and plants come under the eukaryotic group.