The Cell is the fundamental structural and functional unit of living organisms, just as the atom is the fundamental unit in chemical structures.
Cell is the smallest and the basic unit of live. Within this tiny objects many chemical reactions takes place in a regulated and well designed manner.
The term Cell (Gr. Kytos, cell, L. Cella – hollow space) was first used by Robert Hooke (1655) to describe the hollow space or cavities he observed in thin slices of cork in a microscope. Hooke described his observation in the volume ‘Micrographia’.
Anton von leeuwenhoek discovered free cells, as opposed to the ‘walled in’cells of Hooke. Leeuwenhoek described the structures of red blood cells (RBCs), protozoa and bacteria.
Dutch scientists Corti and Fontana (1772) identified the presence of jelly like substance in the cells. Dujardin (1835) named this substance as Sarcode. The term protoplasm was coined by Purkenjee (1840). Robert Brown (1831) reported the presence of spherical ball like structure called ‘nucleus’ in the protoplasm of cells.
The concept of cell become that of a mass of protoplasm limited in space by a cell membrane and possessing a nucleus.
Theodore Schwann (Botanist) created the term ‘cell theory’ and declared that plant consists of cells. Similarly Mathias Schlienden declared that animals are composed of cells. Thus, Schlieden and Schwann established or proposed the cell theory.
The cell theory states that:
1. All living organisms are made up of cells.
2. Cell is the structural and functional unit of all organisms.
A major expansion of the cell theory was expressed by Rudolf Virchow (1855) in his famous aphorism “Omnis cellulae e cellula” i.e., all cells arise from the pre-existing cells, which established cell division as the central phenomenon in the reproduction of organisms. Thus, cell is a hereditary unit of all organisms.
Later discoveries in cell biology led to the modern version of the cell theory, which states that
1. Cells are the morphological and physiological units of all living organisms.
2. The properties of a given organism depend on those of its individual cells.
3. Cell originate only from other cells, and continuity is maintained through the genetic material
4. The smallest unit of life is the cell.
Types of Cells:
Cells are broadly divided into Prokaryotic cells and Eukaryotic cells
Prokaryotic Cells (Pro – before or primitive, karyon – nucleus)
These cells lack a true nucleus and nuclear envelope. They have a single circular naked DNA molecule that occupies a space in the cell called a nucleiod, and is in direct contact with the rest of the protoplasm.
They lack membrane bound cell organelles like Mitochondria, Golgi bodies, Endoplasmic reticulum etc. Many enzymes such as those needed in cellular respiration are attached to the plasma membrane.
The DNA lacks the associated histone proteins found in eukaryotes, thus no chromosome like structures are found. They lack special apparatus for cell division, so they divide a mitotically. They reproduce by binary fission and donot reproduce sexually. E.g., Bacteria and Archaebacteria.
Eukaryotic Cell (Eu-True, Karyon – nucleus)
These are cells with a nucleus, this is where the genetic material is surrounded by a membrane. They have both a cellular membrane and a nuclear membrane. Through the nuclear membrane nucleocytoplasmic interchange takes place.
The DNA associated with histone proteins and form multiple chromosomes. The proteins are involved in regulation.
The protoplasm contains many membrane bound organelles such as Mitochondria, Chloroplasts, Endoplasmic reticulum etc.
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