The Cell is defined as “A unit of biological activity delimited by a differentially permeable membrane and capable of self reproduction in a medium free of other living systems” (Loewy and Siekevitz, 1963) or it may be defined as “A systematically organized community of molecular populations in dynamic interactions. It has a morphological, chemical and physical organization which enables it to assimilate, grow and reproduce.
The cell is the unit of structure and function of living organisms. It is the minimum organization of matter that is capable of all those processes we collectively refer to as life. Robert Hooke (1665), studied a thin slice of bottle cork and found small compartments in it like the bee-hive and called these cells. In 1674 A. Leeuwenhoek studied first living cell. The botanist M.J. Schleiden and the zoologist U. Schwann first propounded the cell theory in 1839, out of their parallel and independent studies of the tissues of plants and animals. Purkinje (1837) named the living matter of the cell as protoplasm. In 1858 Rudolf Virchow confirmed the cell’s unique role as the vessel of living matter when he showed that all cells are necessarily derived from pre-existing cells : Omnis Cellula e Cellula.
In the century that followed, investigators of the cell approached the subject from two fundamentally different directions. Cell biologists working with electron microscope found that the cytoplasm is differentiated into large number of organelles adopted to carry on the diverse processes of life. Biochemists on the other hand disrupted the delicate structure of cell and traced some of the pathways by which the cell carries out the biochemical reactions that underlie the processes of life including those responsible for manufacturing the substances of cell itself. On the basis of recent researches the cell is considered as “highly organised molecular factory”.
Every living organism starts its life with a cell. If it continues its life as such it is called an unicellular organism e.g., Yeast, Diatoms, Bacteria. But in most of the organisms this cell divides and multi cellular organisms are produced.
The cell is made up of an essentially living substance the protoplasm. In plants the protoplasm is enclosed by a dead Cell wall but in animals this wall is absent. The cell wall has got extremely minute pores through which the protoplasm of one cell is connected to that of the other in the form of thin threads. These protoplasmic threads are called plasmodesmata.
Type of cells
The living organisms have two types of cells :
1. Prokaryotic cells.
2. Eukaryotic cells,
1. Prokaryotic cells (Gk. pro = primitive, karyon = nucleus) : From the structural point of view these cells are primitive. In these cells the nucleus is without a nuclear membrane, due to which substances usually present in the nucleus (Proteins, nucleic acids) are diffused in the central part of the cell in direct contact with the cytoplasm. Also fully developed cell organelles are not present in such cells. Bacteria and blue-green algae (recently termed Cyanobacteria) have such type of cells.
2. Eukaryotic cells (Gk. Eu = good or well; karyon = nucleus) : These cells have a definitely organized nucleus (with a definite nuclear membrane) and cell organelles. These type of cells are found in remaining groups of algae and other plants and animals.