All living organisms are composed of one or more cells. A cell is the structural and functional unit of life. The metabolic reactions in a living organism – can occur only in a delicately balanced environment. Through their unique chemical composition, cells provide a specific environment that is necessary for life activities. Cells form the basic unit of life.
Discovery of the cells dates back to 1665, when Robert Hooke observed the thin section of cork and noticed the presence of minute, identical chambers. The improvement in the field of microscopy provided more and more information on the organisation and functioning of cells. Schleiden and Schwann, two German scientists, proposed the cell theory in 1839, which underlined the importance of cells in controlling the life activities.
The modified form of cell theory is known as cell principle. It is applicable to almost all living organisms and it incorporates nearly all the modern findings about cells. In unicellular organisms, the single cell that comprises the body leads a totally autonomous existance. The various cell organelles perform the vital body functions. In multicellular organisms, each individual cell is autonomous with respect to functions like respiration, cell division, growth etc. However, the activities of different types of cells are integrated for the existence and survival of the organism. Multicellularity in organisms permits division of labour through specialization and increases the functional efficiency.
A multicellular organism shows normally three categories of cells – Undifferentiated cells which are in a state of division and yet to undergo specialization, differentiated cells which have become specialized for specific functions and dedifferentiated or transformed cells which have lost their specialization to become embryonic cells. The capacity of any differentiated cell with an active nucleus, to give rise to an entire organism, is called totipotency. Totipotency is well expressed in plant cells and is lost in several types of animal cells because of a high degree of specialization. Every living cell is involved in the flow of energy, which it exchanges with its surrounding medium.
The activities of a cell are regulated in two ways – flow of intrinsic or genetic information and flow of extrinsic or hormonal information. Every cell functions as a compartmentalized unit bound by membranes in the presence of membrane bound cell organelles. Cells in living organisms can be broadly distinguished into prokaryotic cells and eukaryotic cells, mainly based on the arrangement of genetic material and the presence of cell organelles.