The term cell was first used by Robert Hooke, an English microscopist, in 1665 to describe a honey-comb like compartment seen in the structure of cork, the concept of the cell as the unit of structure and function of all living things was recognized about 1839 when the researches of Mathias Jakob Schleiden and Theodor Sghwann came into light as the cell theory.
Virchow, in 1859, confirmed the cellular hypothesis showing that all cells must necessarily be derived from pre-existing cells: Omnis cellula e cellula.
Due to the employment of various improved micro-techniques such as micromanipulators, chromatography, electrophoresis, isotopic techniques, ultra-microtomes, electron microscopy, X-ray microscopy and various micro-methods of dissection, in the study of cells, the validity of the cell theory has become vague and the place of this theory has been taken by the “modern cell theory” which explicitly states that (1) all living matter is composed of cells; (2) all cells arise from preexisting (other) cells; and (3) all the metabolic reactions of a Living organism; including all energy exchanges and all biosynthetic processes, take place within cells.
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It also states that cells contain the hereditary information of the organisms of which they are a part, and that this information is passed from parent cell to daughter cell.
In modern terms the cell may be defined as “a dynamic, self- directed, and highly organised complex system of molecules and molecular aggregates which appropriates and utilizes the energy of its surrounding for the purpose of growth and reproduction.
The energy of the environment is available to most cells for their life activities either as energy stored in the chemical bonds of such substances as carbohydrates, fats, and proteins; or as light energy, a form used exclusively by green plants and some bacteria.
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Loewy and Siekevitz (1965) defined the cell as “a unit of biological activity delimited by a selectively permeable membrane and capable of Self reproduction in a medium free of other living systems”.
The cell, as it fulfills the requirements of a living system in terms of structure and function, has been said to be the fundamental unit of life.
The typical activities of it are an expression of the coordinated behaviour of the most highly integrated and organized group of components or subcellular parts present in the cell.
So to understand the cell we must, therefore, understand its component parts as isolated structures ranging from the gross to the molecular, and as whole structures interacting within the integrity of the intact cell:
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The living cell performs all the functions of life, such as intake of nutrients, metabolism, growth, respiration and reproduction, etc.
To carry out these functions of life the cell has various cellular components and subcellular parts in its interior.
Thus, in unicellular living forms the activities of the cell constitute the activities of the organism.
In multicellular organisms the integrated activities of the various constituent cell types are responsible for the characteristic activities and behaviour of the whole organism. So to understand the essential physiology of organisms we must know the complete physiology of cell.