The living world is comprised of diverse forms of life like bacteria, protozoa, plants and animals which differ considerably in their morphology, function and behavior. They are made up of one or many cells. All these cells exhibit a uniform plan of organisation at their cellular, sub-cellular and molecular levels. The contents of each cell are bounded by a thin living membrane, called. plasmamembrane. This membrane is also present, surrounding each sub-cellular particles called organelles. These organelles include plastids, mitochondria, endoplasmic reticulum, etc. These organelles perform distinct functions inside the cell. Nucleus is one such organelle that is commonly found in all cell types.
The cell is thus, a fundamental structural and functional unit of living organisms; because the cellular function is destroyed with the destruction of cellular organisation. Biochemical studies reveal that all biological structures are made up of combinations of proteins, carbohydrates, lipids and nucleic acids. Certain proteins accelerate chemical reactions in the form of catalysts called enzymes. All cells release energy from food molecules by aerobic or anaerobic respiration. Energy released by such processes is generally utilised and stored as adenosine triphosphate (ATP). New cells are produced by division and redivision of preexisting cells through mitosis and meiosis. These divisions are found to be essentially the same in majority of organisms.
Like that of an atom, which is the fundamental unit in chemical structures, the cell is now accepted as the basic unit of life in all living organisms. This is mainly because, the cell is a physical entity and its ability to multiply, mutate and respond to stimuli, etc. is not found in smaller units of matter. Moreover, like those of atoms, the cells can be broken into their sub cellular fragments and when extracted for study by centrifugation, they can continue to perform many of their activities for some time thereafter. They may utilise oxygen, ferment sugars or even form new molecules.
But these activities individually do not constitute life, as these disrupted fragments are no longer capable of continuing life activity. Therefore, we can conclusively tell that cell is the most elementary unit that sustains life. Compared to atom and molecule of matter, the cell is a unit of far larger size with greater complexity. It is considered as a microcosm bounded by a definite boundary, inside which constant chemical activities and a flow of energy continue.
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“All animals and plants, however, complicated, are composed of a few elements which are repeated in each of them”-such concept was originally put forward by ancient philosophers and naturalists, Aristotle and Paracelsus. They meant these to the macroscopic structures of an organism, such as the roots, leaves, and flowers common to various plants or segments and organs present in animals. Many centuries later, the invention of magnifying lenses led to the discovery of microscopic world and in 1665, the British architect and microscopist, Robert Hooke for the first time saw the remains of dead cells in a piece of cork (cork of oak- Quercus suber) under his own invented microscope. The cork looked “perforated and porous much like a honey comb”. He coined the term cell (Gr.kytos-cell; L.cella-hollow space) to describe the tiny structures as they resembled the unadorned cells occupied by monks.
He published his findings in a book entitled Micrographia. He did not restrict his studies to cork, but observed a wide range of plant materials and concluded that, they had the same structure. The microscopes developed by Hooke and his contemporaries could magnify only 100 to 200 times with great distortion of colour an shape. During subsequent years, better lenses were ground which increased the resolving powers of microscopes. In 1674, another microscopist, Antony van Leeuwenhoek discovered free cells of bacteria, yeast, protozoa, RBC, etc., and observed some organisation within them. By 1800, with the invention of good microscopes, and development of techniques for the fixation and staining of cells, there was a general acceptance that organisms were cellular.
However, there existed a good deal of confusion over the definition of cells as bits of animal matter examined did not contain the contents that were present in plants. Subsequently, in 1831 Robert Brown made the discovery of nucleus, a common occurrence in all cells by observing a small sphere like body in the cells of an orchid root. These observations finally led to the foundation of the cell theory.