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Why are Organisms Composed of Cells?
All living organisms are composed of structural and functional units called cells. The body of a living organism may be composed of a single cell - as in unicellular organisms or many cells - as in multicellular organisms. The number of cells in the body of multicellular organisms is correlated with the size and complexity of their body.
Discovery of the Cell
The history of the discovery of cells dates back to the 17th century and is closely associated with the history of the invention of microscopes. An Italian microscopist Marcello Malpighi studied the structure of some plants and proposed the idea that plants are composed of tiny structural units. He called them as 'Utricles'.
Cell Theory and Cell Principle
Two German biologists M.J.Schleiden (1838) and Theodore Schwann (1839) proposed the cell theory (or cell doctrine), which unified the ideas prevailing at that period on the organisation in plants and animals.
Cell as a Self Contained Unit
The cell whether in a unicellular organism or a multicellular organism is capable of exhibiting some of its functions independently. Thus, the cell shows functional autonomy.
Flow of Energy and Information
Living cells constantly exchange energy and materials with their surrounding environment. The functioning of living cells depends upon the flow of energy and flow of information.
Compartmentalization in Cells
Each cell can be regarded as a compartment by itself since it is bound on all the sides by a cell membrane. It separates the protoplasm within the cell from the surrounding environment. As a result, the cell is able to retain specific molecules and carry out certain reactions in an orderly manner. This situation provides the cell with specific advantages. Apart from isolating the cell contents from outside, the cell membrane allows exchange of materials selectively.
Prokaryotic and Eukaryotic Cells
There are two basic types of cells found in living organisms - the prokaryotic cells and the eukaryotic cells.
Summary
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.
