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The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block oflife.[1] Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). Humans contain about 10 trillion (1013) cells.Most plant and animal cells are between 1 and 100 µm and therefore are visible only under the microscope.[2]
The cell was discovered by Robert Hooke in 1665. In 1835, before the final cell theory wasdeveloped, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and TheodorSchwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain thehereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.[3]
The word cell comes from the Latin cellula, meaning "a small room".The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the smallrooms monks lived in.[4]
There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.Prokaryotic cells
Main article: Prokaryote

Diagram of a typical prokaryotic cell
The prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the otherorganelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar structure.
Nuclear material of prokaryotic cell consist of a single chromosome that is in...
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