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'''Cell theory''' refers to the idea that ]s are the basic unit of structure in every living thing. Development of this ] during the mid 1600s was made possible by advances in ]. This theory is one of the foundations of ]. The theory says that new cells are formed from other existing cells, and that the cell is a fundamental unit of structure, function and organization in all living organisms. '''Cell theory''' refers to the idea that ]s are the basic unit of structure in every living thing. Development of this ] during the mid 1600s was made possible by advances in ]. This theory is one of the foundations of ]. The theory says that new cells are formed from other existing cells, and that the cell is a fundamental unit of structure, function and organization in all living organisms.


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== History ==
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The cell was first discovered by ] in 1665. He examined very thin slices of cork and saw a multitude of tiny pores that he remarked looked like the walled compartments of a honeycomb. Because of this association, Hooke called them cells, the name they still bear. However, Hooke did not know their real structure or function. <ref>{{cite book |author=Inwood, Stephen |title=The man who knew too much: the strange and inventive life of Robert Hooke, 1635-1703 |publisher=Pan |location=London |year=2003 |pages=72 |isbn=0-330-48829-5}}</ref> Hooke's description of these cells (which were actually non-living cell walls) was published in '']''.<ref>{{cite journal | author=Karling JS | title=Schleiden's Contribution to the Cell Theory| journal=The American Naturalist | year=1939 |volume=73 | pages=517–37| doi=10.1086/280862}}</ref>. His cell observations gave no indication of the ] and other ]s found in most living cells.
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The first man to witness a live cell under a microscope was ], who in 1674 described the ] '']'' and named the moving organisms animalcules, meaning "little animals".<ref>{{cite web |url=http://www.euronet.nl/users/warnar/leeuwenhoek.html#references |author=Moll WAW | title= Antonie van Leeuwenhoek |year=2006 |accessdate=2008-11-25}}</ref>. Leeuwenhoek probably also saw ].<ref>{{cite journal |author=Porter JR |title=Antony van Leeuwenhoek: tercentenary of his discovery of bacteria |journal=Bacteriol Rev |volume=40 |issue=2 |pages=260–9 |year=1976 |month=June |pmid=786250 |pmc=413956 |doi= |url=http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=786250}}</ref> Cell theory was in contrast to the ] theories proposed before the discovery of cells.
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The idea that cells were separable into individual units was proposed by ]<ref>Treviranus, Ludolph Christian 1811, "Beyträge zur Pflanzenphysiologie"</ref> and ]<ref>Moldenhawer, Johann Jacob Paul 1812, "Beyträge zur Anatomie der Pflanzen"</ref>. All of this finally led to ] formulating one of the fundamental tenets of modern cell theory by declaring that "The cell is the fundamental element of organization"<ref></ref>
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The observations of Hooke, Leeuwenhoek, Schleiden, Schwann, Virchow, and others led to the development of the cell theory. The cell theory is a widely accepted explanation of the relationship between cells and living things. The cell theory states:
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# All living things are composed of one or more cells.
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# Cells are the basic units of structure and function in living things.
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# New cells are produced from existing cells.
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The cell theory holds true for all living things, no matter how big or small, or how simple or complex. Since according to research, cells are common to all living things, they can provide information about all life. And because all cells come from other cells, scientists can study cells to learn about growth, reproduction, and all other functions that living things perform. By learning about cells and how they function, you can learn about all types of living things.
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Credit for developing cell theory is usually given to three scientists: ], ], and ]. In 1839, Schwann and Schleiden suggested that cells were the basic unit of life. Their theory accepted the first two tenets of ] (see next section, below). However the cell theory of Schleiden differed from modern cell theory in that it proposed a method of spontaneous crystallization that he called "Free Cell Formation"<ref>Schleiden, Matthias Jakob 1839,"Contributions to Phytogenesis"</ref>. In 1858, Rudolf Virchow concluded that all cells come from pre-existing cells, thus completing the classical cell theory.
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=== Classical interpretation ===
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# All organisms are made up of one or more cells.
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# Cells are the fundamental functional and structural unit of life.
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# All cells come from pre-existing cells.
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# The cell is the unit of structure, physiology, and organization in living things.
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# The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
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=== Modern interpretation ===
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The generally accepted parts of modern cell theory include:
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# The ] is the fundamental unit of structure and function in living things.
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# All cells come from pre-existing cells by division.
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# ] (] and ]) occurs within cells.
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# Cells contain hereditary information (DNA) which is passed from cell to cell during cell division
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# All cells are basically the same in chemical composition.
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# All known living things are made up of cells.
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# Some organisms are unicellular, i.e., made up of only one cell.
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# Others are multicellular, composed of a number of cells.
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# The activity of an organism depends on the total activity of independent cells.
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=== Exceptions ===
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{{See also|Origin of life}}
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# ]es are considered alive by some, yet they are not made up of cells. Viruses have many features of life, but by definition of the cell theory, they are not alive.
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# The first cell did not originate from a pre-existing cell. There was no exact first cell since the definition of cell is not that precise. This is an intellectual game that comes from making strict logical symbols out of biological definitions.
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# ] and ]s have their own genetic material, and reproduce independently from the rest of the cell.
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=== Types of cells ===
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Cells can be subdivided into the following subcategories:
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# '']'': Prokaryotes lack a nucleus (though they do have circular DNA) and other membrane-bound organelles (though they do contain ribosomes). ] and ] are two divisions of prokaryotes.
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# '']'': Eukaryotes, on the other hand, have distinct nuclei and membrane-bound organelles (], ], ], ], ]). In addition, they possess organized chromosomes which store genetic material.
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== See also == == See also ==

Revision as of 08:20, 17 September 2009

A prokaryote

Cell theory refers to the idea that cells are the basic unit of structure in every living thing. Development of this theory during the mid 1600s was made possible by advances in microscopy. This theory is one of the foundations of biology. The theory says that new cells are formed from other existing cells, and that the cell is a fundamental unit of structure, function and organization in all living organisms.

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See also

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