Cell theory

Human cancer cells with nuclei (specifically the DNA) stained blue. The central and rightmost cell are in interphase, so the entire nuclei are labeled. The cell on the left is going through mitosis and its DNA has condensed.

In biology, cell theory is a scientific theory that describes the properties of cells, which are the basic unit of structure in all organisms and also the basic unit of reproduction. The initial development of the theory, during the mid-17th century, was made possible by advances in microscopy; the study of cells is called cell biology. Cell theory is one of the foundations of biology.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 three tenets to the cell theory are as described below:

1. All living organisms are composed of one or more cells.
2. The cell is the most basic unit of life.
3. All cells arise from pre-existing, living cells.

The first two tenets were postulated in 1839, and the third was proposed by Rudolf Virchow around 1855.

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The cell was first discovered by Robert Hooke in 1665. He examined (under a coarse, compound microscope) very thin slices of bottle cork and saw a multitude of tiny pores that he remarked looked like the walled compartments a monk would live in. Because of this association, Hooke called them cells, the name they still bear. However, Hooke did not know their real structure or function. What Hooke had thought were cells were actually just empty cell walls of plant tissues, but without ever thinking that cells could be alive, and also since his microscope had a very low magnification, making it difficult to observe the internal organization of the structure he had discovered, he did not think his "cellulae" could be alive. Hooke's description of these cells (which were actually non-living cell walls) was published in Micrographia. His cell observations gave no indication of the nucleus and other organelles found in most living cells.

Robert Hooke's microscope

The first person to make a compound microscope was Zacharias Jansen, while the first to witness living cells under a microscope was Anton van Leeuwenhoek,who made use of a microscope containing much better lenses that could magnify objects almost 300-fold(Becker,kleinsmith,hardin,p. 1). In 1674 Leeuwenhoek described the algae Spirogyra and named the moving organisms animalcules, meaning "little animals". Leeuwenhoek probably also saw bacteria. Bacteria are microscopic (very tiny) organisms that are unicellular (made up of a single cell). Cell theory was in contrast to the vitalism theories proposed before the discovery of cells. The idea that cells were separable into individual units was proposed by Ludolph Christian Treviranus and Johann Jacob Paul Moldenhawer. All of this finally led to Henri Dutrochet formulating one of the fundamental tenets of modern cell theory by declaring that "The cell is the fundamental element of organization".

The cell theory holds true for all living things, no matter how big or small. 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. Cells are the building blocks of life.

Credit for developing cell theory is usually given to three scientists: Theodor Schwann, Matthias Jakob Schleiden, and Rudolf Virchow. In 1839, Schwann and Schleiden suggested that cells were the basic unit of life. Their theory accepted the first two tenets of modern cell theory (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". In fact, Schleiden's theory of free cell formation was refuted in the 1850s by Robert Remark,Rudolf Virchow and Albert Kolliker. In 1855, Rudolf Virchow concluded that all cells come from pre-existing cells, thus completing the classical cell theory. (Note that the idea that all cells come from pre-existing cells had in fact already been proposed by Robert Remak; it has been suggested that Virchow plagiarised Remak.)

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1. All living cells arise from pre-existing cells by division. (This was stated by Rudolf Virchow after studying the growth of cells in human tissues.)
2. The cell is the fundamental unit of structure and function in all living organisms (Wolfe 1972,p. 5).
3. The activity of an organism depends on the total activity of independent cells.
4. Energy flow (metabolism and biochemistry) occurs within cells.
5. Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm (Wolfe 1972, p. 8).
6. All cells are basically the same in chemical composition in organisms of similar species .

Types of cells

A prokaryote

Cells can be subdivided into the following subcategories:

1. Prokaryotes: Prokaryotes are relatively small cells surrounded by the plama membrane, with a characteristic cell wall that may differ in composition depending on the particular organism (Wolfe 1972,p. 11). Prokaryotes lack a nucleus (although they do have circular or linear DNA) and other membrane-bound organelles (though they do contain ribosomes). The protoplasm of a prokaryote contains the chromosomal region that appears as fibrous deposits under the microscope, and the cytoplasm(Wolfe 1972, p. 11). Bacteria and Archaea are the two domains of prokaryotes.
2. Eukaryotes: Eukaryotic cells are also surrounded by the plama membrane, but on the other hand,they have distinct nuclei bound by a nuclear membrane or envelope. Eukaryotic cells also contain membrane-bound organelles, such as (mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, vacuoles)(Wolfe 1972,p. 13). In addition, they possess organized chromosomes which store genetic material.

See also

• Cell adhesion
• Cell biology
• Cellular differentiation
• Germ theory of disease

Further reading

• Turner W (January 1890). "The Cell Theory Past and Present". J Anat Physiol. 24 (Pt 2): 253–87. PMC 1328050. PMID 17231856.
• Tavassoli M (January 1980). "The cell theory: a foundation to the edifice of biology". Am. J. Pathol. 98 (1): 44. PMC 1903404. PMID 6985772.

External links

• Mallery C (2008-02-11). "Cell Theory". Retrieved 2008-11-25.
• "Studying Cells Tutorial". 2004. Retrieved 2008-11-25.