CELLS

arianna g per.4


 * Students know ****how prokaryotic cells, eukaryotic cells (including those from plants and animals), and viruses differ in complexity and general structure.

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 The **cell** is the functional basic unit of life. It was discovered by [|Robert Hooke]and is the 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 of life.[|[1]] Some organisms, such as most [|bacteria], are [|unicellular] (consist of a single cell). Other organisms, such as [|humans], are [|multicellular]. (Humans have about 100 trillion or 1014 cells; a typical cell size is 10 [|µm]; a typical cell mass is 1 [|nanogram]. The largest cells are about 135 µm in the [|anterior horn in the spinal cord] while [|granule cells] in the [|cerebellum], the smallest, can be some 4 µm and the longest cell can reach from the toe to the lower [|brain stem]([|Pseudounipolar cells]).[|[2]]) The largest known cells are unfertilised [|ostrich][|egg cells] which weigh 3.3 pounds.[|[3]][|[4]] In 1835, before the final cell theory was developed, [|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 [|Theodor Schwann], 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 the[|hereditary information] necessary for regulating cell functions and for transmitting information to the next generation of cells.[|[5]] 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 small rooms monks lived in.

The [|prokaryote] cell is simpler, and therefore smaller, than a eukaryote cell, lacking a[|nucleus] and most of the other [|organelles] of eukaryotes. There are two kinds of prokaryotes: [|bacteria] and [|archaea]; these share a similar structure.

A prokaryotic cell has three architectural regions:
 * On the outside, [|flagella] and [|pili] project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells;
 * Enclosing the cell is the [|cell envelope] – generally consisting of a [|cell wall]covering a [|plasma membrane] though some bacteria also have a further covering layer called a [|capsule]. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as //[|Mycoplasma]// (bacteria) and//[|Thermoplasma]// (archaea). The cell wall consists of //[|peptidoglycan]// in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting ([|cytolysis]) from [|osmotic pressure]against a [|hypotonic] environment. Some eukaryote cells ([|plant cells] and [|fungi]cells) also have a cell wall;
 * Inside the cell is the [|cytoplasmic region] that contains the [|cell genome] (DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium //[|Borrelia burgdorferi]//, which causes Lyme disease). Though not forming a //nucleus//, the[|DNA] is condensed in a //nucleoid//. Prokaryotes can carry [|extrachromosomal DNA]elements called //[|plasmids]//, which are usually circular. Plasmids enable additional functions, such as [|antibiotic resistance].

[|Eukaryotic] cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is a [|cell nucleus], a membrane-delineated compartment that houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus." Other differences include:


 * The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
 * The eukaryotic DNA is organized in one or more linear molecules, called[|chromosomes], which are associated with [|histone] proteins. All chromosomal DNA is stored in the //[|cell nucleus]//, separated from the cytoplasm by a membrane. Some eukaryotic [|organelles] such as[|mitochondria] also contain some DNA.
 * Many eukaryotic cells are [|ciliated] with //primary cilia//. Primary cilia play important roles in chemosensation, [|mechanosensation], and thermosensation. Cilia may thus be "viewed as sensory cellular [|antennae] that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[|[7]]
 * Eukaryotes can move using //motile [|cilia]// or //flagella//. The flagella are more complex than those of prokaryotes.

http://en.wikipedia.org/wiki/Cell_(biology)