Cell (biology)
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2008-07-15T01:32:57Z
WadeSimMiser
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Reverted edits by [[Special:Contributions/58.69.66.110|58.69.66.110]] to last version by ClueBot (using [[WP:HG|Huggle]])
[[Image:Cork Micrographia Hooke.png|thumb|right|300px|Drawing of the structure of [[Cork cambium|cork]] as it appeared under the microscope to [[Robert Hooke]] from [[Micrographia]] which is the origin of the word "'''cell'''" being used to describe the smallest unit of a living organism]]
[[Image:Epithelial-cells.jpg|thumb|right|300px|Cells in culture, [[staining (biology)|stain]]ed for [[keratin]] (red) and [[DNA]] (green)]]
The '''cell''' is the structural and functional unit of all known [[Life|living]] [[organism]]s. It is the smallest unit of an organism that is classified as living, and is sometimes called the building block of life.<ref name="Alberts2002">[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=Cell+Movements+and+the+Shaping+of+the+Vertebrate+Body+AND+mboc4%5Bbook%5D+AND+374635%5Buid%5D&rid=mboc4.section.3919 Cell Movements and the Shaping of the Vertebrate Body] in Chapter 21 of ''[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=cell+biology+AND+mboc4%5Bbook%5D+AND+373693%5Buid%5D&rid=mboc4 Molecular Biology of the Cell]'' fourth edition, edited by Bruce Alberts (2002) published by Garland Science.<BR> The Alberts text discusses how the "cellular building blocks" move to shape developing [[embryo]]s. It is also common to describe small molecules such as [[amino acid]]s as "[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=%22all+cells%22+AND+mboc4%5Bbook%5D+AND+372023%5Buid%5D&rid=mboc4.section.4#23 molecular building blocks]".</ref> Some organisms, such as most [[bacteria]], are [[unicellular]] (consist of a single cell). Other organisms, such as [[human]]s, are [[multicellular]]. (Humans have an estimated 100 trillion or 10<sup>14</sup> cells; a typical cell size is 10 [[micrometre|µm]]; a typical cell mass is 1 [[nanogram]].) The largest known cell is an [[ostrich]] [[egg (biology)|egg]].
In 1837 before the final cell theory was developed, a [[Czech Republic|Czech]] [[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. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the [[genetics|hereditary information]] necessary for regulating cell functions and for transmitting information to the next generation of cells.<ref>{{cite book
| last = Maton
| first = Anthea
| authorlink =
| coauthors = Hopkins, Jean Johnson, Susan LaHart, David Quon Warner, Maryanna Wright, Jill D
| title = Cells Building Blocks of Life
| publisher = Prentice Hall
| date = 1997
| location = New Jersey
| pages =
| url =
| doi =
| id =
| isbn = 0-13-423476-6}}</ref>
The word ''cell'' comes from the [[Latin]] ''cellula'', meaning, a small room. The descriptive name for the smallest living biological structure was chosen by [[Robert Hooke]] in a book he published in 1665 when he compared the [[Cork (material)|cork]] cells he saw through his microscope to the small rooms monks lived in.<ref name="Hooke">"<cite>... I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular [..] these pores, or cells, [..] were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this. . .</cite>" – Hooke describing his observations on a thin slice of cork. [http://www.ucmp.berkeley.edu/history/hooke.html Robert Hooke]</ref>
==Preamble==
Each cell is at least somewhat self-contained and self-maintaining: it can take in [[nutrient]]s, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.
[[Image:Cellsize.jpg|thumb|left|300px|Mouse cells grown in a culture dish. These cells grow in large clumps, but each individual cell is about 10 [[micrometre]]s across]]
All cells have several different abilities:<ref name="AlbertsCh1">[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=%22all+cells%22+AND+mboc4%5Bbook%5D+AND+372023%5Buid%5D&rid=mboc4.section.4#23 The Universal Features of Cells on Earth] in Chapter 1 of the Alberts textbook (reference #1, above).</ref>
* Reproduction by [[cell division]]: ([[binary fission]]/[[mitosis]] or [[meiosis]]).
* Use of [[enzyme]]s and other [[protein]]s [[genetic code|coded for]] by [[DNA]] [[gene]]s and made via [[messenger RNA]] intermediates and [[ribosome]]s.
* [[Metabolism]], including taking in raw materials, building cell components, converting [[energy]], [[molecule]]s and releasing [[by-product]]s. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is released and then used in [[metabolic pathway]]s.
* Response to external and internal [[Signal transduction|stimuli]] such as changes in temperature, [[pH]] or levels of nutrients.
* Cell contents are contained within a [[Cell membrane|cell surface membrane]] that is made from a [[lipid bilayer]] with proteins embedded in it.
Some [[prokaryotic]] cells contain important internal membrane-bound compartments,<ref>{{cite journal | last = L.M. | first = Mashburn-Warren | coauthors =Whiteley, M. | title =Special delivery: vesicle trafficking in prokaryotes. | journal =Mol Microbiol | volume =61 | issue =4| pages =839-46| date =2006 | doi =10.1111/j.1365-2958.2006.05272.x | doi = 10.1111/j.1365-2958.2006.05272.x <!--Retrieved from url by DOI bot--> | pmid = 16879642 }}</ref> but [[eukaryotic]] cells have a specialized set of [[endomembrane system|internal membrane compartments]]. Material is moved between these compartments by regulated [[Kinesin|traffic]] and [[TRAPP complex|transport]] of small spheres of membrane-bound material called [[Vesicle (biology)|''vesicles'']].<ref name="Rose2005">A. Rose, S. J. Schraegle, E. A. Stahlberg and I. Meier (2005) "Coiled-coil protein composition of 22 proteomes--differences and common themes in subcellular infrastructure and traffic control" in ''BMC evolutionary biology'' Vulume 5 article 66. {{Entrez Pubmed|16288662}}<BR>Rose ''et al.'' suggest that [[Coiled coil|coiled-coil]] alpha helical vesicle transport proteins are only found in eukaryotic organisms.</ref>
<br clear=all />
==Anatomy of cells==
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|Prokaryote}}
[[Image:Average prokaryote cell- en.svg|thumb|400px|right|Diagram of a typical [[prokaryotic]] cell]]
[[Prokaryote]]s differ from eukaryotes since they lack a nuclear membrane and a [[cell nucleus]]. Prokaryotes also lack most of the intracellular organelles and structures that are seen in eukaryotic cells. There are two kinds of prokaryotes, [[bacteria]] and [[archaea]], but these are similar in the overall structures of their cells. Most functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic cell's plasma membrane. Prokaryotic cells have three architectural regions: appendages called [[flagella]] and [[Pilus|pili]] — proteins attached to the cell surface; a [[cell envelope]] - consisting of a capsule, a [[cell wall]], and a [[plasma membrane]]; and a [[cytoplasm|cytoplasmic region]] that contains the [[genome|cell genome]] (DNA) and ribosomes and various sorts of inclusions. Other differences include:
* The ''plasma membrane'' (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
* Most prokaryotes have a ''[[cell wall]]'' (some exceptions are ''[[Mycoplasma]]'' (bacteria) and ''[[Thermoplasma]]'' (archaea)). This wall consists of ''[[peptidoglycan]]'' in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" ([[cytolysis]]) from [[osmotic pressure]] against a [[Tonicity#Hypotonicity|hypotonic]] environment. A cell wall is also present in some eukaryotes like plants (cellulose) and [[fungi]], but has a different chemical composition.
* A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium ''Borrelia burgdorferi'', which causes [[Lyme disease]]). Even without a real ''nucleus'', the [[DNA]] is condensed in a ''nucleoid''. Prokaryotes can carry [[extrachromosomal DNA]] elements called ''[[plasmid]]s'', which are usually circular. Plasmids can carry additional functions, such as [[antibiotic resistance]].
===Eukaryotic cells===
{{main|Eukaryote}}
[[Image:Biological cell.svg|thumb|400px|Diagram of a typical [[animalia|animal]] ([[eukaryotic]]) cell, showing subcellular components.<br/>
[[Organelle]]s:<br/>
(1) [[nucleolus]]<br/>
(2) [[cell nucleus|nucleus]]<br/>
(3) [[ribosome]]<br/>
(4) [[vesicle (biology)|vesicle]]<br/>
(5) [[rough endoplasmic reticulum]] (ER)<br/>
(6) [[Golgi apparatus]]<br/>
(7) [[Cytoskeleton]]<br/>
(8) [[smooth endoplasmic reticulum]]<br/>
(9) [[mitochondrion|mitochondria]]<br/>
(10) [[vacuole]]<br/>
(11) [[cytoplasm]]<br/>
(12) [[lysosome]]<br/>
(13) [[centriole]]s within [[centrosome]]]]
[[Eukaryote|Eukaryotic]] cells are about 10 times the size of 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 the presence of a [[cell nucleus]], a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that 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 [[chromosome]]s, which are associated with [[histone]] proteins. All chromosomal DNA is stored in the ''[[cell nucleus]]'', separated from the cytoplasm by a membrane. Some eukaryotic [[organelle]]s such as [[mitochondria]] also contain some DNA.
* Eukaryotes can move using ''cilia'' or ''flagella''. The flagella are more complex than those of prokaryotes.
{| align="center" class="toccolours" border="1" style="border:1px solid gray; border-collapse:collapse; clear: both;"
|+'''Table 1: Comparison of features of prokaryotic and eukaryotic cells'''
|-
|
!Prokaryotes
!Eukaryotes
|-
!Typical organisms
|[[bacterium|bacteria]], [[archaea]]
|[[protist]]s, [[fungus|fungi]], [[plant]]s, [[animal]]s
|-
!Typical size
|~ 1-10 [[µm]]
|~ 10-100 [[µm]] ([[spermatozoon|sperm cells]], apart from the tail, are smaller)
|-
!Type of [[cell nucleus|nucleus]]
|[[nucleoid region]]; no real nucleus
|real nucleus with double membrane
|-
!DNA
|circular (usually)
|linear molecules ([[chromosome]]s) with [[histone]] [[protein]]s
|-
!RNA-/protein-synthesis
|coupled in [[cytoplasm]]
|RNA-synthesis inside the nucleus<br />protein synthesis in cytoplasm
|-
![[Ribosome]]s
|50S+30S
|60S+40S
|-
!Cytoplasmatic structure
|very few structures
|highly structured by endomembranes and a [[cytoskeleton]]
|-
![[Chemotaxis|Cell movement]]
|[[Flagellum|flagella]] made of [[flagellin]]
|flagella and [[Cilium|cilia]] containing [[microtubule]]s; [[lamellipodia]] and [[filopodia]] containing [[actin]]
|-
![[Mitochondrium|Mitochondria]]
|none
|one to several thousand (though some lack mitochondria)
|-
![[Chloroplast]]s
|none
|in [[algae]] and [[plant]]s
|-
!Organization
|usually single cells
|single cells, colonies, higher multicellular organisms with specialized cells
|-
![[Cell division]]
|[[Binary fission]] (simple division)
|[[Mitosis]] (fission or budding) <br />[[Meiosis]]
|}
{| align="center" class="toccolours" border="1" style="border:1px solid gray; border-collapse:collapse;"
|+'''Table 2: Comparison of structures between animal and plant cells'''
|-
|
!Typical animal cell
!Typical plant cell
|- valign="top"
!Organelles
|
* [[Cell nucleus|Nucleus]]
** [[Nucleolus]] (within nucleus)
* Rough [[endoplasmic reticulum]] (ER)
* [[Smooth ER]]
* [[Ribosome]]s
* [[Cytoskeleton]]
* [[Golgi apparatus]]
* [[Cytoplasm]]
* [[Mitochondrion|Mitochondria]]
* [[vesicle (biology)|Vesicles]]
* [[Lysosome]]s
* [[Centrosome]]
** [[Centriole]]s
* [[Vacuole]]s
|
* [[Cell nucleus|Nucleus]]
** [[Nucleolus]] (within nucleus)
* Rough ER
* Smooth ER
* [[Ribosomes]]
* [[Cytoskeleton]]
* [[Golgi apparatus]] ([[dictiosome]]s)
* [[Cytoplasm]]
* [[Mitochondrion|Mitochondria]]
* [[vesicle (biology)|Vesicles]]
* [[Chloroplast]] and other [[plastid]]s
* [[Central vacuole]](large)
** [[Tonoplast]] (central vacuole membrane)
* [[Peroxisome]] (e.g. [[Glyoxysome]])
* [[Vacuole]]s
|- valign="top"
!Additional structures
|
* [[Plasma membrane]]
* [[Flagellum]]
* [[Cilium]]
|
* [[Plasma membrane]]
* [[Flagellum]] (only in gametes)
* [[Cell wall]]
* [[Plasmodesmata]]
|}
==Subcellular components==
[[Image:celltypes.svg|thumbnail|300px|The cells of eukaryotes (left) and prokaryotes (right).]]
All cells, whether [[prokaryotic]] or [[eukaryotic]], have a [[cell membrane|membrane]] that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the [[cell potential|electric potential of the cell]]. Inside the membrane, a [[salt]]y [[cytoplasm]] takes up most of the cell volume. All cells possess [[DNA]], the hereditary material of [[gene]]s, and [[RNA]], containing the information necessary to [[gene expression|build]] various [[protein]]s such as [[enzyme]]s, the cell's primary machinery. There are also other kinds of [[biomolecule]]s in cells. This article will list these primary components of the cell, then briefly describe their function.
===Cell membrane: A cell's defining boundary===
{{main|Cell membrane}}
The cytoplasm of a cell is surrounded by a cell membrane or ''plasma membrane''. The plasma membrane in plants and prokaryotes is usually covered by a [[cell wall]]. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a [[lipid bilayer|double layer of lipids]] ([[hydrophobic]] fat-like molecules) and [[hydrophilic]] [[phosphorus]] molecules. Hence, the layer is called a [[phospholipid bilayer]]. It may also be called a fluid mosaic membrane. Embedded within this membrane is a variety of [[protein]] molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance ([[molecule]] or [[ion]]) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain [[Receptor (biochemistry)#Transmembrane receptors|receptor]] proteins that allow cells to detect external signalling molecules such as [[hormone]]s.
===Cytoskeleton: A cell's scaffold===
{{main|Cytoskeleton}}
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during [[endocytosis]], the uptake of external materials by a cell, and [[cytokinesis]], the separation of daughter cells after [[cell division]]; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of [[microfilament]]s, [[intermediate filament]]s and [[microtubule]]s. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.<ref>{{cite journal |author=Michie K, Löwe J |title=Dynamic filaments of the bacterial cytoskeleton |journal=Annu Rev Biochem |volume=75 |issue= |pages=467–92 |year=2006 |pmid=16756499 | doi = 10.1146/annurev.biochem.75.103004.142452 <!--Retrieved from PMID by DOI bot-->}}</ref>
===Genetic material===
Two different kinds of genetic material exist: [[DNA|deoxyribonucleic acid]] (DNA) and [[RNA|ribonucleic acid]] (RNA). Most organisms use DNA for their long-term information storage, but [[RNA virus|some viruses]] (e.g., [[retrovirus]]es) have RNA as their genetic material. The biological information contained in an organism is [[Genetic code|encoded]] in its DNA or RNA sequence. RNA is also used for information transport (e.g., [[mRNA]]) and [[enzyme|enzymatic]] functions (e.g., [[ribosome|ribosomal]] RNA) in organisms that use [[DNA]] for the genetic code itself. [[Transfer RNA]] (tRNA) molecules are used to add specific amino acids during the process of protein [[translation]].
Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial [[chromosome]]) in the [[nucleoid region]] of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called [[chromosome]]s inside a discrete nucleus, usually with additional genetic material in some organelles like [[mitochondria]] and [[chloroplasts]] (see [[endosymbiotic theory]]).
A human cell has genetic material in the nucleus (the [[genome|nuclear genome]]) and in the mitochondria (the [[mitochondrial genome]]). In humans the nuclear genome is divided into 23 pairs of linear DNA molecules called [[chromosome]]s. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the [[mitochondrial DNA]] is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production as well as specific tRNAs.
Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called [[transfection]]. This can be transient, if the DNA is not inserted into the cell's [[genome]], or stable, if it is. Certain [[virus]]es also insert their genetic material into the genome.
===Organelles===
{{main|Organelle}}
The human body contains many different [[Organ (anatomy)|organs]], such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called [[organelle]]s, that are adapted and/or specialized for carrying out one or more vital functions. Membrane-bound organelles are <!--"only" INCORRECT here: misplaced modifier; placed before the prepositional phrase--> found only in eukaryotes.
There are several types of organelles within an animal cell. Some (such as the [[nucleus]] and [[golgi apparatus]]) are typically solitary, while others (such as [[mitochondria]], [[peroxisomes]] and [[lysosomes]]) can be numerous (hundreds to thousands). The [[cytoplasm]] is the gelatinous fluid that fills the cell and surrounds the organelles.
{|
|-
|
; Cell nucleus (a cell's information center) : The [[cell nucleus]] is the most conspicuous organelle found in a [[eukaryotic]] cell. It houses the cell's [[chromosomes]], and is the place where almost all [[DNA]] replication and [[RNA]] [[synthesis]] ([[Transcription (genetics)|transcription]]) occur. The nucleus is spherical in shape and separated from the cytoplasm by a double membrane called the [[nuclear envelope]]. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, [[DNA]] is [[Transcription (genetics)|transcribed]], or copied into a special [[RNA]], called [[mRNA]]. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The [[nucleolus]] is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the [[cytoplasm]].
|[[Image:Diagram human cell nucleus no text.png|300px|Diagram of a cell nucleus]]
|-
| colspan="2" |
; Mitochondria and Chloroplasts (the power generators) : [[Mitochondrion|Mitochondria]] are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy by the process of [[oxidative phosphorylation]], utilizing [[oxygen]] to release energy stored in cellular nutrients (typically pertaining to [[glucose]]) to generate [[adenosine triphosphate|ATP]]. Mitochondria multiply by splitting in two.
Organelles that are modified chloroplasts are broadly called [[plastid]]s, and are involved in energy storage through the process of [[photosynthesis]], which utilizes solar energy to generate carbohydrates and oxygen from carbon dioxide and water.
Mitochondria and chloroplasts each contain their own genome, which is separate and distinct from the nuclear genome of a cell. Both of these organelles contain this DNA in circular plasmids, much like prokaryotic cells, strongly supporting the evolutionary theory of [[endosymbiosis]]; since these organelles contain their own genomes and have other similarities to prokaryotes, they are thought to have developed through a symbiotic relationship after being engulfed by a primitive cell.
|-
|
; Endoplasmic reticulum (eukaryotes only) : The [[endoplasmic reticulum]] (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in calcium sequestration and release.
|-
|
; Golgi apparatus (eukaryotes only) : The primary function of the Golgi apparatus is to process and package the [[macromolecule]]s such as [[protein]]s and [[lipid]]s that are synthesized by the cell. It is particularly important in the processing of proteins for [[secretion]]. The Golgi apparatus forms a part of the [[endomembrane system]] of eukaryotic cells. [[Vesicles]] that enter the Golgi apparatus are processed in a cis to trans direction, meaning they coalesce on the cis side of the apparatus and after processing pinch off on the opposite (trans) side to form a new vesicle in the animal cell.
|[[Image:Endomembrane system diagram no text nucleus.png|300px|Diagram of an endomembrane system]]
|-
| colspan="2" |
; Ribosomes (the protein production centers in the cell) : The [[ribosome]] is a large complex of [[RNA]] and [[protein]], composed of many molecules. In prokaryotes, ribosomes only exist floating freely in the cytosol, whereas in eukaryotes they can be found either free or bound to membranes.
|-
| colspan="2" |
; [[Lysosomes]] and [[Peroxisomes]] (eukaryotes only) : Lysosomes contain [[digestive enzyme]]s (acid [[hydrolase]]s). They digest excess or worn-out [[organelle]]s, food particles, and engulfed [[virus]]es or [[bacteria]]. Peroxisomes have enzymes that rid the cell of toxic [[peroxide]]s. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. These organelles are often called a "suicide bag" because of their ability to detonate and destroy the cell.
|-
| colspan="2" |
; Centrosome (the cytoskeleton organiser) : The [[centrosome]] produces the [[microtubules]] of a cell - a key component of the [[cytoskeleton]]. It directs the transport through the [[endoplasmic reticulum|ER]] and the [[Golgi apparatus]]. Centrosomes are composed of two [[centrioles]], which separate during [[cell division]] and help in the formation of the [[mitotic spindle]]. A single centrosome is present in the [[animal cell]]s. They are also found in some fungi and algae cells.
|-
| colspan="2" |
; Vacuoles : [[Vacuole]]s store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably ''[[Amoeba]]'', have contractile vacuoles, which are able to pump water out of the cell if there is too much water.
==Structures outside the cell wall==
===Capsule===
It is present only in some bacteria outside the cell wall. It is gelatinous in nature. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as bacillus anthracis or hyaluronic acid as in streptococci.
Capsules not stained by ordinary stain and can detected by special stain. The capsule is antigenic. The capsule has antiphagocytic function so it determines the virulence of many bacteria. It also plays a role in attachment of the organism to mucous membranes.
===Flagella===
[[Flagella]] are the organ of mobility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread like appendages, protein in nature, formed of flagellin protein (antigenic). They can not be stained by gram stain. They have a special stain. According to their arrangement they may be monotrichate, amphitrichate, lophotrichate, peritrichate.
===Fimbriae (pili)===
They are short and thin hair like filaments, formed of protein called pilin (antigenic). [[Fimbriae]] are responsible for attachement of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili) involved in the process of conjunction.
==Cell functions==
===Cell growth and metabolism===
{{main|Cell growth|Metabolism}}
Between successive cell divisions, cells grow through the functioning of cellular metabolism.
Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: [[catabolism]], in which the cell breaks down complex molecules to produce energy and reducing power, and [[anabolism]], in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.
Complex sugars consumed by the organism can be broken down into a less chemically-complex sugar molecule called [[glucose]]. Once inside the cell, glucose is broken down to make adenosine triphosphate ([[adenosine triphosphate|ATP]]), a form of energy, via two different pathways.
The first pathway, [[glycolysis]], requires no oxygen and is referred to as [[Fermentation (biochemistry)|anaerobic metabolism]]. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.
The second pathway, called the Krebs cycle, or [[citric acid cycle]], occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.
[[Image:Proteinsynthesis.png|frame|300px|An overview of protein synthesis.<br/>Within the [[cell nucleus|nucleus]] of the cell (''light blue''), [[gene]]s (DNA, ''dark blue'') are [[transcription (genetics)|transcribed]] into [[RNA]]. This RNA is then subject to post-transcriptional modification and control, resulting in a mature [[mRNA]] (''red'') that is then transported out of the nucleus and into the [[cytoplasm]] (''peach''), where it undergoes [[translation (genetics)|translation]] into a protein. mRNA is translated by [[ribosome]]s (''purple'') that match the three-base [[codon]]s of the mRNA to the three-base anti-codons of the appropriate [[transfer RNA|tRNA]]. Newly-synthesized proteins (''black'') are often further modified, such as by binding to an effector molecule (''orange''), to become fully active.]]
===Creation of new cells===
{{main|Cell division}}
Cell division involves a single cell (called a ''mother cell'') dividing into two daughter cells. This leads to growth in [[multicellular organism]]s (the growth of [[biological tissue|tissue]]) and to procreation ([[vegetative reproduction]]) in [[unicellular organism]]s.
[[Prokaryote|Prokaryotic]] cells divide by [[binary fission]]. [[Eukaryote|Eukaryotic]] cells usually undergo a process of nuclear division, called [[mitosis]], followed by division of the cell, called [[cytokinesis]]. A [[diploid]] cell may also undergo [[meiosis]] to produce haploid cells, usually four. [[Haploid]] cells serve as [[gamete]]s in multicellular organisms, fusing to form new diploid cells.
[[DNA replication]], or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.
===Protein synthesis===
{{main|Protein biosynthesis}}
Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from [[amino acid]] building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: [[transcription (genetics)|transcription]] and [[translation (genetics)|translation]].
Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give [[messenger RNA]] (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called [[ribosome]]s located in the [[cytosol]], where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to [[transfer RNA]] (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.
==Cell movement or motility ==
Cell has the ability to move spontaneously during the process of wound healing, immune response and cancer metastasis. The fastest moving cells in the human body are the spermaculi, which enter and exit through the penis. This was proven when Professor Julia Ertmann of UCLA observed changes in the reproductive spermaculi in a lab environment (circa 2005).
For wound healing to occur, white blood cells and cells that ingest bacteria move to the wound site to kill the microorganisms that cause infection. A the same time fibroblasts (connective tissue cells) move there to remodel damaged structures. In the case of tumor development, cells from a primary tumor move away and spread to other parts of the body. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.<ref>[http://www.biolsci.org/v03p0303.htm The Forces Behind Cell Movement<!-- Bot generated title -->]</ref> The process is divided into three steps - protrusion of the leading edge of the cell, adhesion of the leading edge and deadhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each of these steps is driven by physical forces generated by unique segments of the cytoskeleton.<ref name="AlbertsB">Alberts B, Johnson A, Lewis J. et al. Molecular Biology of the Cell, 4e. Garland Science. 2002</ref><ref name="Ananthakrishnan">Ananthakrishnan R, Ehrlicher A. The Forces Behind Cell Movement. Int J Biol Sci 2007; 3:303-317. http://www.biolsci.org/v03p0303.htm</ref>
==Origins of cells==<!-- This section is linked from [[Timeline of evolution]] -->
{{main|Origin of life}}
The origin of cells has to do with the origin of life, which began the [[timeline of evolution|history of life]] on Earth. The birth of the cell marked the passage from prebiotic chemistry to biological life.
===Origin of the first cell===
:''For more information, [[RNA world hypothesis]]''
:''For more information, [[Last universal ancestor]]''
The [[unit of selection]] in modern organisms and populations of organisms is not clear, with [[natural selection]] being proposed to work at the level of genes, cells, individual organisms, groups of organisms and even species.<ref name=Gould>{{cite journal |author=Gould SJ |title=Gulliver's further travels: the necessity and difficulty of a hierarchical theory of selection |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9533127 |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=353 |issue=1366 |pages=307–14 |year=1998 |pmid=9533127 |doi=10.1098/rstb.1998.0211}}</ref><ref>{{cite journal |author=Mayr E |title=The objects of selection |url=http://www.pnas.org/cgi/content/full/94/6/2091 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=94 |issue=6 |pages=2091–94 |year=1997 |pmid=9122151 | doi = 10.1073/pnas.94.6.2091 }}</ref> None of these models are mutually-exclusive and selection may act on multiple levels simultaneously.<ref>{{cite journal |author=Maynard Smith J |title=The units of selection |journal=Novartis Found. Symp. |volume=213 |pages=203–11; discussion 211–17 |year=1998 |pmid=9653725}}</ref> However, in a [[gene-centered view of evolution]], life is regarded in terms of [[Survival machine|replicator]]s—that is the [[DNA]] molecules in the organism. If freely-floating DNA molecules that code for [[enzyme]]s are not enclosed in cells, the enzymes that benefit a given replicator (for example, by producing nucleotides) may do so less efficiently, and may in fact benefit competing replicators. If the entire DNA molecule of a replicator is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The replicator will directly benefit from its encoded enzymes.
Biochemically, cell-like spheroids formed by [[proteinoid]]s are observed by heating [[amino acid]]s with [[phosphoric acid]] as a [[catalyst]]. They bear many of the basic features provided by [[cell membrane]]s. Proteinoid-based protocells enclosing RNA molecules may have been the first cellular life forms on Earth. Some [[amphiphiles]] have the tendency to spontaneously form membranes in water. A spherically closed membrane contains water and is a hypothetical precursor to the modern cell membrane composed of [[proteins]] and [[phospholipid]] [[Lipid bilayer|bilayer]] membranes.
===Origin of eukaryotic cells===
The eukaryotic cell seems to have evolved from a [[symbiosis|symbiotic community]] of prokaryotic cells. It is almost certain that DNA-bearing organelles like the [[mitochondria]] and the [[chloroplasts]] are what remains of ancient symbiotic oxygen-breathing [[proteobacteria]] and [[cyanobacteria]], respectively, where the rest of the cell seems to be derived from an ancestral [[archaea]]n prokaryote cell – a theory termed the [[endosymbiotic theory]].
There is still considerable debate about whether organelles like the [[hydrogenosome]] predated the origin of [[mitochondria]], or viceversa: see the [[hydrogen hypothesis]] for the origin of eukaryotic cells.
Sex, as the stereotyped choreography of meiosis and syngamy that persists in nearly all extant eukaryotes, may have played a role in the transition from prokaryotes to eukaryotes. An 'origin of sex as vaccination' theory suggests that the eukaryote genome accreted from prokaryan parasite genomes in numerous rounds of lateral gene transfer. Sex-as-syngamy (fusion sex) arose when infected hosts began swapping nuclearized genomes containing coevolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.<ref name="sterrer">{{cite journal | author = Sterrer W | title = On the origin of sex as vaccination | journal = Journal of Theoretical Biology | volume = 216 | pages = 387-396 | year = 2002 | pmid = 12151256 | doi = 10.1006/jtbi.2002.3008 }}</ref>
==History==
* 1632 – 1723: [[Antonie van Leeuwenhoek]] teaches himself to grind [[Lens (optics)|lenses]], builds a [[microscope]] and draws [[protozoa]], such as ''[[Vorticella]]'' from rain water, and [[Bacterium|bacteria]] from his own mouth.
* 1665: [[Robert Hooke]] discovers cells in cork, then in living plant tissue using an early microscope.<ref name="Hooke" />
* 1839: [[Theodor Schwann]] and [[Matthias Jakob Schleiden]] elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
* The belief that life forms are able to occur spontaneously (''[[Abiogenesis|generatio spontanea]]'') is contradicted by [[Louis Pasteur]] (1822 – 1895) (although [[Francesco Redi]] had performed an experiment in 1668 that suggested the same conclusion).
* 1855: [[Rudolph Virchow]] states that cells always emerge from [[cell division]]s (''omnis cellula ex cellula'').
* 1931: [[Ernst Ruska]] builds first [[transmission electron microscope]] (TEM) at the [[University of Berlin]]. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously-unresolvable organelles.
* 1953: [[James D. Watson|Watson]] and [[Francis Crick|Crick]] made their first announcement on the double-[[helix]] structure for DNA on February 28.
* 1981: [[Lynn Margulis]] published ''Symbiosis in Cell Evolution'' detailing the [[endosymbiotic theory]].
==See also==
<div class="references-small" style="-moz-column-count:2; column-count:2;">
* [[Cell biology]]
* [[List of basic cell biology topics]]
* [[Cell culture]]
* [[Cell type]]s
* [[Cellular component]]
* [[Cytorrhysis]]
* [[Cytotoxicity]]
* [[Plant cell]]
* [[Plasmolysis]]
* [[Stem cell]]
* [[Syncytium]]
</div>
==References==
{{reflist}}
* {{NCBI-scienceprimer}}
==External links==
{{commons|Cell (biology)}}
{{wikibookspar||Cell Biology}}
* [http://vcell.ndsu.nodak.edu/animations/ Virtual Cell's Educational Animations].
* [http://www.ixedu.com/es/celula/Introduccion.html Ixedu.com] 3D Animations, Virtual Microscope, Activities, a Game and more! All about the cells.
* [http://www.studiodaily.com/main/searchlist/6850.html The Inner Life of A Cell], a flash video showing what happens inside of a cell
* [http://www.ibiblio.org/virtualcell/tour/cell/cell.htm The Virtual Cell]
* [http://www.cellsalive.com/ Cells Alive!]
* [http://www.jcb.org/ Journal of Cell Biology]
* [http://members.optusnet.com.au/exponentialist/Cells.htm A comparison of the generational and exponential growth of cell populations]
* [http://brainmaps.org/index.php?q=cell High-resolution images of brain cells]
* [http://www.biology.arizona.edu/cell_bio/cell_bio.html The Biology Project > Cell Biology]
* [http://cellimages.ascb.org/ The Image & Video Library] of [http://www.ascb.org/ The American Society for Cell Biology], a collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function and biology of the cell.
* [http://www.centreofthecell.org/ Centre of the Cell online]
* [http://publications.nigms.nih.gov/insidethecell/ Inside the Cell]
===Online textbooks===
* {{cite book | author = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P | title = Molecular Biology of the Cell | edition = 4th ed.| publisher = Garland | year = 2002 | url = http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.TOC&depth=2 | isbn = 0815332181}}
* {{cite book | author = Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipurksy SL, Darnell J | title = Molecular Cell Biology | edition = 5th ed. | publisher = WH Freeman: New York, NY | year = 2004 | url = http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.TOC | isbn = 978-0716743668}}
* {{cite book |author=Cooper GM |title=The cell: a molecular approach | edition = 2nd ed. | publisher=ASM Press |location=Washington, D.C |year=2000 |pages= |isbn=0-87893-102-3 | url = http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=cooper.TOC&depth=2 }}
* Gall JG, McIntosh JR, eds (2001).''[http://cellimages.ascb.org/cdm4/browse.php?CISOROOT=/p4041coll11 Landmark Papers in Cell Biology]''. Bethesda, MD and Cold Spring Harbor, NY: The American Society for Cell Biology and Cold Spring Harbor Laboratory Press; 2001. Commentaries and links to original research papers published in the ASCB Image & Video Library
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