Editing Cell (biology) (section)

== Cell types ==

{{main|Cell type}}

Cells are broadly categorized into two types: [[eukaryotic cell]]s, which possess a [[Cell nucleus|nucleus]], and [[prokaryotic|prokaryotic cell]]s, which lack a nucleus but have a nucleoid region. Prokaryotes are [[single-celled organism]]s, whereas eukaryotes can be either single-celled or [[multicellular organism|multicellular]].

=== Prokaryotic cells ===

{{main|Prokaryote}}

[[File:Prokaryote cell.svg|thumb|upright=1.25|Structure of a typical [[prokaryotic]] cell]]

[[Prokaryote]]s include [[bacteria]] and [[archaea]], two of the [[three domain system|three]] [[Domain (biology)|domains of life]]. Prokaryotic cells were the first form of [[life]] on Earth, characterized by having vital [[biological process]]es including [[cell signaling]]. They are simpler and smaller than eukaryotic cells, and lack a [[cell nucleus|nucleus]], and other membrane-bound [[organelle]]s. The [[DNA]] of a prokaryotic cell consists of a single [[Circular prokaryote chromosome|circular chromosome]] that is in direct contact with the [[cytoplasm]]. The nuclear region in the cytoplasm is called the [[nucleoid]]. Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0&nbsp;μm in diameter.<ref name="Black 2004 p. ">{{cite book |last=Black |first=Jacquelyn G. |title=Microbiology |publisher=Wiley |publication-place=New York Chichester |date=2004 |isbn=978-0-471-42084-2 |page=}}</ref>{{rp|78}}

A prokaryotic cell has three regions:

* Enclosing the cell is the [[cell envelope]], generally consisting of a [[plasma membrane]] covered by a [[cell wall]] which, for some bacteria, may be further covered by a third layer called a [[bacterial capsule|capsule]]. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as ''[[Mycoplasma]]'' (bacteria) and ''[[Thermoplasma]]'' (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. 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 bursting ([[cytolysis]]) from [[osmotic pressure]] due to a [[hypotonic]] environment. Some eukaryotic cells ([[plant cell]]s and [[fungal]] cells) also have a cell wall.
* Inside the cell is the [[cytoplasm|cytoplasmic region]] that contains the [[genome]] (DNA), ribosomes and various sorts of inclusions.<ref name="NCBI">{{NCBI-scienceprimer |article=What Is a Cell? |url=https://web.archive.org/web/20130503014839/http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html |access-date=3 May 2013 |date=30 March 2004}}</ref> The genetic material is freely found in the cytoplasm. Prokaryotes can carry [[extrachromosomal DNA]] elements called [[plasmid]]s, which are usually circular. Linear bacterial plasmids have been identified in several species of [[spirochete]] bacteria, including members of the genus ''[[Borrelia]]'' notably ''[[Borrelia burgdorferi]]'', which causes Lyme disease.<ref>European Bioinformatics Institute, [http://www.ebi.ac.uk/2can/genomes/bacteria/Borrelia_burgdorferi.html Karyn's Genomes: Borrelia burgdorferi] {{Webarchive|url=https://web.archive.org/web/20130506040937/http://www.ebi.ac.uk/2can/genomes/bacteria/Borrelia_burgdorferi.html |date=2013-05-06 }}, part of 2can on the EBI-EMBL database. Retrieved 5 August 2012</ref> Though not forming a nucleus, the [[DNA]] is condensed in a [[nucleoid]]. Plasmids encode additional genes, such as [[antibiotic resistance]] genes.
* On the outside, some prokaryotes have [[flagella]] and [[Pilus|pili]] that project from the cell's surface. These are structures made of proteins that facilitate movement and communication between cells.

=== Eukaryotic cells ===

{{main|Eukaryote}}

[[File:Animal cell structure en.svg|thumb|upright=1.25|Structure of a typical animal cell]]
[[File:Plant cell structure-en.svg|thumb|upright=1.25|Structure of a typical [[plant cell]]]]

[[Plants]], [[animals]], [[fungi]], [[slime mould]]s, [[protozoa]], and [[algae]] are all [[eukaryotic]]. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is [[Cellular compartment|compartmentalization]]: the presence of membrane-bound [[organelle]]s (compartments) in which specific activities take place. Most important among these is a [[cell nucleus]],<ref name="NCBI"/> an organelle that houses the cell's [[DNA]]. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Some of the other differences are:

* 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.<ref name="NCBI"/> Some eukaryotic organelles such as [[mitochondria]] also contain some DNA.
* Many eukaryotic cells are [[cilium|ciliated]] with [[primary cilia]]. Primary cilia play important roles in chemosensation, [[mechanosensation]], and [[thermosensation]]. Each cilium may thus be "viewed as a sensory cellular [[Antenna (biology)|antennae]] that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."<ref name="Christenson2008">{{cite journal |last1=Satir |first1=P. |last2=Christensen |first2=Søren T. |title=Structure and function of mammalian cilia |journal=Histochemistry and Cell Biology |volume=129 |issue=6 |pages=687–693 |date=June 2008 |pmid=18365235 |pmc=2386530 |doi=10.1007/s00418-008-0416-9 |id=1432-119X }}</ref>
* Motile eukaryotes can move using [[motile cilia]] or [[flagella]]. Motile cells are absent in [[conifer]]s and [[flowering plant]]s.{{cn|date=September 2023}} Eukaryotic flagella are more complex than those of prokaryotes.<ref>{{cite journal |last1=Blair |first1=D. F. |last2=Dutcher |first2=S. K. |title=Flagella in prokaryotes and lower eukaryotes |journal=Current Opinion in Genetics & Development |volume=2 |issue=5 |pages=756–767 |date=October 1992 |pmid=1458024 |doi=10.1016/S0959-437X(05)80136-4 }}</ref>

{|class="wikitable" style="margin-left: auto; margin-right: auto;"
|+Comparison of features of prokaryotic and eukaryotic cells
|-
!
![[Prokaryote]]s
![[Eukaryote]]s
|-
!Typical organisms
|[[bacterium|bacteria]], [[archaea]]
|[[protist]]s, [[algae]], [[fungus|fungi]], [[plant]]s, [[animal]]s
|-
!Typical size
|~ 1–5&nbsp;[[Micrometre|μm]]<ref name="CampbellBiology320">{{cite book |title=Campbell Biology{{snd}}Concepts and Connections |publisher=Pearson Education |year=2009 |page=320}}</ref>
|~ 10–100&nbsp;μm<ref name=CampbellBiology320 />
|-
![[DNA]]
|In [[nucleoid region]]
|In [[cell nucleus|nucleus]] with double membrane
|-
![[Chromosome]]s
|Single, usually [[Circular prokaryote chromosome|circular]]
|Multiple paired linear chromosomes with [[histone]] [[protein]]s
|-
![[RNA]]/[[protein]] synthesis
|coupled in the [[cytoplasm]]
|[[Transcription (genetics)|RNA synthesis]] in the nucleus<br />[[Translation (biology)|protein synthesis]] in the cytoplasm
|-
![[Ribosome]]s
|[[50S]] and [[30S]]
|[[60S]] and [[40S]]
|-
!Cytoplasmic structure
|Microcompartments with proteins, [[cytoskeleton]]
| [[Endomembrane system]], cytoskeleton
|-
![[Chemotaxis|Cell movement]]
|[[Flagellum|flagella]]
|flagella and [[Cilium|cilia]]; [[lamellipodia]] and [[filopodia]]
|-
![[Mitochondrion|Mitochondria]]
|none
|one to several thousand
|-
![[Chloroplast]]s
|none
|in [[algae]] and [[plant]]s
|-
!Organization
|single cells, colonies, [[biofilm]]s
|single cells, colonies, [[Multicellular organism|multicellular organisms]] with specialized cells
|-
![[Cell division]]
|[[binary fission]] (simple division)
|[[mitosis]] (fission or budding) <br />[[meiosis]]
|-
![[Membrane]]s
|[[cell membrane]]
|Cell membrane and membrane-bound organelles
|}

Many groups of eukaryotes are single-celled. Among the many-celled groups are animals and plants. The number of cells in these groups vary with species; it has been estimated that the [[human body]] contains around 37 trillion (3.72×10<sup>13</sup>) cells,<ref>{{Cite journal |last1=Bianconi |first1=Eva |last2=Piovesan |first2=Allison |last3=Facchin |first3=Federica |last4=Beraudi |first4=Alina |last5=Casadei |first5=Raffaella |last6=Frabetti |first6=Flavia |last7=Vitale |first7=Lorenza |last8=Pelleri |first8=Maria Chiara |last9=Tassani |first9=Simone |last10=Piva |first10=Francesco |last11=Perez-Amodio |first11=Soledad |date=2013-11-01 |title=An estimation of the number of cells in the human body |journal=Annals of Human Biology |volume=40 |issue=6 |pages=463–471 |doi=10.3109/03014460.2013.807878 |issn=0301-4460 |pmid=23829164 |hdl=11585/152451 |s2cid=16247166 |doi-access=free }}</ref> and more recent studies put this number at around 30 trillion (~36 trillion cells in the male, ~28 trillion in the female).<ref name=":0">{{Cite journal |last1=Hatton |first1=Ian A. |last2=Galbraith |first2=Eric D. |last3=Merleau |first3=Nono S. C. |last4=Miettinen |first4=Teemu P. |last5=Smith |first5=Benjamin McDonald |last6=Shander |first6=Jeffery A. |date=2023-09-26 |title=The human cell count and size distribution |journal=Proceedings of the National Academy of Sciences |language=en |volume=120 |issue=39 |pages=e2303077120 |doi=10.1073/pnas.2303077120 |issn=0027-8424 |pmc=10523466 |pmid=37722043|bibcode=2023PNAS..12003077H }}</ref>