Editing Cell (biology)

{{Short description|Basic unit of many life forms}}
{{About|the basic unit of lifeforms|the branch of biology that studies them|Cell biology}}
{{Pp|small=yes}}
{{pp-move-indef}}
{{Infobox anatomy
|Name =Cell
|Image =Wilson1900Fig2.jpg
|Caption =[[Onion]] (''[[Allium cepa]]'') root cells in different phases of the [[cell cycle]] (drawn by [[Edmund Beecher Wilson|E.&nbsp;B. Wilson]], 1900)
|Image2 =celltypes.svg
|Caption2 =A [[eukaryotic]] cell (left) and [[prokaryotic]] cell (right)
}}

The '''cell''' is the basic structural and functional unit of all [[life|forms of life]]. Every cell consists of [[cytoplasm]] enclosed within a [[Cell membrane|membrane]]; many cells contain [[organelle]]s, each with a specific function. The term comes from the [[Latin]] word {{lang|la|cellula}} meaning 'small room'. Most cells are only visible under a [[light microscope|microscope]]. Cells [[Abiogenesis|emerged on Earth]] about 4 billion years ago. All cells are capable of [[Self-replication|replication]], [[protein synthesis]], and [[cell motility|motility]]. 

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 such as [[bacteria]], whereas eukaryotes can be either single-celled, such as [[amoeba]]e, or [[multicellular organism|multicellular]], such as some [[algae]], [[plant]]s, [[animal]]s, and [[fungi]]. Eukaryotic cells contain organelles including [[Mitochondrion|mitochondria]], which provide energy for cell functions, [[chloroplast]]s, which in plants create sugars by [[photosynthesis]], and [[ribosome]]s, which synthesise proteins.

Cells were discovered by [[Robert Hooke]] in 1665, who named them after their resemblance to [[Monastic cell|cells]] inhabited by [[Christian monasticism|Christian monks]] in a monastery. [[Cell theory]], developed in 1839 by [[Matthias Jakob Schleiden]] and [[Theodor Schwann]], states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells.

== 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>

== Subcellular components ==

All cells, whether [[prokaryotic]] or [[eukaryotic]], have a [[cell membrane|membrane]] that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the [[Membrane potential|electric potential of the cell]]. Inside the membrane, the [[cytoplasm]] takes up most of the cell's volume. Except [[red blood cell]]s, which lack a cell nucleus and most organelles to accommodate maximum space for [[hemoglobin]], 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 lists these primary [[cellular component]]s, then briefly describes their function.

=== Cell membrane ===

{{main|Cell membrane}}

[[File: Cell membrane detailed diagram en.svg|thumb|upright=1.35|Detailed diagram of lipid bilayer of cell membrane]]

The [[cell membrane]], or plasma membrane, is a selectively permeable{{Cn|date=May 2024}} [[biological membrane]] that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it 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 phospholipids]], which are [[amphiphilic]] (partly [[hydrophobic]] and partly [[hydrophilic]]). Hence, the layer is called a [[phospholipid bilayer]], or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the [[porosome]] the universal secretory portal in cells and a variety of [[protein]] molecules that act as channels and pumps that move different molecules into and out of the cell.<ref name="NCBI"/> The membrane is semi-permeable, and selectively permeable, in that it can either let a substance ([[molecule]] or [[ion]]) pass through freely, to a limited extent or not at all.{{Cn|date=May 2024}} Cell surface membranes also contain [[Receptor (biochemistry)#Transmembrane receptors|receptor]] proteins that allow cells to detect external signaling molecules such as [[hormone]]s.<ref name="Guyton Hall 2016">{{cite book |last1=Guyton |first1=Arthur C. |last2=Hall |first2=John E. |year=2016 |title=Guyton and Hall Textbook of Medical Physiology |location=Philadelphia |publisher=Elsevier Saunders |pages=930–937 |url=https://books.google.com/books?id=3sWNCgAAQBAJ |oclc=1027900365 |isbn=978-1-4557-7005-2}}</ref>

=== Cytoskeleton ===

{{main|Cytoskeleton}}
{{further|Morphogenesis}}

[[File:DAPIMitoTrackerRedAlexaFluor488BPAE.jpg|thumb|A fluorescent image of an endothelial cell. Nuclei are stained blue, [[mitochondria]] are stained red, and microfilaments are stained green.]]

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 [[microtubule]]s, [[intermediate filament]]s and [[microfilament]]s. In the cytoskeleton of a [[neuron]] the intermediate filaments are known as [[neurofilament]]s. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.<ref name="NCBI"/> The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, [[cell polarity|polarity]] and cytokinesis.<ref>{{cite journal |last1=Michie |first1=K. A. |last2=Löwe |first2=J. |title=Dynamic filaments of the bacterial cytoskeleton |journal=Annual Review of Biochemistry |volume=75 |pages=467–492 |year=2006 |pmid=16756499 |doi=10.1146/annurev.biochem.75.103004.142452 |s2cid=4550126 }}</ref> The subunit protein of microfilaments is a small, monomeric protein called [[actin]]. The subunit of microtubules is a dimeric molecule called [[tubulin]]. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. Some of the subunit proteins of intermediate filaments include [[vimentin]], [[desmin]], [[lamin]] (lamins A, B and C), [[keratin]] (multiple acidic and basic keratins), and [[neurofilament proteins]] ([[NEFL|NF–L]], [[NEFM|NF–M]]).

=== Genetic material ===

{{main|DNA|RNA}}

[[File:DNA orbit animated.gif|thumb|[[DNA|Deoxyribonucleic acid]] (DNA)]]

Two different kinds of genetic material exist: [[DNA|deoxyribonucleic acid]] (DNA) and [[RNA|ribonucleic acid]] (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is [[Genetic code|encoded]] in its DNA sequence.<ref name="NCBI"/> RNA is used for information transport (e.g., [[mRNA]]) and [[enzyme|enzymatic]] functions (e.g., [[ribosome|ribosomal]] RNA). [[Transfer RNA]] (tRNA) molecules are used to add amino acids during protein [[Translation (biology)|translation]].

Prokaryotic genetic material is organized in a simple [[circular bacterial chromosome]] in the [[nucleoid region]] of the cytoplasm. Eukaryotic genetic material is divided into different,<ref name="NCBI"/> 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 contained in the [[cell nucleus]] (the [[genome|nuclear genome]]) and in the mitochondria (the [[mitochondrial genome]]). In humans, the nuclear genome is divided into 46 linear DNA molecules called [[chromosome]]s, including 22 [[homologous chromosome]] pairs and a pair of [[sex chromosomes]]. The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the [[mitochondrial DNA]] is very small compared to nuclear chromosomes,<ref name="NCBI"/> it codes for 13 proteins involved in mitochondrial energy production and 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}}

Organelles are parts of the cell that are adapted and/or specialized for carrying out one or more vital functions, analogous to the [[Organ (biology)|organs]] of the human body (such as the heart, lung, and kidney, with each organ performing a different function).<ref name="NCBI"/> Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the [[Cell nucleus|nucleus]] and [[Golgi apparatus]]) are typically solitary, while others (such as [[mitochondria]], [[chloroplasts]], [[peroxisomes]] and [[lysosomes]]) can be numerous (hundreds to thousands). The [[cytosol]] is the gelatinous fluid that fills the cell and surrounds the organelles.

==== Eukaryotic ====

[[File:HeLa cells stained with Hoechst 33258.jpg|thumb|Human cancer cells, specifically [[HeLa cells]], with DNA stained blue. The central and rightmost cell are in [[interphase]], so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going through [[mitosis]] and its chromosomes have condensed.]]

* '''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 and separated from the cytoplasm by a double membrane called the [[nuclear envelope]], space between these two membrane is called perinuclear space. 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 [[messenger RNA]] (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]].<ref name="NCBI"/>
* '''Mitochondria and chloroplasts''': generate energy for the cell. [[Mitochondrion|Mitochondria]] are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.<ref name="NCBI"/> [[Cellular respiration|Respiration]] occurs in the cell mitochondria, which generate the cell's energy by [[oxidative phosphorylation]], using [[oxygen]] to release energy stored in cellular nutrients (typically pertaining to [[glucose]]) to generate [[adenosine triphosphate|ATP]] ([[aerobic respiration]]). Mitochondria multiply by [[binary fission]], like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through [[photosynthesis]].

[[File:Endomembrane system diagram en.svg|thumb|upright=1.25|Diagram of the [[endomembrane system]] ]]

* '''Endoplasmic reticulum''': The [[endoplasmic reticulum]] (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes.<ref name="NCBI"/> The smooth ER plays a role in calcium sequestration and release and also helps in synthesis of [[lipid]].
* '''Golgi apparatus''': 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.
* '''Lysosomes and peroxisomes''': [[Lysosome]]s contain [[digestive enzyme]]s (acid [[hydrolase]]s). They digest excess or worn-out [[organelle]]s, food particles, and engulfed [[virus]]es or [[bacteria]]. [[Peroxisome]]s have enzymes that rid the cell of toxic [[peroxide]]s, Lysosomes are optimally active in an acidic environment. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.<ref name="NCBI"/>
* '''Centrosome''': the cytoskeleton organizer: 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 lie perpendicular to each other in which each has an organization like a [[Cart wheel|cartwheel]], which separate during [[cell division]] and help in the formation of the [[mitotic spindle]]. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
* '''Vacuoles''': [[Vacuole]]s sequester waste products and in plant cells store water. They are often described as liquid filled spaces and are surrounded by a membrane. Some cells, most notably ''[[Amoeba (genus)|Amoeba]]'', have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells. Vacuoles of plant cells are surrounded by a membrane which transports ions against concentration gradients.

==== Eukaryotic and prokaryotic ====

* '''Ribosomes''': The [[ribosome]] is a large complex of [[RNA]] and [[protein]] molecules.<ref name="NCBI"/> They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).<ref>{{cite journal  |last1=Ménétret |first1=Jean-François |last2=Schaletzky |first2=Julia |last3=Clemons |first3=William M. |last4=Osborne |first4=Andrew R. |last5=Skånland |first5=Sigrid S. |last6=Denison |first6=Carilee |last7=Gygi |first7=Steven P. |last8=Kirkpatrick |first8=Don S. |last9=Park |first9=Eunyong |last10=Ludtke |first10=Steven J. |last11=Rapoport |first11=Tom A. |last12=Akey |first12=Christopher W. |display-authors=3 |title=Ribosome binding of a single copy of the SecY complex: implications for protein translocation |journal=Molecular Cell |volume=28 |issue=6 |pages=1083–1092 |date=December 2007 |pmid=18158904 |doi=10.1016/j.molcel.2007.10.034 |url=https://authors.library.caltech.edu/90566/2/1-s2.0-S1097276507008258-mmc1.pdf |doi-access=free |access-date=2020-09-01 |archive-date=2021-01-21 |archive-url=https://web.archive.org/web/20210121115905/https://authors.library.caltech.edu/90566/2/1-s2.0-S1097276507008258-mmc1.pdf |url-status=live }}</ref>
* '''Plastids''': [[Plastid]] are membrane-bound organelle generally found in plant cells and [[Euglenid|euglenoids]] and contain specific ''pigments'', thus affecting the colour of the plant and organism. And these pigments also helps in food storage and tapping of light energy. There are three types of plastids based upon the specific pigments. [[Chloroplast]]s contain [[chlorophyll]] and some carotenoid pigments which helps in the tapping of light energy during photosynthesis. [[Chromoplast]]s contain fat-soluble [[carotenoid]] pigments like orange carotene and yellow xanthophylls which helps in synthesis and storage. [[Leucoplast]]s are non-pigmented plastids and helps in storage of nutrients.<ref>{{cite book |last=Sato |first=N. |year=2006 |pages=75–102 |title=The Structure and Function of Plastids |volume=23 |editor1=Wise, R. R. |editor2=Hoober, J. K. |publisher=Springer |chapter=Origin and Evolution of Plastids: Genomic View on the Unification and Diversity of Plastids |isbn=978-1-4020-4060-3 |doi=10.1007/978-1-4020-4061-0_4 |series=Advances in Photosynthesis and Respiration}}</ref>

== Structures outside the cell membrane ==

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

=== Cell wall ===

{{further|Cell wall}}

Many types of prokaryotic and eukaryotic cells have a [[cell wall]]. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of [[cellulose]], fungi cell walls are made up of [[chitin]] and bacteria cell walls are made up of [[peptidoglycan]].

=== Prokaryotic ===

==== Capsule ====

A gelatinous [[bacterial capsule|capsule]] is present in some bacteria outside the cell membrane and cell wall. The capsule may be [[polysaccharide]] as in [[pneumococci]], [[meningococci]] or [[polypeptide]] as ''[[Bacillus anthracis]]'' or [[hyaluronic acid]] as in [[streptococci]].
Capsules are not marked by normal staining protocols and can be detected by [[India ink#Uses other than writing|India ink]] or [[methyl blue]], which allows for higher contrast between the cells for observation.<ref>{{cite book |url=https://books.google.com/books?id=N2GU-DYKkk0C&q=Prokaryotic+india+ink&pg=PA87 |title=Prokaryotes |publisher=Newnes |date=1996 |isbn=978-0080984735 |access-date=November 9, 2020 |archive-date=April 14, 2021 |archive-url=https://web.archive.org/web/20210414134256/https://books.google.com/books?id=N2GU-DYKkk0C&q=Prokaryotic+india+ink&pg=PA87 |url-status=live }}</ref>{{rp|87}}

==== Flagella ====

[[Flagella]] are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.

==== Fimbriae ====

A [[fimbria (bacteriology)|fimbria]] (plural fimbriae also known as a [[pilus]], plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called [[pilin]] ([[antigenic]]) and are responsible for the attachment of bacteria to specific receptors on human cells ([[cell adhesion]]). There are special types of pili involved in [[bacterial conjugation]].

== Cellular processes ==

[[File:Three cell growth types.svg|thumb|upright=1.25|[[Prokaryotes]] divide by [[binary fission]], while [[eukaryotes]] divide by [[mitosis]] or [[meiosis]].]]

=== Replication ===

{{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]], while [[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,<ref name="NCBI"/> always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the [[cell cycle]].

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before [[meiosis I]]. DNA replication does not occur when the cells divide the second time, in [[meiosis II]].<ref>{{cite book|title=Campbell Biology{{snd}}Concepts and Connections|year=2009|publisher=Pearson Education|page=138}}</ref> Replication, like all cellular activities, requires specialized proteins for carrying out the job.<ref name="NCBI"/>

=== DNA repair ===

{{main|DNA repair}}

Cells of all organisms contain enzyme systems that scan their DNA for [[DNA damage (naturally occurring)|damage]] and carry out [[DNA repair|repair processes]] when it is detected. Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damage that could lead to [[mutation]]. [[Escherichia coli|''E. coli'']] bacteria are a well-studied example of a cellular organism with diverse well-defined [[DNA repair]] processes. These include: [[nucleotide excision repair]], [[DNA mismatch repair]], [[non-homologous end joining]] of double-strand breaks, [[homologous recombination|recombinational repair]] and light-dependent repair ([[photolyase|photoreactivation]]).<ref>{{cite book |last1=Snustad |first1=D. Peter |last2=Simmons |first2=Michael J. |title=Principles of Genetics |edition=5th |at=DNA repair mechanisms, pp. 364–368}}</ref>

=== Growth and metabolism ===

{{main|Cell growth|Metabolism|Photosynthesis}}

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 agent|reducing power]], and [[anabolism]], in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.

Complex sugars can be broken down into simpler sugar molecules called [[monosaccharides]] such as [[glucose]]. Once inside the cell, glucose is broken down to make adenosine triphosphate ([[adenosine triphosphate|ATP]]),<ref name="NCBI"/> a molecule that possesses readily available energy, through two different pathways. In plant cells, [[chloroplast]]s create sugars by [[photosynthesis]], using the energy of light to join molecules of water and [[carbon dioxide]].

=== 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.

=== Motility ===

{{main|Motility}}

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include [[flagella]] and [[cilia]].

In multicellular organisms, cells can move during processes such as wound healing, the immune response and [[cancer metastasis]]. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.<ref name="Ananthakrishnan Ehrlicher 2007">{{cite journal |last1=Ananthakrishnan |first1=R. |last2=Ehrlicher |first2=A. |title=The forces behind cell movement |journal=International Journal of Biological Sciences |volume=3 |issue=5 |pages=303–317 |date=June 2007 |pmid=17589565 |pmc=1893118 |doi=10.7150/ijbs.3.303 |publisher=Biolsci.org }}</ref> The process is divided into three steps: protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.<ref name="Alberts2">{{cite book |last1=Alberts |first1=Bruce |title=Molecular biology of the cell |date=2002 |publisher=Garland Science |isbn=0815340729 |pages=973–975 |edition=4th}}</ref><ref name="Ananthakrishnan Ehrlicher 2007"/>

==== Navigation, control and communication ====

{{See also|Cybernetics#In biology}}

In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to [[Chemotaxis|navigate]] efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused [[chemoattractant]]s which enable them to sense upcoming maze junctions before reaching them, including around corners.<ref>{{cite news |last1=Willingham |first1=Emily |title=Cells Solve an English Hedge Maze with the Same Skills They Use to Traverse the Body |url=https://www.scientificamerican.com/article/cells-solve-an-english-hedge-maze-with-the-same-skills-they-use-to-traverse-the-body/ |access-date=7 September 2020 |work=Scientific American |language=en |archive-date=4 September 2020 |archive-url=https://web.archive.org/web/20200904102655/https://www.scientificamerican.com/article/cells-solve-an-english-hedge-maze-with-the-same-skills-they-use-to-traverse-the-body/ |url-status=live }}</ref><ref>{{cite news |title=How cells can find their way through the human body |url=https://phys.org/news/2020-08-cells-human-body.html |access-date=7 September 2020 |work=phys.org |language=en |archive-date=3 September 2020 |archive-url=https://web.archive.org/web/20200903220400/https://phys.org/news/2020-08-cells-human-body.html |url-status=live }}</ref><ref>{{cite journal |last1=Tweedy |first1=Luke |last2=Thomason |first2=Peter A. |last3=Paschke |first3=Peggy I. |last4=Martin |first4=Kirsty |last5=Machesky |first5=Laura M. |last6=Zagnoni |first6=Michele |last7=Insall |first7=Robert H.|title=Seeing around corners: Cells solve mazes and respond at a distance using attractant breakdown |journal=Science |volume=369 |issue=6507 |date=August 2020 |page=eaay9792 |pmid=32855311 |doi=10.1126/science.aay9792 |s2cid=221342551 |url=https://www.science.org/doi/10.1126/science.aay9792 |access-date=2020-09-13 |archive-date=2020-09-12 |archive-url=https://web.archive.org/web/20200912234645/https://science.sciencemag.org/content/369/6507/eaay9792 |url-status=live }}</ref>

== Multicellularity ==

{{main|Multicellular organism}}

=== Cell specialization/differentiation ===

{{main|Cellular differentiation}}

[[File:C elegans stained.jpg|thumb|upright|Staining of a ''[[Caenorhabditis elegans]]'' highlights the nuclei of its cells.]]

Multicellular organisms are [[organism]]s that consist of more than one cell, in contrast to [[single-celled organism]]s.<ref>{{cite book |last=Becker |first=Wayne M. |display-authors=etal |title=The world of the cell |publisher=[[Benjamin Cummings|Pearson Benjamin Cummings]] |year=2009 |isbn=978-0321554185 |page=480}}</ref>

In complex multicellular organisms, cells specialize into different [[cell type]]s that are adapted to particular functions. In mammals, major cell types include [[skin cells]], [[muscle cells]], [[neuron]]s, [[blood cell]]s, [[fibroblast]]s, [[stem cells]], and others. Cell types differ both in appearance and function, yet are [[Genetics|genetically]] identical. Cells are able to be of the same [[genotype]] but of different cell type due to the differential [[Regulation of gene expression|expression]] of the [[gene]]s they contain.

Most distinct cell types arise from a single [[totipotent]] cell, called a [[zygote]], that [[Cellular differentiation|differentiates]] into hundreds of different cell types during the course of [[Development (biology)|development]]. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of [[molecule]]s during [[cell division|division]]).

=== Origin of multicellularity ===

Multicellularity has evolved independently at least 25 times,<ref name="Grosberg2007">{{cite journal |last1= Grosberg |first1=R. K. |last2=Strathmann |first2=R. R. |url=http://www-eve.ucdavis.edu/grosberg/Grosberg%20pdf%20papers/2007%20Grosberg%20%26%20Strathmann.AREES.pdf |title=The evolution of multicellularity: A minor major transition? |journal=Annu Rev Ecol Evol Syst |year=2007 |volume=38 |pages=621–654 |doi=10.1146/annurev.ecolsys.36.102403.114735 |access-date=2013-12-23 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304121329/http://www-eve.ucdavis.edu/grosberg/Grosberg%20pdf%20papers/2007%20Grosberg%20%26%20Strathmann.AREES.pdf |url-status=dead }}</ref> including in some prokaryotes, like [[cyanobacteria]], [[myxobacteria]], [[actinomycetes]], or ''[[Methanosarcina]]''. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.<ref name="pmid21351878">{{cite journal |last1=Popper |first1=Zoë A. |last2=Michel |first2=Gurvan |last3=Hervé |first3=Cécile |last4=Domozych |first4=David S. |last5=Willats |first5=William G.T. |last6=Tuohy |first6=Maria G. |last7=Kloareg |first7=Bernard |last8=Stengel |first8=Dagmar B. |display-authors=3 |title=Evolution and diversity of plant cell walls: from algae to flowering plants |journal=Annual Review of Plant Biology |volume=62 |pages=567–590 |date=2011 |pmid=21351878 |doi=10.1146/annurev-arplant-042110-103809 |url=http://public.wsu.edu/~lange-m/Documnets/Teaching2011/Popper2011.pdf |hdl=10379/6762 |s2cid=11961888 |hdl-access=free |access-date=2013-12-23 |archive-date=2016-07-29 |archive-url=https://web.archive.org/web/20160729224035/http://public.wsu.edu/~lange-m/Documnets/Teaching2011/Popper2011.pdf |url-status=live }}</ref> It evolved repeatedly for plants ([[Chloroplastida]]), once or twice for [[animal]]s, once for [[brown algae]], and perhaps several times for [[fungi]], [[Mycetozoa|slime molds]], and [[red algae]].<ref>{{cite journal|last=Bonner |first=John Tyler |author-link=John Tyler Bonner |year=1998 |title=The Origins of Multicellularity |journal=Integrative Biology |volume=1 |issue=1 |pages=27–36 |url=http://courses.cit.cornell.edu/biog1101/outlines/Bonner%20-Origin%20of%20Multicellularity.pdf |format=PDF, 0.2 MB |issn=1093-4391 |doi=10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6 |url-status=dead |archive-url=https://web.archive.org/web/20120308175112/http://courses.cit.cornell.edu/biog1101/outlines/Bonner%20-Origin%20of%20Multicellularity.pdf |archive-date=March 8, 2012 }}</ref> Multicellularity may have evolved from [[Colony (biology)|colonies]] of interdependent organisms, from [[cellularization]], or from organisms in [[Symbiosis|symbiotic relationships]].

The first evidence of multicellularity is from [[cyanobacteria]]-like organisms that lived between 3 and 3.5 billion years ago.<ref name="Grosberg2007"/> Other early fossils of multicellular organisms include the contested ''[[Grypania]] spiralis'' and the fossils of the black shales of the [[Palaeoproterozoic]] [[Francevillian Group Fossil]] B Formation in [[Gabon]].<ref>{{cite journal |last1=Albani |first1=Abderrazak El |last2=Bengtson |first2=Stefan |last3=Canfield |first3=Donald E. |last4=Bekker |first4=Andrey |last5=Macchiarelli |first5=Roberto |last6=Mazurier |first6=Arnaud |last7=Hammarlund |first7=Emma U. |last8=Boulvais |first8=Philippe |last9=Dupuy |first9=Jean-Jacques |last10=Fontaine |first10=Claude |last11=Fürsich |first11=Franz T. |last12=Gauthier-Lafaye |first12=François |last13=Janvier |first13=Philippe |last14=Javaux |first14=Emmanuelle |last15=Ossa |first15=Frantz Ossa |last16=Pierson-Wickmann |first16=Anne-Catherine |last17=Riboulleau |first17=Armelle |last18=Sardini |first18=Paul |last19=Vachard |first19=Daniel |last20=Whitehouse |first20=Martin |last21=Meunier |first21=Alain |display-authors=3 |title=Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago |journal=Nature |volume=466 |issue=7302 |pages=100–104 |date=July 2010 |pmid=20596019 |doi=10.1038/nature09166 |author-link=Abderrazak El Albani |s2cid=4331375 |bibcode=2010Natur.466..100A }}</ref>

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in [[experimental evolution|evolution experiments]] using predation as the [[selective pressure]].<ref name="Grosberg2007"/>

== Origins ==<!-- This section is linked from [[Timeline of evolution]] -->

{{main|Evolutionary history of life}}

The origin of cells has to do with the [[Abiogenesis|origin of life]], which began the [[timeline of evolution|history of life]] on Earth.

=== Origin of life ===

{{further|Abiogenesis|Evolution of cells}}

[[File:Stromatolites.jpg|thumb|[[Stromatolites]] are left behind by [[cyanobacteria]], also called blue-green algae. They are among the oldest fossils of life on Earth. This one-billion-year-old fossil is from [[Glacier National Park (U.S.)|Glacier National Park]] in the United States.]]

Small molecules needed for life may have been carried to Earth on meteorites, created at [[Hydrothermal vent|deep-sea vents]], or [[Miller–Urey experiment|synthesized by lightning in a reducing atmosphere]]. There is little experimental data defining what the first self-replicating forms were. [[RNA]] may have been [[RNA world hypothesis|the earliest self-replicating molecule]], as it can both store genetic information and catalyze chemical reactions.<ref name=OrgelLE>{{cite journal |last= Orgel |first=L. E. |title=The origin of life--a review of facts and speculations |journal=Trends in Biochemical Sciences |volume=23 |issue=12 |pages=491–495 |date=December 1998 |pmid=9868373 |doi=10.1016/S0968-0004(98)01300-0 }}</ref>

Cells emerged around 4 billion years ago.<ref name="NAT-20170301">{{cite journal |last1=Dodd |first1=Matthew S. |last2=Papineau |first2=Dominic |last3=Grenne |first3=Tor |last4=Slack |first4=John F. |last5=Rittner |first5=Martin |last6=Pirajno |first6=Franco |last7=O'Neil |first7=Jonathan |last8=Little |first8=Crispin T.S. |display-authors=3 |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates |journal=[[Nature (journal)|Nature]] |date=1 March 2017 |volume=543 |issue=7643 |pages=60–64 |doi=10.1038/nature21377 |doi-access=free |pmid=28252057 |bibcode=2017Natur.543...60D |url=http://eprints.whiterose.ac.uk/112179/ |access-date=2 March 2017 |url-status=live |archive-url=https://web.archive.org/web/20170908201821/http://eprints.whiterose.ac.uk/112179/ |archive-date=8 September 2017}}</ref><ref>{{Cite journal |last1=Betts |first1=Holly C. |last2=Puttick |first2=Mark N. |last3=Clark |first3=James W. |last4=Williams |first4=Tom A. |last5=Donoghue |first5=Philip C. J. |last6=Pisani |first6=Davide |date=20 August 2018 |title=Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin |journal=Nature Ecology & Evolution |volume=2 |issue=10 |pages=1556–1562 |doi=10.1038/s41559-018-0644-x |pmid=30127539|pmc=6152910 |bibcode=2018NatEE...2.1556B }}</ref> The first cells were most likely [[heterotroph]]s. The early cell membranes were probably simpler and more permeable than modern ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayered [[Vesicle (biology and chemistry)|vesicles]] in water, and could have preceded RNA.<ref name="Griffiths 2007">{{cite journal |last=Griffiths |first=G. |title=Cell evolution and the problem of membrane topology |journal=Nature Reviews. Molecular Cell Biology |volume=8 |issue=12 |pages=1018–1024 |date=December 2007 |pmid=17971839 |doi=10.1038/nrm2287 |s2cid=31072778 |doi-access=free }}</ref><ref name="ScienceDaily 2021">{{cite web |title=First cells may have emerged because building blocks of proteins stabilized membranes |url=https://www.sciencedaily.com/releases/2019/08/190812155502.htm |access-date=2021-09-18 |website=ScienceDaily |archive-date=2021-09-18 |archive-url=https://web.archive.org/web/20210918102211/https://www.sciencedaily.com/releases/2019/08/190812155502.htm |url-status=live }}</ref>

=== First eukaryotic cells ===

{{main|Eukaryogenesis}}

[[File:Symbiogenesis 2 mergers.svg|thumb|upright=1.35|In the theory of [[symbiogenesis]], a merger of an [[archaea]]n and an aerobic bacterium created the eukaryotes, with aerobic [[Mitochondrion|mitochondria]], some 2.2 billion years ago. A second merger, 1.6 billion years ago, added [[chloroplast]]s, creating the green plants.<ref name=latorre/>]]

[[Eukaryote|Eukaryotic]] cells were created some 2.2 billion years ago in a process called [[eukaryogenesis]]. This is widely agreed to have involved [[symbiogenesis]], in which [[archaea]] and [[bacteria]] came together to create the first eukaryotic common ancestor. This cell had a new level of complexity and capability, with a nucleus<ref name="McGrath 2022">{{cite journal |last=McGrath |first=Casey |title=Highlight: Unraveling the Origins of LUCA and LECA on the Tree of Life |journal=Genome Biology and Evolution |volume=14 |issue=6 |date=31 May 2022 |pages=evac072 |doi=10.1093/gbe/evac072|pmc=9168435 }}</ref><ref name="Weiss et al 2016">{{cite journal |last1=Weiss |first1=Madeline C. |last2=Sousa |first2=F. L. |last3=Mrnjavac |first3=N. |last4=Neukirchen |first4=S. |last5=Roettger |first5=M. |last6=Nelson-Sathi |first6=S. |last7=Martin |first7=William F. |author7-link=William F. Martin |display-authors=3 |s2cid=2997255 |year=2016 |title=The physiology and habitat of the last universal common ancestor |journal=Nature Microbiology |volume=1 |issue=9 |page=16116 |doi=10.1038/nmicrobiol.2016.116 |pmid=27562259 |url=http://complexityexplorer.s3.amazonaws.com/supplemental_materials/3.6+Early+Metabolisms/Weiss_et_al_Nat_Microbiol_2016.pdf }}</ref> and facultatively aerobic [[Mitochondrion|mitochondria]].<ref name=latorre>{{cite book |last1=Latorre |first1=A. |last2=Durban |first2=A |last3=Moya |first3=A. |last4=Pereto |first4=J. |chapter-url=https://books.google.com/books?id=m3oFebknu1cC&pg=PA326 |chapter=The role of symbiosis in eukaryotic evolution |title=Origins and Evolution of Life: An astrobiological perspective |editor1=Gargaud, Muriel |editor2=López-Garcìa, Purificacion |editor3=Martin, H. |year=2011 |location=Cambridge |publisher=Cambridge University Press |pages=326–339 |isbn=978-0-521-76131-4 |access-date=27 August 2017 |archive-date=24 March 2019 |archive-url=https://web.archive.org/web/20190324055723/https://books.google.com/books?id=m3oFebknu1cC&pg=PA326 |url-status=live }}</ref> It evolved some 2 billion years ago into a population of single-celled organisms that included the last eukaryotic common ancestor, gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. It featured at least one [[centriole]] and [[cilium]], sex ([[meiosis]] and [[syngamy]]), [[peroxisome]]s, and a dormant [[cyst]] with a cell wall of [[chitin]] and/or [[cellulose]].<ref>{{cite journal |last=Leander |first=B. S. |title=Predatory protists |journal=Current Biology |volume=30 |issue=10 |pages=R510–R516 |date=May 2020 |pmid=32428491 |doi=10.1016/j.cub.2020.03.052 |s2cid=218710816 |doi-access=free }}</ref><ref name="Strassert Irisarri Williams Burki 2021">{{cite journal |last1=Strassert |first1=Jürgen F. H. |last2=Irisarri |first2=Iker |last3=Williams |first3=Tom A. |last4=Burki |first4=Fabien |title=A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids |journal=Nature Communications |volume=12 |issue=1 |date=25 March 2021 |page=1879 |doi=10.1038/s41467-021-22044-z|pmid=33767194 |pmc=7994803 |bibcode=2021NatCo..12.1879S }}</ref> In turn, the last eukaryotic common ancestor gave rise to the eukaryotes' [[crown group]], containing the ancestors of [[animal]]s, [[Fungus|fungi]], [[plant]]s, and a diverse range of single-celled organisms.<ref name="Gabaldón">{{cite journal |last=Gabaldón |first=T. |title=Origin and Early Evolution of the Eukaryotic Cell |journal=Annual Review of Microbiology |volume=75 |issue=1 |pages=631–647 |date=October 2021 |pmid=34343017 |doi=10.1146/annurev-micro-090817-062213 |s2cid=236916203 }}</ref><ref name="w1990">{{cite journal |last1=Woese |first1=C.R. |author1-link=Carl Woese |last2=Kandler |first2=Otto |author2-link=Otto Kandler |last3=Wheelis |first3=Mark L. |author3-link=Mark Wheelis |title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=87 |issue=12 |pages=4576–4579 |date=June 1990 |pmid=2112744 |pmc=54159 |doi=10.1073/pnas.87.12.4576 |bibcode=1990PNAS...87.4576W |doi-access=free }}</ref> The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that added [[chloroplast]]s, derived from [[cyanobacteria]].<ref name=latorre/>

== History of research ==

{{main|Cell theory}}

[[File:RobertHookeMicrographia1665.jpg|thumb|upright|Robert Hooke's drawing of cells in [[Cork (material)|cork]], 1665]]

In 1665, [[Robert Hooke]] examined a thin slice of cork under his [[microscope]], and saw a structure of small enclosures. He wrote "I could exceeding plainly perceive it to be all perforated and porous, much like a [[Honeycomb|Honey-comb]], but that the pores of it were not regular".<ref name="Hooke 1665">{{cite book |last1=Hooke |first1=Robert |author1-link=Robert Hooke |title=[[Micrographia]] |date=1665 |chapter=Observation 18 |chapter-url=https://en.wikisource.org/wiki/Micrographia/Chapter_18}}</ref> To further support his theory, [[Matthias Schleiden]] and [[Theodor Schwann]] both also studied cells of both animal and plants. What they discovered were significant differences between the two types of cells. This put forth the idea that cells were not only fundamental to plants, but animals as well.<ref name="Maton 1997">{{cite book |last=Maton |first=Anthea |url=https://archive.org/details/cellsbuildingblo00mato |title=Cells Building Blocks of Life |publisher=Prentice Hall |year=1997 |isbn=978-0134234762 |location=New Jersey |pages=44-45 The Cell Theory}}</ref>

* 1632–1723: [[Antonie van Leeuwenhoek]] taught himself to make [[Lens (optics)|lenses]], constructed basic [[optical microscope]]s and drew protozoa, such as ''[[Vorticella]]'' from rain water, and [[Bacterium|bacteria]] from his own mouth.<ref name="Gest 2004">{{cite journal |last=Gest |first=H. |year=2004 |title=The discovery of microorganisms by Robert Hooke and Antoni Van Leeuwenhoek, fellows of the Royal Society |journal=Notes and Records of the Royal Society of London |volume=58 |issue=2 |pages=187–201 |doi=10.1098/rsnr.2004.0055 |pmid=15209075 |s2cid=8297229}}</ref>
* 1665: [[Robert Hooke]] discovered cells in [[Cork (material)|cork]], then in living plant tissue using an early compound microscope. He coined the term ''cell'' (from [[Latin]] ''cellula'', meaning "small room"<ref name="npr12">{{Multiref|{{Cite web |date=September 17, 2010 |title=The Origins Of The Word 'Cell' |url=https://www.npr.org/templates/story/story.php?storyId=129934828&t=1628175572746 |url-status=live |archive-url=https://web.archive.org/web/20210805150111/https://www.npr.org/templates/story/story.php?storyId=129934828&t=1628175572746 |archive-date=2021-08-05 |access-date=2021-08-05 |website=[[National Public Radio]] }}|{{cite encyclopedia |title=cellŭla |encyclopedia=[[A Latin Dictionary]] |year=1879 |publisher=Charlton T. Lewis and Charles Short |url= http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0059:entry=cellula|access-date= 5 August 2021|isbn= 978-1999855789 |archive-date=7 August 2021|archive-url= https://web.archive.org/web/20210807122358/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dcellula |url-status= live}} }}</ref>) in his book ''[[Micrographia]]'' (1665).<ref name="Hooke">{{cite book |last=Hooke |first=Robert |author-link=Robert Hooke |title=Micrographia: ...|date=1665 |publisher=Royal Society of London |location=London |page=113 |url=https://archive.org/stream/mobot31753000817897#page/113/mode/2up|quote= ... 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 ... |postscript=none}} – Hooke describing his observations on a thin slice of cork. See also: [http://www.ucmp.berkeley.edu/history/hooke.html Robert Hooke] {{Webarchive|url=https://web.archive.org/web/19970606013455/http://www.ucmp.berkeley.edu/history/hooke.html |date=1997-06-06 }}</ref><ref name="Gest 2004"/>
* 1839: [[Theodor Schwann]]<ref>{{cite book |last=Schwann |first=Theodor |author-link=Theodor Schwann |year=1839 |title=Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen |publisher=Sander |place=Berlin |url=http://www.deutschestextarchiv.de/book/show/schwann_mikroskopische_1839}}</ref> and [[Matthias Jakob Schleiden]] elucidated 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.
* 1855: [[Rudolf Virchow]] stated that new cells come from pre-existing cells by [[cell division]] (''omnis cellula ex cellula'').
* 1931: [[Ernst Ruska]] built the first [[transmission electron microscope]] (TEM) at the [[University of Berlin]].<ref name="Ruska">{{cite book |title=The Early Development of Electron Lenses and Electron Microscopy |series=Applied Optics |volume=25 |issue=6 |pages=820 |author1=Ernst Ruska |translator=T. Mulvey |isbn=978-3-7776-0364-3 |date=January 1980 |doi=10.1364/AO.25.000820 |bibcode=1986ApOpt..25..820R }}</ref> By 1935, he had built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
* 1981: [[Lynn Margulis]] published ''Symbiosis in Cell Evolution'' detailing how eukaryotic cells were created by [[symbiogenesis]].<ref name="Cornish-Bowden 2017">{{Cite journal |last=Cornish-Bowden |first=Athel |title=Lynn Margulis and the origin of the eukaryotes |journal=[[Journal of Theoretical Biology]] |series=The origin of mitosing cells: 50th anniversary of a classic paper by Lynn Sagan (Margulis) |date=7 December 2017 |volume=434 |page=1 |doi=10.1016/j.jtbi.2017.09.027 |pmid=28992902 |bibcode=2017JThBi.434....1C |url=https://www.sciencedirect.com/science/article/pii/S0022519317304459 }}</ref>

== See also ==

{{Div col}}
* [[Cytotoxicity]]
* [[List of human cell types]]
* ''[[The Inner Life of the Cell]]''
* [[Outline of cell biology]]
* ''[[Parakaryon myojinensis]]''
* [[Syncytium]]
{{div col end}}

== References ==

{{Reflist}}

== Further reading ==

{{Refbegin}}
* {{Cite book |last=Alberts |first=B. |display-authors=etal |title=Molecular Biology of the Cell |edition=6th |publisher=Garland |year=2014 |isbn=978-0815344322 }}; The [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.TOC&depth=2 fourth edition is freely available] {{Webarchive|url=https://web.archive.org/web/20091011113848/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.TOC&depth=2 |date=2009-10-11 }} from [[National Center for Biotechnology Information]] Bookshelf.
* {{Cite book |last=Lodish |first=Harvey |display-authors=etal |title=Molecular Cell Biology |edition=5th |publisher=WH Freeman |location= New York |year=2004 |url=https://archive.org/details/molecularcellbio00harv |isbn=978-0716743668 }}
* {{Cite book |last=Cooper |first=G. M. |title=The cell: a molecular approach |edition=2nd |publisher=ASM Press |location=Washington, D.C. |year=2000 |isbn=978-0878931026 |url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=cooper.TOC&depth=2 |access-date=2017-08-30 |archive-date=2009-06-30 |archive-url=https://web.archive.org/web/20090630030426/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=cooper.TOC&depth=2 |url-status=live }}
{{Refend}}

== External links ==

{{Commons category|Cells}}
* {{Cite web|title=The Inner Life of the Cell |url=https://xvivo.com/examples/the-inner-life-of-the-cell/ |website=[[XVIVO]] website}} – 2006 animation of molecular mechanisms inside cells
* {{Cite web|title=Inside the Cell|url=http://publications.nigms.nih.gov/insidethecell/ |archive-date=2017-07-20 |archive-url=https://web.archive.org/web/20170720164847/http://publications.nigms.nih.gov/insidethecell/ |url-status=dead }} – 2005 science education booklet by [[National Institutes of Health]] in PDF and [[ePub]].

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