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"Eukaryotic cell" redirects here. For the journal, seeEukaryotic Cell(journal).

Eukaryota
Temporal range:Statherian–Present1650–0 Ma
Scientific classificationEdit this classification
Domain: Eukaryota
(Chatton,1925)Whittaker&Margulis,1978
Subgroups
Synonyms

Theeukaryotes(/juːˈkærioʊts,-əts/yoo-KARR-ee-ohts, -⁠əts)[4]constitute thedomainofEukaryotaorEukarya,organismswhosecellshave a membrane-boundnucleus.Allanimals,plants,fungi,and manyunicellular organismsare eukaryotes. They constitute a major group oflife formsalongside the two groups ofprokaryotes:theBacteriaand theArchaea.Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collectiveglobal biomassis much larger than that of prokaryotes.

The eukaryotes seemingly emerged within theAsgard archaea,and are closely related to theHeimdallarchaeia.[5]This implies that there are onlytwo domains of life,Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during thePaleoproterozoic,likely asflagellatedcells. The leadingevolutionarytheory is they were created bysymbiogenesisbetween an anaerobic Asgard archaean and an aerobicproteobacterium,which formed themitochondria.A second episode of symbiogenesis with acyanobacteriumcreated the plants, withchloroplasts.

Eukaryotic cells containmembrane-bound organellessuch as thenucleus,theendoplasmic reticulum,and theGolgi apparatus.Eukaryotes may be eitherunicellularormulticellular.In comparison, prokaryotes are typically unicellular. Unicellular eukaryotes are sometimes calledprotists.Eukaryotes can reproduce bothasexuallythroughmitosisandsexuallythroughmeiosisandgametefusion (fertilization).

Diversity

Further information:Organism

Eukaryotes areorganismsthat range from microscopic singlecells,such aspicozoansunder 3 micrometres across,[6]toanimalslike theblue whale,weighing up to 190tonnesand measuring up to 33.6 metres (110 ft) long,[7]orplantslike thecoast redwood,up to 120 metres (390 ft) tall.[8]Many eukaryotes are unicellular; the informal grouping calledprotistsincludes many of these, with some multicellular forms like thegiant kelpup to 200 feet (61 m) long.[9]The multicellular eukaryotes include the animals, plants, andfungi,but again, these groups too contain many unicellularspecies.[10]Eukaryotic cells are typically much larger than those ofprokaryotes—thebacteriaand thearchaea—having a volume of around 10,000 times greater.[11][12]Eukaryotes represent a small minority of the number oforganisms,but, as many of them are much larger, their collective globalbiomass(468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass ofEarth.[13]

The eukaryotes are a diverse lineage, consisting mainly ofmicroscopic organisms.[14]Multicellularity in some form hasevolved independentlyat least 25 times within the eukaryotes.[15][16]Complex multicellular organisms, not counting the aggregation ofamoebaeto formslime molds,have evolved within only six eukaryotic lineages:animals,symbiomycotan fungi,brown algae,red algae,green algae,andland plants.[17]Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics.[14]

Distinguishing features

Further information:Cell (biology) § Eukaryotic cells

Nucleus

The defining feature of eukaryotes is thattheir cellshavenuclei.This gives them their name, from theGreekεὖ(eu,"well" or "good" ) andκάρυον(karyon,"nut" or "kernel", here meaning "nucleus" ).[18]Eukaryotic cells have a variety of internal membrane-bound structures, calledorganelles,and acytoskeletonwhich defines the cell's organization and shape. The nucleus stores the cell'sDNA,which is divided into linear bundles calledchromosomes;[19]these are separated into two matching sets by amicrotubular spindleduring nuclear division, in the distinctively eukaryotic process ofmitosis.[20]

Biochemistry

Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such assteranesynthesis.[21]The eukaryotic signatureproteinshave no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton, the complextranscriptionmachinery, the membrane-sorting systems, thenuclear pore,and someenzymesin the biochemical pathways.[22]

Internal membranes

Further information:Endomembrane system
Prokaryote,to same scale
Eukaryotic cell withendomembrane system
Eukaryotic cells are some 10,000 times larger than prokaryotic cells by volume, and containmembrane-bound organelles.

Eukaryote cells include a variety of membrane-bound structures, together forming theendomembrane system.[23]Simple compartments, calledvesiclesandvacuoles,can form by budding off other membranes. Many cells ingest food and other materials through a process ofendocytosis,where the outer membraneinvaginatesand then pinches off to form a vesicle.[24]Some cell products can leave in a vesicle throughexocytosis.[25]

The nucleus is surrounded by a double membrane known as thenuclear envelope,withnuclear poresthat allow material to move in and out.[26]Various tube- and sheet-like extensions of the nuclear membrane form theendoplasmic reticulum,which is involved inprotein transportand maturation. It includes the rough endoplasmic reticulum, covered inribosomeswhich synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum.[27]In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles (cisternae), theGolgi apparatus.[28]

Vesicles may be specialized; for instance,lysosomescontaindigestive enzymesthat break downbiomoleculesin the cytoplasm.[29]

Mitochondria

Main article:Mitochondrion
Mitochondriaare essentially universal in the eukaryotes, and with their ownDNAsomewhat resemble prokaryotic cells.

Mitochondriaare organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell",[30]for its function providing energy by oxidising sugars or fats to produce the energy-storing moleculeATP.[31][32]Mitochondria have two surroundingmembranes,each aphospholipid bilayer,theinnerof which is folded into invaginations calledcristaewhereaerobic respirationtakes place.[33]

Mitochondria containtheir own DNA,which has close structural similarities tobacterial DNA,from which it originated, and which encodesrRNAandtRNAgenes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA.[34]

Some eukaryotes, such as themetamonadsGiardiaandTrichomonas,and the amoebozoanPelomyxa,appear to lack mitochondria, but all contain mitochondrion-derived organelles, likehydrogenosomesormitosomes,having lost their mitochondria secondarily.[35]They obtain energy by enzymatic action in the cytoplasm.[36][35]

Plastids

Main article:Plastid
The most common type ofplastidis thechloroplast,which containschlorophylland produces organic compounds byphotosynthesis.

Plants and various groups ofalgaehaveplastidsas well as mitochondria. Plastids, like mitochondria, havetheir own DNAand are developed fromendosymbionts,in this casecyanobacteria.They usually take the form ofchloroplastswhich, like cyanobacteria, containchlorophylland produce organic compounds (such asglucose) throughphotosynthesis.Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others throughsecondary endosymbiosisor ingestion.[37]The capture and sequestering of photosynthetic cells and chloroplasts,kleptoplasty,occurs in many types of modern eukaryotic organisms.[38][39]

Cytoskeletal structures

Main article:Cytoskeleton
The cytoskeleton.Actin filamentsare shown in red,microtubulesin green. (The nucleus is in blue.)

Thecytoskeletonprovides stiffening structure and points of attachment for motor structures that enable the cell to move, change shape, or transport materials. The motor structures aremicrofilamentsofactinandactin-binding proteins,including α-actinin,fimbrin,andfilaminare present in submembranouscortical layersand bundles.Motor proteinsof microtubules,dyneinandkinesin,andmyosinof actin filaments, provide dynamic character of the network.[40][41]

Many eukaryotes have long slender motile cytoplasmic projections, calledflagella,or multiple shorter structures calledcilia.These organellesare variously involved in movement, feeding, and sensation. They are composed mainly oftubulin,and are entirely distinct from prokaryotic flagella. They are supported by a bundle ofmicrotubulesarising from acentriole,characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs (mastigonemes), as in manyStramenopiles.Their interior is continuous with the cell'scytoplasm.[42][43]

Centrioles are often present, even in cells and groups that do not have flagella, butconifersandflowering plantshave neither. They generally occur in groups that give rise to various microtubular roots. These form a primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division.[44]

Cell wall

Main article:Cell wall

The cells of plants, algae, fungi and mostchromalveolates,but not animals, are surrounded by a cell wall. This is a layer outside thecell membrane,providing the cell with structural support, protection, and a filtering mechanism. The cell wall also preventsover-expansionwhen water enters the cell.[45]

The majorpolysaccharidesmaking up the primary cell wall ofland plantsarecellulose,hemicellulose,andpectin.The cellulosemicrofibrilsare linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in the primary cell wall isxyloglucan.[46]

Sexual reproduction

Further information:Evolution of sexual reproduction
Sexual reproductionrequires alife cyclethat alternates between ahaploidphase, with one copy of eachchromosomein the cell, and adiploidphase, with two copies. In eukaryotes, haploidgametesare produced bymeiosis;two gametes fuse to form a diploidzygote.

Eukaryotes have a life cycle that involvessexual reproduction,alternating between ahaploidphase, where only one copy of each chromosome is present in each cell, and adiploidphase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such aseggsandspermatozoa,to form azygote;this may grow into a body, with its cells dividing bymitosis,and at some stage produce haploid gametes throughmeiosis,a division that reduces the number of chromosomes and createsgenetic variability.[47]There is considerable variation in this pattern. Plants have bothhaploid and diploid multicellular phases.[48]Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have a smaller surface area to volume ratio.[49]

Theevolution of sexual reproductionmay be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks andRogerhave proposed that facultative sex was present in the group's common ancestor.[50]A core set of genes that function in meiosis is present in bothTrichomonas vaginalisandGiardia intestinalis,two organisms previously thought to be asexual.[51][52]Since these two species are descendants of lineages that diverged early from the eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes.[51][52]Species once thought to be asexual, such asLeishmaniaparasites, have a sexual cycle.[53]Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently.[54]

Evolution

Tree of eukaryotes showing major subgroups and thumbnail diagrams of representative members of each group. Updated synthesis based on recent (as of 2023) phylogenomic reconstructions.[55]

History of classification

Further information:History of taxonomy

Inantiquity,the two lineages ofanimalsandplantswere recognized byAristotleandTheophrastus.The lineages were given thetaxonomic rankofKingdombyLinnaeusin the 18th century. Though he included thefungiwith plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom.[56]The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologistGeorg A. Goldfusscoined the wordprotozoato refer to organisms such asciliates,[57]and this group was expanded untilErnst Haeckelmade it a kingdom encompassing all single-celled eukaryotes, theProtista,in 1866.[58][59][60]The eukaryotes thus came to be seen as four kingdoms:

The protists were at that time thought to be "primitive forms", and thus anevolutionary grade,united by their primitive unicellular nature.[59]Understanding of the oldest branchings in thetree of lifeonly developed substantially withDNA sequencing,leading to a system ofdomainsrather than kingdoms as top level rank being put forward byCarl Woese,Otto Kandler,andMark Wheelisin 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that"'eukaryotes' will continue to be an acceptable common synonym ".[2][61]In 1996, the evolutionary biologistLynn Margulisproposed to replace Kingdoms and Domains with "inclusive" names to create a "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)".[3]

Phylogeny

By 2014, a rough consensus started to emerge from the phylogenomic studies of the previous two decades.[10][62]The majority of eukaryotes can be placed in one of two large clades dubbedAmorphea(similar in composition to theunikonthypothesis) and theDiphoda(formerly bikonts), which includes plants and most algal lineages. A third major grouping, theExcavata,has been abandoned as a formal group as it isparaphyletic.[63]The proposed phylogeny below includes only one group of excavates (Discoba),[64]and incorporates the 2021 proposal thatpicozoansare close relatives of rhodophytes.[65]TheProvoraare a group of microbial predators discovered in 2022.[1]


Eukaryotes
2200 mya

One view of the great kingdoms and their stem groups.[64][66][67][14]TheMetamonadaare hard to place, being sister possibly toDiscobaor toMalawimonada[14]or being a paraphyletic group external toall other eukaryotes.[68]

Origin of eukaryotes

Main article:Eukaryogenesis
In the theory ofsymbiogenesis,a merger of anarchaeanand an aerobic bacterium created the eukaryotes, with aerobicmitochondria;a second merger addedchloroplasts,creating thegreen plants.[69]

The origin of the eukaryotic cell, oreukaryogenesis,is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. Thelast eukaryotic common ancestor(LECA) is the hypothetical origin of all living eukaryotes,[70]and was most likely abiological population,not a single individual.[71]The LECA is believed to have been a protist with a nucleus, at least onecentrioleandflagellum,facultatively aerobic mitochondria, sex (meiosisandsyngamy), a dormantcystwith a cell wall ofchitinorcellulose,andperoxisomes.[72][73][74]

Anendosymbiotic unionbetween a motileanaerobicarchaean and an aerobicAlpha proteobacteriumgave rise to the LECA and all eukaryotes, withmitochondria.A second, much later endosymbiosis with a cyanobacterium gave rise to the ancestor of plants, withchloroplasts.[69]

The presence of eukaryotic biomarkers in archaea points towards an archaeal origin. The genomes ofAsgardarchaea have plenty ofEukaryotic signature proteingenes, which play a crucial role in the development of thecytoskeletonand complex cellular structures characteristic of eukaryotes. In 2022,cryo-electron tomographydemonstrated that Asgard archaea have a complexactin-based cytoskeleton, providing the first direct visual evidence of the archaeal ancestry of eukaryotes.[75]

Fossils

The timing of the origin of eukaryotes is hard to determine but the discovery ofQingshania magnificia,the earliest multicelluar eukaryote from North China which lived during 1.635 billion years ago, suggests that the crown group eukaryotes would have originated from the latePaleoproterozoic(Statherian); the earliest unequivocal unicellular eukaryotes which lived during approximately 1.65 billion years ago are also discovered from North China:Tappania plana,Shuiyousphaeridium macroreticulatum,Dictyosphaera macroreticulata,Germinosphaera alveolata,andValeria lophostriata.[76]

Someacritarchsare known from at least 1.65 billion years ago, and a fossil,Grypania,which may be an alga, is as much as 2.1 billion years old.[77][78]The"problematic"[79]fossilDiskagmahas been found inpaleosols2.2 billion years old.[79]

Reconstruction of the problematic[79]Diskagma buttonii,a terrestrial fossil less than 1mm high, from rocks around 2.2 billion years old

Structures proposed to represent "large colonial organisms" have been found in theblack shalesof thePalaeoproterozoicsuch as theFrancevillian B Formation,inGabon,dubbed the "Francevillian biota"which is dated at 2.1 billion years old.[80][81]However, the status of these structures as fossils is contested, with other authors suggesting that they might representpseudofossils.[82]The oldest fossils than can unambiguously be assigned to eukaryotes are from the Ruyang Group of China, dating to approximately 1.8-1.6 billion years ago.[83]Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form ofred algae,though recent work suggests the existence of fossilizedfilamentous algaein theVindhyabasin dating back perhaps to 1.6 to 1.7 billion years ago.[84]

The presence ofsteranes,eukaryotic-specificbiomarkers,inAustralianshalespreviously indicated that eukaryotes were present in these rocks dated at 2.7 billion years old,[21][85]but these Archaean biomarkers have been rebutted as later contaminants.[86]The oldest valid biomarker records are only around 800 million years old.[87]In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago.[88]The nature of steranes as eukaryotic biomarkers is further complicated by the production ofsterolsby some bacteria.[89][90]

Whenever their origins, eukaryotes may not have become ecologically dominant until much later; a massive increase in thezinc compositionof marine sediments800million years agohas been attributed to the rise of substantial populations of eukaryotes, which preferentially consume and incorporatezincrelative to prokaryotes, approximately a billion years after their origin (at the latest).[91]

See also

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