The wordeukaryote is derived from theGreek words "eu" (εὖ) meaning "true" or "good" and "karyon" (κάρυον) meaning "nut" or "kernel", referring to thenucleus of acell.[5]
Eukaryotes areorganisms that range from microscopic singlecells, such aspicozoans under 3 micrometres across,[6] toanimals like theblue whale, weighing up to 190tonnes and measuring up to 33.6 metres (110 ft) long,[7] orplants like thecoast redwood, up to 120 metres (390 ft) tall.[8] Many eukaryotes are unicellular; the informal grouping calledprotists includes many of these, with some multicellular forms like thegiant kelp up 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—thebacteria and 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]
Eukaryotes range in size from single-celled organisms to huge whales
The defining feature of eukaryotes is thattheir cells have a well-defined, membrane-boundnuclei, distinguishing them fromprokaryotes that lack such a structure. Eukaryotic cells have a variety of internal membrane-bound structures, calledorganelles, and acytoskeleton which defines the cell's organization and shape. The nucleus stores the cell'sDNA, which is divided into linear bundles calledchromosomes;[18] these are separated into two matching sets by amicrotubular spindle during nuclear division, in the distinctively eukaryotic process ofmitosis.[19]
Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such assterane synthesis.[20] The eukaryotic signatureproteins have no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton, the complextranscription machinery, the membrane-sorting systems, thenuclear pore, and someenzymes in the biochemical pathways.[21]
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 the endomembrane system.[22] Simple compartments, calledvesicles andvacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process ofendocytosis, where the outer membraneinvaginates and then pinches off to form a vesicle.[23] Some cell products can leave in a vesicle throughexocytosis.[24]
The nucleus is surrounded by a double membrane known as thenuclear envelope, withnuclear pores that allow material to move in and out.[25] Various tube- and sheet-like extensions of the nuclear membrane form theendoplasmic reticulum, which is involved inprotein transport and maturation. It includes the rough endoplasmic reticulum, covered inribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum.[26] In most eukaryotes, these protein-carrying vesicles are released and their contents further modified in stacks of flattened vesicles (cisternae), theGolgi apparatus.[27]
Mitochondria are essentially universal in the eukaryotes, and with their ownDNA somewhat resemble prokaryotic cells.
Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell",[29] for its function providing energy by oxidising sugars or fats to produce the energy-storing moleculeATP.[30][31] Mitochondria have two surroundingmembranes, each aphospholipid bilayer, theinner of which is folded into invaginations calledcristae whereaerobic respiration takes place.[32]
Mitochondria containtheir own DNA, which has close structural similarities tobacterial DNA, from which it originated, and which encodesrRNA andtRNA genes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA.[33]
Some eukaryotes, such as themetamonadsGiardia andTrichomonas, and the amoebozoanPelomyxa, appear to lack mitochondria, but all contain mitochondrion-derived organelles, likehydrogenosomes ormitosomes, having lost their mitochondria secondarily.[34] They obtain energy by enzymatic action in the cytoplasm.[35][34] It is thought that mitochondria developed fromprokaryotic cells which becameendosymbionts living inside eukaryotes.[36]
Plants and various groups ofalgae have plastids as well as mitochondria. Plastids, like mitochondria, havetheir own DNA and are developed fromendosymbionts, in this casecyanobacteria. They usually take the form ofchloroplasts which, like cyanobacteria, containchlorophyll and 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 endosymbiosis or ingestion.[37] The capture and sequestering of photosynthetic cells and chloroplasts,kleptoplasty, occurs in many types of modern eukaryotic organisms.[38][39]
Many eukaryotes have long slender motile cytoplasmic projections, calledflagella, or multiple shorter structures calledcilia.These organelles are 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 ofmicrotubules arising 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, butconifers andflowering plants have 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]
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-expansion when water enters the cell.[45]
Eukaryotes have a life cycle that involvessexual reproduction, alternating between ahaploid phase, where only one copy of each chromosome is present in each cell, and adiploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such aseggs andspermatozoa, 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 reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks andRoger have 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 vaginalis andGiardia 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 asLeishmania parasites, have a sexual cycle.[53] Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently.[54]
Inantiquity, the two lineages ofanimals andplants were recognized byAristotle andTheophrastus. The lineages were given thetaxonomic rank of kingdom byLinnaeus in the 18th century. Though he included thefungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom.[55] The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologistGeorg A. Goldfuss coined the wordProtozoa to refer to organisms such asciliates,[56] and this group was expanded untilErnst Haeckel made it a kingdom encompassing all single-celled eukaryotes, theProtista, in 1866.[57][58][59] 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.[58] Understanding of the oldest branchings in thetree of life only developed substantially withDNA sequencing, leading to a system ofdomains rather than kingdoms as top level rank being put forward byCarl Woese,Otto Kandler, andMark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that"'eukaryotes' will continue to be an acceptable common synonym".[1][60] In 1996, the evolutionary biologistLynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create a "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)".[2]
Tree of eukaryotes showing major subgroups and thumbnail diagrams of representative members of each group, based on 2023 phylogenomic reconstructions.[61]
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 theunikont hypothesis) 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 was found to beparaphyletic.[63] The proposed phylogeny below includes two groups of excavates (Discoba andMetamonada),[64] and incorporates the 2021 proposal thatpicozoans are close relatives of rhodophytes.[65] TheProvora are a group of microbial predators discovered in 2022.[66]
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,[71] and was most likely abiological population, not a single individual.[72] The LECA is believed to have been a protist with a nucleus, at least onecentriole andflagellum, facultatively aerobic mitochondria, sex (meiosis andsyngamy), a dormantcyst with a cell wall ofchitin orcellulose, andperoxisomes.[73][74][75]
The presence of eukaryotic biomarkers in archaea points towards an archaeal origin. The genomes of Promethearchaeati archaea have plenty ofeukaryotic signature protein genes, which play a crucial role in the development of thecytoskeleton and complex cellular structures characteristic of eukaryotes. In 2022,cryo-electron tomography demonstrated that Promethearchaeati archaea have a complexactin-based cytoskeleton, providing the first direct visual evidence of the archaeal ancestry of eukaryotes.[76]
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 1.635 billion years ago, suggests that the crown group eukaryotes originated from the latePaleoproterozoic (Statherian). The earliest unequivocal unicellular eukaryotes,Tappania plana,Shuiyousphaeridium macroreticulatum,Dictyosphaera macroreticulata,Germinosphaera alveolata, andValeria lophostriata from North China, lived approximately 1.65 billion years ago.[77]
Reconstruction of the problematic[80]Diskagma buttonii, a terrestrial fossil less than 1mm high, from rocks around 2.2 billion years old
TheNeoarchean fossilThuchomyces shares similarities with eukaryotes, specifically fungi. It especially resembles the problematic fossilDiskagma,[80] with hyphae and multiple differentiated layers.[81] However, it is over 600 million years older than all other possible eukaryotes, and many of its "eukaryote features" are not specific to the clade, meaning it is almost certainly a microbial mat instead.[82]
Structures proposed to represent "large colonial organisms" have been found in theblack shales of thePalaeoproterozoic such as theFrancevillian B Formation, inGabon, dubbed the "Francevillian biota" which is dated at 2.1 billion years old.[83][84] However, the status of these structures as fossils is contested, with other authors suggesting that they might representpseudofossils.[85] The oldest fossils that can unambiguously be assigned to eukaryotes are from the Ruyang Group of China, dating to approximately 1.8-1.6 billion years ago.[86] 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 algae in theVindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.[87]
The presence ofsteranes, eukaryotic-specificbiomarkers, inAustralianshales previously indicated that eukaryotes were present in these rocks dated at 2.7 billion years old,[20][88] but these Archaean biomarkers have been rebutted as later contaminants.[89] The oldest valid biomarker records are only around 800 million years old.[90] In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago.[91] The nature of steranes as eukaryotic biomarkers is further complicated by the production ofsterols by some bacteria.[92][93]
Whenever their origins, eukaryotes may not have become ecologically dominant until much later; a massive increase in thezinc composition of marine sediments800 million years ago has been attributed to the rise of substantial populations of eukaryotes, which preferentially consume and incorporatezinc relative to prokaryotes, approximately a billion years after their origin (at the latest).[94]
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