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Eukaryotes () are
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological me ...

organism
s whose
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
have a
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
enclosed within a
nuclear envelope The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive electr ...

nuclear envelope
. Eukaryotes belong to the
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
Eukaryota or Eukarya; their name comes from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
εὖ (''eu'', "well" or "good") and κάρυον (''karyon'', "nut" or "kernel"). The domain Eukaryota makes up one of the three domains of life;
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
(the
prokaryotes A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...

prokaryotes
) make up the other two domains. The eukaryotes are usually now regarded as having emerged in the Archaea or as a sister of the now cultivated
Asgard Asgard (Old Norse Old Norse, Old Nordic, or Old Scandinavian is a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants ...
archaea. Eukaryotes represent a tiny minority of the number of organisms; however, due to their generally much larger size, their collective global
biomass Biomass is plant or animal material used as fuel to produce electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position ...
is estimated to be about equal to that of prokaryotes. Eukaryotes emerged approximately 2.1–1.6 billion years ago, during the
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 541million years ago. It is the most recent part of the Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest par ...
eon, likely as
flagellated A flagellum (; plural: flagella) is a lash-like appendage that protrudes from the soma (biology), cell body of certain cells termed as flagellates. A flagellate can have one or several flagella. The primary function of a flagellum is that of ani ...
phagotrophs. Eukaryotic cells typically contain other membrane-bound organelles such as
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
and
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that stud ...

Golgi apparatus
; and
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s can be found in
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s and
algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grouping that includes species from multiple distinct s. Included organisms range from , such as '','' and the s, to forms, such as the , a large whi ...

algae
. Prokaryotic cells may contain primitive organelles. Eukaryotes may be either
unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Or ...
or
multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biolo ...
, and include many
cell type A cell type is a classification used to distinguish between morphologically or phenotypically distinct cell forms within a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank o ...
s forming different kinds of
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
; in comparison, prokaryotes are typically unicellular.
Animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

Animal
s,
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s, and
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
are the most familiar eukaryotes; other eukaryotes are sometimes called
protist A protist () is any (that is, an organism whose contain a ) that is not an , , or . While it is likely that protists share a (the ), the exclusion of other eukaryotes means that protists do not form a natural group, or . Therefore, some pro ...
s. Eukaryotes can reproduce both asexually through
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
and sexually through
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
and
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis,
DNA replication In , DNA replication is the of producing two identical replicas of DNA from one original molecule. DNA replication occurs in all acting as the most essential part for . This is essential for cell division during growth and repair of damaged tis ...

DNA replication
is followed by two rounds of
cell division Cell division is the process by which a parent cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religiou ...

cell division
to produce four
haploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
daughter cells. These act as sex cells or gametes. Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental
chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A mole ...

chromosome
s resulting from
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, ph ...
during meiosis.


History of the concept

The concept of the eukaryote has been attributed to the French biologist Edouard Chatton (1883–1947). The terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist
Roger Stanier Roger Yate Stanier (22 October 1916 – 29 January 1982) was a Canadian microbiologist who was influential in the development of modern microbiology. As a member of the Delft School and former student of C. B. van Niel, he made important contrib ...
and the Dutch-American microbiologist
C. B. van Niel Cornelis Bernardus van Niel (November 4, 1897, Haarlem Haarlem (; predecessor of ''Harlem'' in English) is a city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. a ...
in 1962. In his 1937 work ''Titres et Travaux Scientifiques'', Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes. However he mentioned this in only one paragraph, and the idea was effectively ignored until his statement was rediscovered by Stanier and van Niel. In 1905 and 1910, the Russian biologist
Konstantin Mereschkowski Konstantin Sergeevich Mereschkowski ( rus, Константи́н Серге́евич Мережко́вский, p=mʲɪrʲɪˈʂkofskʲɪj; – 9 January 1921) was a prominent Russian biologist Francesco Redi, the founder of biology, is re ...

Konstantin Mereschkowski
(1855–1921) argued that
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
s were reduced
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a of that obtain energy via . The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", though modern botanists restrict the term ' to s and do not ...

cyanobacteria
in a
symbiosis Symbiosis (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...
with a non-
photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthetic
(
heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following peri ...
ic) host that was itself formed by symbiosis between an amoeba-like host and a bacterium-like cell that formed the nucleus. Plants had thus inherited photosynthesis from cyanobacteria. In 1967,
Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. ...

Lynn Margulis
provided microbiological evidence for
endosymbiosis An endosymbiont or endobiont is any organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mol ...
as the origin of chloroplasts and mitochondria in eukaryotic cells in her paper, ''On the origin of mitosing cells.'' In the 1970s,
Carl Woese Carl Richard Woese (; July 15, 1928 – December 30, 2012) was an American microbiologist A microbiologist (from Greek ) is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an B ...

Carl Woese
explored microbial
phylogenetics In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

phylogenetics
, studying variations in
16S ribosomal RNA 16 S ribosomal RNA (or 16 S rRNA Ribosomal ribonucleic acid (rRNA) is a type of which is the primary component of s, essential to all cells. rRNA is a which carries out in ribosomes. Ribosomal RNA is transcribed from (rDNA) and then ...
. This helped to uncover the origin of the eukaryotes and the
symbiogenesis Symbiogenesis, endosymbiotic theory, or serial endosymbiotic theory is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly oth ...

symbiogenesis
of two important eukaryote
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s,
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
and
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s. In 1977, Woese and George Fox introduced a "third form of life", which they called the Archaebacteria; in 1990, Woese,
Otto Kandler Otto Kandler (23 October 1920 in Deggendorf - 29 August 2017 in Munich Munich ( ; german: München ; bar, Minga ) is the capital and most populous city of Bavaria. With a population of 1,558,395 inhabitants as of 31 July 2020, it is the ...
and Mark L. Wheelis renamed this the Archaea. In 1979, G. W. Gould and G. J. Dring suggested that the eukaryotic cell's
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
came from the ability of Firmicute
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
to form
endospore An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), dom ...
s. In 1987 and later papers,
Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology in the Department of Zoolog ...
proposed instead that the membranes of the nucleus and
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
first formed by infolding a prokaryote's plasma membrane. In the 1990s, several other biologists proposed endosymbiotic origins for the nucleus, effectively reviving Mereschkowski's theory.


Cell features

Eukaryotic cells Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

Eukaryotic cells
are typically much larger than those of
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...
s, having a volume of around 10,000 times greater than the prokaryotic cell. They have a variety of internal membrane-bound structures, called
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s, and a
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...

cytoskeleton
composed of
microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, rep ...

microtubule
s,
microfilament Actin cytoskeleton of mouse embryo fibroblasts, stained with Fluorescein isothiocyanate-phalloidin, 250px Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cell (biology), cells that form part of the ...
s, and
intermediate filament Intermediate filaments (IFs) are cytoskeletal 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking prote ...

intermediate filament
s, which play an important role in defining the cell's organization and shape. Eukaryotic
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
is divided into several linear bundles called
chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A mole ...

chromosome
s, which are separated by a during nuclear division.


Internal membranes

Eukaryote cells include a variety of membrane-bound structures, collectively referred to as the
endomembrane system The endomembrane system is composed of the different membranes that are suspended in the cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life ...

endomembrane system
. Simple compartments, called
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features of ...
and
vacuole A vacuole () is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts o ...
s, can form by budding off other membranes. Many cells ingest food and other materials through a process of
endocytosis Endocytosis is a in which are brought into the cell. The material to be internalized is surrounded by an area of , which then buds off inside the cell to form a containing the ingested material. Endocytosis includes (cell drinking) and (cell ...

endocytosis
, where the outer membrane and then pinches off to form a vesicle. It is probable that most other membrane-bound organelles are ultimately derived from such vesicles. Alternatively some products produced by the cell can leave in a vesicle through
exocytosis Exocytosis () is a form of active transport In cellular biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, phys ...
. The nucleus is surrounded by a double membrane known as the
nuclear envelope The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive electr ...

nuclear envelope
, with
nuclear pore A nuclear pore is a part of a large complex of proteins, known as a nuclear pore complex that spans the nuclear envelope, which is the double membrane surrounding the eukaryotic Eukaryotes () are organism In biology, an organism ( ...
s that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
, which is involved in protein transport and maturation. It includes the rough endoplasmic reticulum where
ribosome Ribosomes ( ), also called Palade granules, are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (mRNA translation). Ribosomes link amino acids ...

ribosome
s are attached to synthesize proteins, which enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles (
cisterna A cisterna (plural cisternae) is a flattened membrane vesicle of the endoplasmic reticulum and Golgi apparatus. A Golgi stack may contain anywhere from three to twenty cisternae, but most contain about six cisternae. Golgi cisternae can be sepa ...

cisterna
e), the
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that stud ...

Golgi apparatus
. Vesicles may be specialized for various purposes. For instance,
lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

lysosome
s contain digestive
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s that break down most
biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a Nobel Prize in Chemistry, Nobel Prize i ...
s in the cytoplasm.
Peroxisome Image:Peroxisome.svg, 300px, Basic structure of a peroxisome A peroxisome () is a membrane-bound organelle a type of microbody, found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular ...

Peroxisome
s are used to break down
peroxide Peroxides are a group of compounds with the structure R−O−O−R, where R = any element. The O−O group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen p ...

peroxide
, which is otherwise toxic. Many
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

protozoa
ns have contractile vacuoles, which collect and expel excess water, and extrusomes, which expel material used to deflect predators or capture prey. In higher plants, most of a cell's volume is taken up by a central vacuole, which mostly contains water and primarily maintains its .


Mitochondria

Mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

Mitochondria
are organelles found in all but one eukaryote. Mitochondria provide energy to the eukaryote cell by oxidising sugars or fats and releasing energy as
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
. They have two surrounding
membranes Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membr ...

membranes
, each a phospholipid bi-layer; the inner of which is folded into invaginations called
cristae A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it al ...
where
aerobic respiration Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
takes place. The outer mitochondrial membrane is freely permeable and allows almost anything to enter into the
intermembrane space #REDIRECT Intermembrane space#REDIRECT Intermembrane space The intermembrane space (IMS) is the space occurring between or involving two or more membranes. In cell biology, it is most commonly described as the region between the inner membrane and ...
while the inner mitochondrial membrane is semi permeable so allows only some required things into the mitochondrial matrix. Mitochondria contain , which has close structural similarities to bacterial DNA, and which encodes
rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA A non-coding RNA (ncRNA) is an RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consis ...
and
tRNA Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an e ...

tRNA
genes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA. They are now generally held to have developed from
endosymbiotic An endosymbiont or endobiont is any organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synony ...

endosymbiotic
prokaryotes, probably
proteobacteria Proteobacteria is a major phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plu ...
. Some eukaryotes, such as the
metamonad The metamonads are microscopic eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, prope ...
s such as ''
Giardia ''Giardia'' ( or ) is a genus of anaerobic flagellated protozoan parasites of the phylum metamonada that colonise and reproduce in the small intestines of several vertebrates, causing giardiasis. Their life cycle alternates between a swimmi ...

Giardia
'' and ''
Trichomonas ''Trichomonas'' is a genus of anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also re ...

Trichomonas
'', and the amoebozoan '' Pelomyxa'', appear to lack mitochondria, but all have been found to contain mitochondrion-derived organelles, such as
hydrogenosome A hydrogenosome is a membrane-enclosed organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are par ...

hydrogenosome
s and
mitosome A mitosome is an organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ ( ...
s, and thus have lost their mitochondria secondarily. They obtain energy by enzymatic action on nutrients absorbed from the environment. The metamonad ''
Monocercomonoides ''Monocercomonoides'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also ...
'' has also acquired, by
lateral gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring ( reproduction). ...
, a cytosolic
sulfur Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
mobilisation system which provides the clusters of iron and sulfur required for protein synthesis. The normal mitochondrial iron-sulfur cluster pathway has been lost secondarily.


Plastids

Plants and various groups of
algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grouping that includes species from multiple distinct s. Included organisms range from , such as '','' and the s, to forms, such as the , a large whi ...

algae
also have
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
s. Plastids also have their own DNA and are developed from
endosymbionts An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualism (biology), mutualistic relationship. (The term endosymbiosis is from the Greek language, Greek: ἔ ...

endosymbionts
, in this case
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a of that obtain energy via . The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", though modern botanists restrict the term ' to s and do not ...

cyanobacteria
. They usually take the form of
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s which, like cyanobacteria, contain
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally ...

chlorophyll
and produce organic compounds (such as
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
) through
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
. 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 through
secondary endosymbiosis Symbiogenesis, endosymbiotic theory, or serial endosymbiotic theory is the leading evolutionary theory of the origin of eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ' ...
or ingestion. The capture and sequestering of photosynthetic cells and chloroplasts occurs in many types of modern eukaryotic organisms and is known as
kleptoplasty Kleptoplasty or kleptoplastidy is a symbiotic phenomenon whereby plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other euka ...
. Endosymbiotic origins have also been proposed for the nucleus, and for eukaryotic
flagella A flagellum (; ) is a hairlike appendage that protrudes from a wide range of microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and ...
.


Cytoskeletal structures

Many eukaryotes have long slender motile cytoplasmic projections, called
flagella A flagellum (; ) is a hairlike appendage that protrudes from a wide range of microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and ...
, or similar structures called
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...
. Flagella and cilia are sometimes referred to as undulipodia, and are variously involved in movement, feeding, and sensation. They are composed mainly of
tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular protein 300px, 3-dimensional structure of hemoglobin, a globular protein. Globular proteins or spheroproteins are spherical ("globe-like") protein ...

tubulin
. These are entirely distinct from prokaryotic flagellae. They are supported by a bundle of microtubules arising from a
centriole In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...

centriole
, characteristically arranged as nine doublets surrounding two singlets. Flagella also may have hairs, or
mastigonemes A chrysomonad ( Heterokonta: Chrysophyceae) under Transmission electron microscopy">TEM, with a smooth flagellum (1) and a long flagellum covered with mastigonemes (3) Mastigonemes are lateral "hairs" that attach to protistan flagella A fla ...
, and scales connecting membranes and internal rods. Their interior is continuous with the cell's
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
. Microfilamental structures composed of
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments. It is found in essentially all Eukaryote, eukaryotic cells, where it may be present at a concentration of over 100 Micromolar, μ ...
and actin binding proteins, e.g., α-
actinin Actinin is a microfilament Actin cytoskeleton of mouse embryo fibroblasts, stained with Fluorescein isothiocyanate-phalloidin, 250px Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cell (biology), ...
,
fimbrin Fimbrin also known as is plastin 1 is a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within org ...
,
filamin Filamins are a class of proteins that hold two actin filaments at large angles. Filamin protein in mammals is made up of an actin-binding domain at its N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or ...
are present in submembranous cortical layers and bundles, as well. Motor proteins of microtubules, e.g., dynein or kinesin and actin, e.g., myosins provide dynamic character of the network. Centrioles are often present even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form a primary component of the cytoskeletal structure, 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. The significance of cytoskeletal structures is underlined in the determination of shape of the cells, as well as their being essential components of migratory responses like chemotaxis and chemokinesis. Some protists have various other microtubule-supported organelles. These include the radiolaria and heliozoa, which produce Pseudopodia#Morphology, axopodia used in flotation or to capture prey, and the haptophytes, which have a peculiar flagellum-like organelle called the haptonema.


Cell wall

The cells of plants and algae, fungi and most Chromalveolata, chromalveolates have a cell wall, a layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major polysaccharides making up the primary cell wall of land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.


Differences among eukaryotic cells

There are many different types of eukaryotic cells, though animals and plants are the most familiar eukaryotes, and thus provide an excellent starting point for understanding eukaryotic structure. Fungi and many protists have some substantial differences, however.


Animal cell

All animals are eukaryotic. Animal cells are distinct from those of other eukaryotes, most notably plant cells, plants, as they lack cell walls and
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s and have smaller
vacuole A vacuole () is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts o ...
s. Due to the lack of a cell wall, animal cells can transform into a variety of shapes. A phagocyte, phagocytic cell can even engulf other structures.


Plant cell

Plant cells are quite different from the cells of the other eukaryotic organisms. Their distinctive features are: * A large central vacuole (enclosed by a membrane, the tonoplast), which maintains the cell's turgor and controls movement of molecules between the cytosol and plant sap, sap * A primary cell wall containing cellulose, hemicellulose and pectin, deposited by the protoplast on the outside of the cell membrane; this contrasts with the cell walls of fungus, fungi, which contain chitin, and the cell envelopes of prokaryotes, in which peptidoglycans are the main structural molecules * The plasmodesmata, pores in the cell wall that link adjacent cells and allow plant cells to communicate with adjacent cells. Animals have a different but functionally analogous system of gap junctions between adjacent cells. * Plastids, especially
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s,
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s that contain
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally ...

chlorophyll
, the pigment that gives
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s their green color and allows them to perform
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
* Bryophytes and pteridophyte, seedless vascular plants only have flagellae and centrioles in the sperm cells. Sperm of cycads and ''Ginkgo'' are large, complex cells that swim with hundreds to thousands of flagellae. * Pinophyta, Conifers (Pinophyta) and flowering plants (Angiospermae) lack the flagellum, flagellae and
centriole In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...

centriole
s that are present in animal cells.


Fungal cell

The cells of fungi are similar to animal cells, with the following exceptions: * A cell wall that contains chitin * Less compartmentation between cells; the hyphae of higher fungi have porous partitions called septum, septa, which allow the passage of cytoplasm, organelles, and, sometimes, nuclei; so each organism is essentially a giant multinucleate supercell – these fungi are described as coenocytic. Primitive fungi have few or no septa. * Only the most primitive fungi, chytrids, have flagella.


Other eukaryotic cells

Some groups of eukaryotes have unique organelles, such as the cyanelles (unusual plastids) of the glaucophytes, the haptonema of the haptophytes, or the ejectosomes of the cryptomonads. Other structures, such as pseudopodia, are found in various eukaryote groups in different forms, such as the lobose amoebozoans or the reticulose foraminiferans.


Reproduction

Cell division generally takes place asexually by
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
, a process that allows each daughter nucleus to receive one copy of each
chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A mole ...

chromosome
. Most eukaryotes also have a life cycle that involves sexual reproduction, alternation of generations, alternating between a haploid phase, where only one copy of each chromosome is present in each cell and a diploid phase, wherein two copies of each chromosome are present in each cell. The diploid phase is formed by fusion of two haploid gametes to form a zygote, which may divide by mitosis or undergo chromosome reduction by
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
. There is considerable variation in this pattern. Animals have no multicellular haploid phase, but each plant generation can consist of haploid and diploid multicellular phases. Eukaryotes have a smaller surface area to volume ratio than prokaryotes, and thus have lower metabolic rates and longer generation times. The evolution of sexual reproduction may be a primordial and fundamental characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Andrew J. Roger, Roger proposed that facultative sex was present in the common ancestor of all eukaryotes. A core set of genes that function in meiosis is present in both ''Trichomonas vaginalis'' and ''Giardia intestinalis'', two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from the eukaryotic evolutionary tree, it was inferred that core meiotic genes, and hence sex, were likely present in a common ancestor of all eukaryotes. Eukaryotic species once thought to be asexual, such as parasitic protozoa of the genus ''Leishmania'', have been shown to have a sexual cycle. Also, evidence now indicates that amoebae, previously regarded as asexual, are anciently sexual and that the majority of present-day asexual groups likely arose recently and independently.


Classification

In Ancient history, antiquity, the two lineages of animals and
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s were recognized. They were given the taxonomic rank of Kingdom (biology), Kingdom by Carl Linnaeus, Linnaeus. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom, the composition of which was not entirely clear until the 1980s. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word ''
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

protozoa
'' to refer to organisms such as ciliates, and this group was expanded until it encompassed all single-celled eukaryotes, and given their own kingdom, the Protista, by Ernst Haeckel in 1866. The eukaryotes thus came to be composed of four kingdoms: * Kingdom Protista * Kingdom Plantae * Kingdom Fungi * Kingdom Animalia The protists were understood to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature. The disentanglement of the deep splits in the tree of life (biology), tree of life only really started with Nucleic acid sequence, DNA sequencing, leading to a system of domain (biology), domains rather than kingdoms as top level rank being put forward by
Carl Woese Carl Richard Woese (; July 15, 1928 – December 30, 2012) was an American microbiologist A microbiologist (from Greek ) is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an B ...

Carl Woese
, uniting all the eukaryote kingdoms under the eukaryote domain. At the same time, work on the protist tree intensified, and is still actively going on today. Several alternative classifications have been forwarded, though there is no consensus in the field. Eukaryotes are a clade usually assessed to be sister to Heimdallarchaeota in the
Asgard Asgard (Old Norse Old Norse, Old Nordic, or Old Scandinavian is a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants ...
grouping in the Archaea. In one proposed system, the basal groupings are the Opimoda, Diphoda, the Discoba, and the Loukozoa. The Eukaryote root is usually assessed to be near or even in Discoba. A Kingdom (biology)#International Society of Protistologists Classification 2005, classification produced in 2005 for the International Society of Protistologists, which reflected the consensus of the time, divided the eukaryotes into six supposedly monophyletic 'supergroups'. However, in the same year (2005), doubts were expressed as to whether some of these supergroups were monophyletic, particularly the Chromalveolata, and a review in 2006 noted the lack of evidence for several of the supposed six supergroups. A revised classification in 2012 recognizes five supergroups. There are also smaller groups of eukaryotes whose position is uncertain or seems to fall outside the major groups – in particular, Haptophyta, Cryptophyta, Centrohelida, Telonemia, Picozoa, Apusomonadida, Ancyromonadida, Breviata, Breviatea, and the genus ''Collodictyon''. Overall, it seems that, although progress has been made, there are still very significant uncertainties in the evolutionary history and classification of eukaryotes. As Andrew J. Roger, Roger & Simpson said in 2009 "with the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution." Newly identified protists, purported to represent novel, deep-branching lineages, continue to be described well into the 21st century; recent examples including ''Rhodelphis'', putative sister group to Rhodophyta, and ''Anaeramoeba'', anaerobic amoebaflagellates of uncertain placement.


Phylogeny

The
rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA A non-coding RNA (ncRNA) is an RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consis ...
trees constructed during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group (not technically a true crown group, crown), which was usually divided by the form of the mitochondrial cristae; see crown eukaryotes. The few groups that lack
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
branched separately, and so the absence was believed to be primitive; but this is now considered an artifact of long-branch attraction, and they are known to have lost them secondarily. It has been estimated that there may be 75 distinct lineages of eukaryotes. Most of these lineages are protists. The known eukaryote genome sizes vary from 8.2 megabases (Mb) in ''Babesia bovis'' to 112,000–220,050 Mb in the dinoflagellate ''Prorocentrum micans'', showing that the genome of the ancestral eukaryote has undergone considerable variation during its evolution. The last common ancestor of all eukaryotes is believed to have been a phagotrophic protist with a nucleus, at least one
centriole In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...

centriole
and cilium, facultatively aerobic mitochondria, sex (
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
and syngamy), a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. Later endosymbiosis led to the spread of plastids in some lineages. Although there is still considerable uncertainty in global eukaryote phylogeny, particularly regarding the position of the root, a rough consensus has started to emerge from the phylogenomic studies of the past two decades. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea (similar in composition to the unikont hypothesis) and the Diaphoretickes, which includes plants and most algal lineages. A third major grouping, the Excavata, has been abandoned as a formal group in the most recent classification of the International Society of Protistologists due to growing uncertainty as to whether its constituent groups belong together. The proposed phylogeny below includes only one group of excavates (Discoba, and incorporates the recent proposal that picozoans are close relatives of rhodophytes. In some analyses, the Hacrobia group (Haptophyta + Cryptophyta) is placed next to Archaeplastida, but in others it is nested inside the Archaeplastida. However, several recent studies have concluded that Haptophyta and Cryptophyta do not form a monophyletic group. The former could be a sister group to the SAR supergroup, SAR group, the latter cluster with the Archaeplastida (plants in the broad sense). The division of the eukaryotes into two primary clades, bikonts (Archaeplastida + SAR supergroup, SAR + Excavata) and unikonts (Amoebozoa + Opisthokonta), derived from an ancestral biflagellar organism and an ancestral uniflagellar organism, respectively, had been suggested earlier. A 2012 study produced a somewhat similar division, although noting that the terms "unikonts" and "bikonts" were not used in the original sense. A highly converged and congruent set of trees appears in Derelle et al. (2015), Ren et al. (2016), Yang et al. (2017) and Cavalier-Smith (2015) including the supplementary information, resulting in a more conservative and consolidated tree. It is combined with some results from Cavalier-Smith for the basal Opimoda. The main remaining controversies are the root, and the exact positioning of the Rhodophyta and the bikonts Rhizaria, Haptista, Cryptista, Picozoa and Telonemia, many of which may be endosymbiotic eukaryote-eukaryote hybrids. Archaeplastida acquired
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s probably by endosymbiosis of a prokaryotic ancestor related to a currently extant Cyanobacteria, cyanobacterium, ''Gloeomargarita lithophora''.


Cavalier-Smith's tree

Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology in the Department of Zoolog ...
2010, 2013, 2014, 2017 and 2018 places the eukaryotic tree's root between Excavata (with ventral feeding groove supported by a microtubular root) and the grooveless Euglenozoa, and monophyletic Chromista, correlated to a single endosymbiotic event of capturing a red-algae. He et al. specifically supports rooting the eukaryotic tree between a monophyletic Discoba (Discicristata + Jakobidae, Jakobida) and an Amorphea-Diaphoretickes clade.


Origin of eukaryotes

The origin of the eukaryotic cell is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. A number of approaches have been used to find the first eukaryote and their closest relatives. The last eukaryotic common ancestor (LECA) is the hypothetical Most recent common ancestor, last common ancestor of all eukaryotes that have ever lived, and was most likely a Population, biological population. Eukaryotes have a number of features that differentiate them from prokaryotes, including an
endomembrane system The endomembrane system is composed of the different membranes that are suspended in the cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life ...

endomembrane system
, and unique biochemical pathways such as sterane synthesis. A set of proteins called eukaryotic signature proteins (ESPs) was proposed to identify eukaryotic relatives in 2002: they have no homology to proteins known in other domains of life by then, but they appear to be universal among eukaryotes. They include proteins that make up the cytoskeleton, the complex transcription machinery, membrane-sorting systems, the nuclear pore, as well as some enzymes in the biochemical pathways.


Fossils

The timing of this series of events is hard to determine; Andrew H. Knoll, Knoll (2006) suggests they developed approximately 1.6–2.1 billion years ago. Some acritarchs are known from at least 1.65 billion years ago, and the possible alga ''Grypania'' has been found as far back as 2.1 billion years ago. The ''Geosiphon''-like fossil fungus ''Diskagma'' has been found in paleosols 2.2 billion years old. Organized living structures have been found in the black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, dated at 2.1 billion years old. Eukaryotic life could have evolved at that time. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of a red algae#Fossil record, red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago. The presence of eukaryotic-specific Biomarker (petroleum), biomarkers (steranes) in Australian shales previously indicated that eukaryotes were present in these rocks dated at 2.7 billion years old, which was even 300 million years older than the first geological records of the appreciable amount of molecular oxygen during the Great Oxidation Event. However, these Archaean biomarkers were eventually rebutted as later contaminants. Currently, putatively the oldest biomarker records are only ~800 million years old. In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago, and thus there is a huge gap between molecular data and geological data, which hinders a reasonable inference of the eukaryotic evolution through biomarker records before 800 million years ago. The nature of steranes as eukaryotic biomarkers is further complicated by the production of sterols by some bacteria. Whenever their origins, eukaryotes may not have become ecologically dominant until much later; a massive uptick in the zinc composition of marine sediments has been attributed to the rise of substantial populations of eukaryotes, which preferentially consume and incorporate zinc relative to prokaryotes, approximately a billion years after their origin (at the latest). In April 2019, biologists reported that the very large medusavirus, or a relative, may have been responsible, at least in part, for the Evolution, evolutionary emergence of complex eukaryotic cells from simpler Prokaryote, prokaryotic cells.


Relationship to Archaea

The nuclear DNA and genetic machinery of eukaryotes is more similar to Archaea than Bacteria, leading to a controversial suggestion that eukaryotes should be grouped with Archaea in the clade Neomura. In other respects, such as membrane composition, eukaryotes are similar to Bacteria. Three main explanations for this have been proposed: * Eukaryotes resulted from the complete fusion of two or more cells, wherein the cytoplasm formed from a bacterium, and the nucleus from an archaeon, Viral Eukaryogenesis, from a virus, or from a pre-cell. * Eukaryotes developed from Archaea, and acquired their bacterial characteristics through the Symbiogenesis, endosymbiosis of a proto-mitochondrion of bacterial origin. * Eukaryotes and Archaea developed separately from a modified bacterium. Alternative proposals include: * The chronocyte hypothesis postulates that a primitive eukaryotic cell was formed by the endosymbiosis of both archaea and bacteria by a third type of cell, termed a chronocyte. This is mainly to account for the fact that eukaryotic signature proteins were not found anywhere else by 2002. * The universal common ancestor (UCA) of the current tree of life was a complex organism that survived a mass extinction event rather than an early stage in the evolution of life. Eukaryotes and in particular akaryotes (Bacteria and Archaea) evolved through reductive loss, so that similarities result from differential retention of original features. Assuming no other group is involved, there are three possible phylogenies for the Bacteria, Archaea and Eukaryota in which each is monophyletic. These are labelled 1 to 3 in the table below. The eocyte hypothesis is a modification of hypothesis 2 in which the Archaea are paraphyletic. (The table and the names for the hypotheses are based on Harish and Kurland, 2017.) In recent years, most researchers have favoured either the three domains (3D) or the eocyte hypothesis. An Ribosomal RNA, rRNA analysis supports the eocyte scenario, apparently with the Eukaryote root in Excavata. A cladogram supporting the eocyte hypothesis, positioning eukaryotes within Archaea, based on phylogenomic analyses of the
Asgard Asgard (Old Norse Old Norse, Old Nordic, or Old Scandinavian is a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants ...
archaea, is: In this scenario, the Asgard group is seen as a sister taxon of the TACK group, which comprises Crenarchaeota (formerly named Eocyte hypothesis, eocytes), Thaumarchaeota, and others. This group is reported contain many of the eukaryotic signature proteins and produce vesicles. In 2017, there was significant pushback against this scenario, arguing that the eukaryotes did not emerge within the Archaea. Cunha ''et al.'' produced analyses supporting the three domains (3D) or Woese hypothesis (2 in the table above) and rejecting the eocyte hypothesis (4 above). Harish and Kurland found strong support for the earlier two empires (2D) or Mayr hypothesis (1 in the table above), based on analyses of the coding sequences of protein domains. They rejected the eocyte hypothesis as the least likely. A possible interpretation of their analysis is that the universal common ancestor (UCA) of the current tree of life was a complex organism that survived an evolutionary bottleneck, rather than a simpler organism arising early in the history of life. On the other hand, the researchers who came up with Asgard re-affirmed their hypothesis with additional Asgard samples. Since then, the publication of additional Asgard archaeal genomes and the independent reconstruction of phylogenomic trees by multiple independent laboratories have provided additional support for an Asgard archaeal origin of eukaryotes. Details of the relation of Asgard archaea members and eukaryotes are still under consideration, although, in January 2020, scientists reported that ''Candidatus Prometheoarchaeum syntrophicum'', a type of cultured Asgard archaea, may be a possible link between simple prokaryotic and complex eukaryotic microorganisms about two billion years ago.


Endomembrane system and mitochondria

The origins of the endomembrane system and mitochondria are also unclear. The phagotrophic hypothesis proposes that eukaryotic-type membranes lacking a cell wall originated first, with the development of endocytosis, whereas mitochondria were acquired by ingestion as endosymbionts. The syntrophic hypothesis proposes that the proto-eukaryote relied on the proto-mitochondrion for food, and so ultimately grew to surround it. Here the membranes originated after the engulfment of the mitochondrion, in part thanks to mitochondrial genes (the hydrogen hypothesis is one particular version). In a study using genomes to construct supertrees, Pisani ''et al.'' (2007) suggest that, along with evidence that there was never a mitochondrion-less eukaryote, eukaryotes evolved from a syntrophy between an archaea closely related to Thermoplasmatales and an alphaproteobacterium, likely a symbiosis driven by sulfur or hydrogen. The mitochondrion and its genome is a remnant of the alphaproteobacterial endosymbiont. The majority of the genes from the symbiont have been transferred to the nucleus. They make up most of the metabolic and energy-related pathways of the eukaryotic cell, while the information system (DNA polymerase, transcription, translation) is retained from archaea.


Hypotheses

Different hypotheses have been proposed as to how eukaryotic cells came into existence. These hypotheses can be classified into two distinct classes – autogenous models and chimeric models.


Autogenous models

Autogenous models propose that a proto-eukaryotic cell containing a Cell nucleus, nucleus existed first, and later acquired Mitochondrion, mitochondria. According to this model, a large
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...
developed invaginations in its plasma membrane in order to obtain enough surface area to volume ratio, surface area to service its cytoplasmic volume. As the invaginations differentiated in function, some became separate compartments – giving rise to the
endomembrane system The endomembrane system is composed of the different membranes that are suspended in the cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life ...

endomembrane system
, including the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
, golgi apparatus, nuclear membrane, and single membrane structures such as
lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

lysosome
s. Mitochondria are proposed to come from the endosymbiosis of an aerobic proteobacterium, and it is assumed that all the eukaryotic lineages that did not acquire mitochondria became extinct,. Chloroplasts came about from another endosymbiotic event involving
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a of that obtain energy via . The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", though modern botanists restrict the term ' to s and do not ...

cyanobacteria
. Since all known eukaryotes have mitochondria, but not all have chloroplasts, the serial endosymbiotic theory proposes that mitochondria came first.


Chimeric models

Chimera (genetics), Chimeric models claim that two prokaryotic cells existed initially – an Archaea, archaeon and a Bacteria, bacterium. The closest living relatives of these appears to be Asgard (archaea), Asgardarchaeota and (distantly related) the alphaproteobacteria called the proto-mitochondrion. These cells underwent a merging process, either by a physical fusion or by Endosymbiont, endosymbiosis, thereby leading to the formation of a eukaryotic cell. Within these chimeric models, some studies further claim that mitochondria originated from a bacterial ancestor while others emphasize the role of endosymbiotic processes behind the origin of mitochondria.


The inside-out hypothesis

The inside-out hypothesis suggests that the fusion between free-living mitochondria-like bacteria, and an archaeon into a eukaryotic cell happened gradually over a long period of time, instead of in a single phagocytosis#In protists, phagocytotic event. In this scenario, an archaeon would trap aerobic bacteria with Bleb (cell biology), cell protrusions, and then keep them alive to draw energy from them instead of digesting them. During the early stages the bacteria would still be partly in direct contact with the environment, and the archaeon would not have to provide them with all the required nutrients. But eventually the archaeon would engulf the bacteria completely, creating the internal membrane structures and nucleus membrane in the process. It is assumed the archaean group called halophiles went through a similar procedure, where they acquired as much as a thousand genes from a bacterium, way more than through the conventional horizontal gene transfer that often occurs in the microbial world, but that the two microbes separated again before they had fused into a single eukaryote-like cell. An expanded version of the inside-out hypothesis proposes that the eukaryotic cell was created by physical interactions between two prokaryotic organisms and that the last common ancestor of eukaryotes got its genome from a whole population or community of microbes participating in cooperative relationships to thrive and survive in their environment. The genome from the various types of microbes would complement each other, and occasional horizontal gene transfer between them would be largely to their own benefit. This accumulation of beneficial genes gave rise to the genome of the eukaryotic cell, which contained all the genes required for independence.


The serial endosymbiotic hypothesis

According to Symbiogenesis, serial endosymbiotic theory (championed by
Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. ...

Lynn Margulis
), a union between a motile anaerobic organism, anaerobic bacterium (like ''Spirochaeta'') and a thermoacidophilic crenarchaeon (like ''Thermoplasma'' which is sulfidogenic in nature) gave rise to the present day eukaryotes. This union established a motile organism capable of living in the already existing acidic and sulfurous waters. Oxygen is known to cause toxicity to organisms that lack the required Metabolism, metabolic machinery. Thus, the archaeon provided the bacterium with a highly beneficial reduced environment (sulfur and sulfate were reduced to sulfide). In microaerophilic conditions, oxygen was reduced to water thereby creating a mutual benefit platform. The bacterium on the other hand, contributed the necessary Fermentation (biochemistry), fermentation products and electron acceptors along with its motility feature to the archaeon thereby gaining a swimming motility for the organism. From a consortium of bacterial and archaeal DNA originated the nuclear genome of eukaryotic cells. Spirochetes gave rise to the motile features of eukaryotic cells. Endosymbiotic unifications of the ancestors of alphaproteobacteria and cyanobacteria, led to the origin of mitochondria and
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spa ...
s respectively. For example, ''Thiodendron'' has been known to have originated via an Ectosymbiosis, ectosymbiotic process based on a similar syntrophy of sulfur existing between the two types of bacteria – ''Desulfobacter'' and ''Spirochaeta''. However, such an association based on motile symbiosis has never been observed practically. Also there is no evidence of archaeans and spirochetes adapting to intense acid-based environments.


The hydrogen hypothesis

In the hydrogen hypothesis, the symbiotic linkage of an anaerobic and autotrophic methanogenic archaeon (host) with an alphaproteobacterium (the symbiont) gave rise to the eukaryotes. The host utilized hydrogen (H2) and carbon dioxide () to produce methane while the symbiont, capable of aerobic respiration, expelled H2 and as byproducts of anaerobic fermentation process. The host's methanogenic environment worked as a sink for H2, which resulted in heightened bacterial fermentation. Endosymbiotic gene transfer acted as a catalyst for the host to acquire the symbionts' carbohydrate metabolism and turn
heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following peri ...
ic in nature. Subsequently, the host's methane forming capability was lost. Thus, the origins of the heterotrophic organelle (symbiont) are identical to the origins of the eukaryotic Lineage (evolution), lineage. In this hypothesis, the presence of H2 represents the selective force that forged eukaryotes out of prokaryotes.


The syntrophy hypothesis

The syntrophy hypothesis was developed in contrast to the hydrogen hypothesis and proposes the existence of two symbiotic events. According to this model, the origin of eukaryotic cells was based on metabolic symbiosis (syntrophy) between a methanogenic archaeon and a deltaproteobacterium. This syntrophic symbiosis was initially facilitated by H2 transfer between different species under anaerobic environments. In earlier stages, an alphaproteobacterium became a member of this integration, and later developed into the mitochondrion. Gene transfer from a deltaproteobacterium to an archaeon led to the methanogenic archaeon developing into a nucleus. The archaeon constituted the genetic apparatus, while the deltaproteobacterium contributed towards the
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
ic features. This theory incorporates two selective forces at the time of nucleus evolution * presence of metabolic partitioning to avoid the harmful effects of the co-existence of Anabolism, anabolic and Catabolism, catabolic cellular pathways, and * prevention of abnormal protein biosynthesis due to a vast spread of introns in the archaeal genes after acquiring the mitochondrion and losing methanogenesis.


6+ serial endosymbiosis scenario

A complex scenario of 6+ serial endosymbiotic events of archaea and bacteria has been proposed in which mitochondria and an asgard related archaeota were acquired at a late stage of eukaryogenesis, possibly in combination, as a secondary endosymbiont. The findings have been rebuked as an artefact.


See also

* Eukaryote hybrid genome * Evolution of sexual reproduction * List of sequenced eukaryotic genomes * ''Parakaryon myojinensis'' * Prokaryote * Thaumarchaeota * Vault (organelle)


Notes


References


External links


"Eukaryotes"
(Tree of Life Web Project) *
Attraction and sex among our microbial Last Eukaryotic Common Ancestors
The Atlantic, November 11, 2020 {{Authority control Eukaryotes, Articles containing video clips Domains (biology), Eukaryote