A
eukaryote ( or ) is an
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
whose
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
contain complex structures enclosed within
membranesA biological membrane or biomembrane is an enclosing or separating amphipathic layer that acts as a barrier within or around a cell. It is almost invariably a lipid bilayer, composed of a double layer of lipid molecules and proteins that may constitute close to 50% of membrane...
. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the
nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
, or
nuclear envelopeThe nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...
, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
ευ (eu, "good", "noble" & "true") and κάρυον (karyon, "nut" & "kernel"). Most eukaryotic cells also contain other membrane-bound
organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
s such as
mitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
,
chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s and the
Golgi apparatusThe Golgi apparatus is an organelle found in most eukaryotic cells...
. Almost all
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
of large organisms are eukaryotes, including
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s,
plantPlants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants,...
s and
fungiA fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants, animals and bacteria...
, although most species of eukaryotic
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s are
microorganismA microorganism or microbe is an organism that is microscopic...
s.
Cell divisionCell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing...
in eukaryotes is different from that in organisms without a nucleus (prokaryotes). It involves separating the duplicated
chromosomeA chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions...
s, through movements directed by microtubules. There are two types of division processes. In
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
, one cell divides to produce two genetically identical cells. In
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
, which is required in
sexual reproductionSexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration...
, one diploid cell (having two instances of each chromosome, one from each parent) undergoes
recombinationGenetic recombination is the process by which a strand of genetic material is broken and then joined to a different DNA molecule. In eukaryotes recombination occurs in mitosis as a common mechanism of DNA repair and in meiosis as a way of facilitating chromosomal crossover...
of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.
Eukaryotes appear to be monophyletic, and so make up one of the three
domainsIn biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...
of life. The two other domains,
BacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and
ArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, are prokaryotes and have none of the above features.
Cell features
Eukaryotic cells are typically much larger than
prokaryoteThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles...
s. They have a variety of internal membranes and structures, called
organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
s, and a
cytoskeletonThe cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm. The cytoskeleton is present in all cells; it was once thought this structure was unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
composed of
microtubuleMicrotubules are one of the components of the cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers. Microtubules serve as structural components within cells and are involved in many cellular processes including mitosis, cytokinesis, and vesicular...
s,
microfilamentMicrofilaments are the thinnest filaments of the cytoskeleton found in the cytoplasm of all eukaryotic cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces...
s, and
intermediate filamentIntermediate filaments are a family of related proteins that share common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of actin and microtubules, although they were initially designated 'intermediate' because their...
s, which play an important role in defining the cell's organization and shape. Eukaryotic
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
is divided into several linear bundles called
chromosomeA chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions...
s, which are separated by a microtubular spindle during nuclear division.
Internal membrane
Eukaryotic cells include a variety of membrane-bound structures, collectively referred to as the
endomembrane systemThe endomembrane system is composed of the different membranes that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles...
. Simple compartments, called
vesiclesA vesicle is a bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell. Vesicles form naturally because of the properties of lipid membranes ...
or
vacuolethumb|400px|Plant cell structurethumb|400px|Animal cell structureA vacuole is a membrane organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells...
s, can form by budding off other membranes. Many cells ingest food and other materials through a process of
endocytosisEndocytosis is the process by which cells absorb molecules from outside the cell by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma membrane or cell membrane...
, where the outer membrane invaginates and then pinches off to form a vesicle. It is probable that most other membrane-bound organelles are ultimately derived from such vesicles.
The nucleus is surrounded by a double membrane (commonly referred to as a
nuclear envelopeThe nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...
), with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form what is called the
endoplasmic reticulumThe endoplasmic reticulum is an eukaryotic organelle that forms an interconnected network of tubules, vesicles, and cisternae within cells. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F...
or ER, which is involved in protein transport and maturation. It includes the rough ER where
ribosomeRibosomes are complexes of RNA and protein that are found in all cells. The ribosome is part of the mechanism that translates the DNA sequence into the protein sequence. Ribosomes from bacteria, archaea and eukaryotes , have significantly different structure and RNA...
s are attached, and the proteins they synthesize enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth ER. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, called
Golgi bodiesThe Golgi apparatus is an organelle found in most eukaryotic cells...
or dictyosomes.
Vesicles may be specialized for various purposes. For instance,
lysosomethumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelle]]s:
[[nucleolus]]
[[cell nucleus|nucleus]]
[[ribosomes]]
[[vesicle |vesicle]]
...
s contain enzymes that break down the contents of food vacuoles, and
peroxisomePeroxisomes are organelles from the microbody family and are present in almost all eukaryotic cells. They participate in the metabolism of fatty acids and many other metabolites. Peroxisomes harbor enzymes that rid the cell of toxic peroxides...
s are used to break down
peroxideA peroxide is a compound containing an oxygen-oxygen single bond. The simplest stable peroxide is hydrogen peroxide. Superoxides, dioxygenyls, ozones and ozonides compound are considered separately.- Organic chemistry :...
, which is toxic otherwise. Many
protozoaProtozoa or Cornelius protozoans Protozoa or Cornelius protozoans Protozoa or Cornelius protozoans (from Greek πρῶτον proton "first" and ζῷα zoa "animals"; singular protozoon; (the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes....
have contractile vacuoles, which collect and expel excess water, and
extrusomeExtrusomes are membrane-bound structures in some eukaryotes which, under certain conditions, discharge their contents outside the cell. There are a variety of different types, probably not homologous, and serving various functions....
s, which expel material used to deflect predators or capture prey. In multicellular organisms,
hormoneA hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms...
s are often produced in vesicles. In higher plants, most of a cell's volume is taken up by a central vacuole, which primarily maintains its osmotic pressure.
Mitochondria and plastids
MitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
are organelles found in nearly all eukaryotes. They are surrounded by double membranes (known as the
phospholipid bi-layerA lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus and...
), the inner of which is folded into invaginations called cristae, where aerobic respiration takes place. Mitochondria contain their own DNA and ribosomes and are only formed by the fission of other mitochondria. They are now generally held to have developed from endosymbiotic prokaryotes, probably
proteobacteriaThe Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
. The few protozoa that lack mitochondria have been found to contain mitochondrion-derived organelles, such as
hydrogenosomeA hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA...
s and
mitosomeA mitosome is an organelle found in some unicellular eukaryotic organisms. The mitosome has only recently been found and named, and its function has not yet been well characterized. It was termed a crypton by another group but that name is not being used any more.The mitosome has been detected only...
s.
Plants and various groups of
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
also have
plastidPlastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s. Again, these have their own DNA and developed from endosymbiotes, in this case
cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
. They usually take the form of
chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s, which like cyanobacteria contain
chlorophyllChlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
and produce energy through
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
. Others are involved in storing food. Although plastids likely had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion.
Endosymbiotic origins have also been proposed for the nucleus, for which see below, and for eukaryotic
flagellaA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
, supposed to have developed from
spirochaeteSpirochaetes belong to a phylum of distinctive Gram-negative bacteria, which have long, helically coiled cells...
s. This is not generally accepted, both from a lack of
cytologicalCell biology is an academic discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
evidence and difficulty in reconciling this with cellular reproduction.
Cytoskeletal structures
Many eukaryotes have long slender motile cytoplasmic projections, called
flagellaA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
, or similar, but shorter structures called
ciliaA cilium is an organelle found in eukaryotic cells. Cilia are tail-like projections.There are two types of cilia: motile cilia and non-motile, or primary, cilia, which typically serve as sensory organelles...
. Flagella and cilia are sometimes referred to as undulipodia, and are variously involved in movement, feeding, and sensation. They are composed mainly of
tubulinTubulin is one of several members of a small family of globular proteins. The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up microtubules. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from dimers of α- and...
. These are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a
basal body.A basal body is an organelle formed from a centriole, a short cylindrical array of microtubules. It is found at the base of a eukaryotic undulipodium and serves as a nucleation site for the growth of the axoneme microtubules...
, also called a
kinetosomeThe axoneme arises from the kinetosome. A kinetosome is NOT the same as a kinetoplast, which is in fact the mitochondrial DNA of kinetoplastids. The kinetoplast stains darkly and is disc-like in shape.-Sources:* www.biology-online.org/dictionary/Kinetosome...
or
centrioleA centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
, characteristically arranged as nine doublets surrounding two singlets. Flagella also may have hairs, or
mastigonemesMastigonemes are lateral "hairs" found covering the flagella of heterokont and cryptophyte algae. They are approximately 15 nm in diameter, and usually consist of a tubular shaft that itself terminates in smaller "hairs". It is believed that they assist in locomotion by increasing the surface...
, and scales connecting membranes and internal rods. Their interior is continuous with the cell's
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
.
Microfilamental structures composed by
actinActin is a globular, roughly 42-kDa protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
and actin binding proteins, e.g., α-
actininActinin is a microfilament protein. α-Actinin is necessary for the attachment of actin filaments to the Z-lines in skeletal muscle cells, and to the dense bodies in smooth muscle cells...
,
fimbrinFimbrin is an actin cross-linking protein important in the formation of filopodia.-Structure:Fimbrin belongs to the calponin homology domain superfamily of actin cross-linking proteins...
, filamin are present in submembraneous cortical layers and bundles, as well. Motor proteins of microtubules, e.g.,
dyneinDynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell...
or
kinesinKinesins are a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule cables powered by the dephosphorylation of ATP...
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. They generally occur in groups of one or two, called kinetids, 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 may also be associated in the formation of a spindle during nuclear division.
Significance of cytoskeletal structures is underlined in determination of shape of the cells, as well as their being essential components of migratory responses like
chemotaxisChemotaxis is the phenomenon in which bodily cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules, or...
and
chemokinesisMotile response of unicellular prokaryotic or eukaryotic organisms to chemicals that cause the cell to make some kind of change in their migratory/swimming behaviour. Changes involve an increase or decrease of speed, alterations of amplitude or frequency of motile character, or direction of migration...
. Some protists have various other microtubule-supported organelles. These include the radiolaria and
heliozoaHeliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...
, which produce axopodia used in flotation or to capture prey, and the
haptophyteThe haptophytes, classed either as the Prymnesiophyta or Haptophyta, are a phylum of algae.The term "Haptophyceae" is sometimes used. This ending implies classification at a lower level...
s, which have a peculiar flagellum-like organelle called the haptonema.
Plant cell wall
Plant cells have a cell wall, a fairly rigid layer outside the
cell membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
, 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 carbohydrates making up the primary cell wall of land plants are
celluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
,
hemicelluloseA hemicellulose can be any of several heteropolymers present in almost all plant cell walls along with cellulose. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...
, and
pectinPectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
. The cellulose
microfibrilBasic structural unit of the plant cell wall, made of cellulose in higher plants and most algae, chitin in some fungi and mannan or xylan in a few algae. Higher plant microfibrils are about 10nm in diameter and extremely long in relation to their width...
s 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
xyloglucanXyloglucan is the most abundant hemicellulose in the primary cell wall of many dicotyledonous plants, and occurs in the primary cell walls of all vascular plants. Xyloglucan binds to the surface of cellulose microfibrils and may link them together...
.
Differences between 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
An
animal cell is a form of eukaryotic cell that makes up many
tissuesTissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function...
in
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s. The animal cell is distinct from other eukaryotes, most notably plant cells, as they lack
cell wallA cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a...
s and
chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s, and they have smaller
vacuolethumb|400px|Plant cell structurethumb|400px|Animal cell structureA vacuole is a membrane organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells...
s. Due to the lack of a rigid
cell wallA cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a...
, animal cells can adopt a variety of shapes, and a
phagocyticPhagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek kutos, "hollow vessel". They are...
cell can even engulf other structures.
There are many different cell types. For instance, there are approximately 210 distinct cell types in the adult human body.
Plant cell
Plant
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
are quite different from the cells of the other eukaryotic organisms. Their distinctive features are:
- A large central vacuole
thumb|400px|Plant cell structurethumb|400px|Animal cell structureA vacuole is a membrane organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells...
(enclosed by a membrane, the tonoplast), which maintains the cell's turgor and controls movement of moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
s between the cytosolThe cytosol or intracellular fluid is the liquid found inside cells. In eukaryotes this liquid is separated by cell membranes from the contents of the organelles suspended in the cytosol, such as the mitochondrial matrix inside the mitochondrion...
and sapSap is a fluid transported in xylem cells or phloem sieve tube elements of a plant. Fluid found in the vacuoles of other cells is sometimes referred to as "cell sap"...
- A primary cell wall
A cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a...
containing celluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, hemicelluloseA hemicellulose can be any of several heteropolymers present in almost all plant cell walls along with cellulose. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...
and pectinPectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
, deposited by the protoplastProtoplast, from the ancient Greek πρῶτον + verb πλάθω or πλάττω , initially referred to the first organized body of a species...
on the outside of the cell membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
; this contrasts with the cell walls of fungiA fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants, animals and bacteria...
, which contain chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods, such as crustaceans Chitin...
, and the cell envelopeThe cell envelope is the cell membrane and cell wall plus an outer membrane, if one is present.Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative. These are differentiated by their Gram staining characteristics....
s of prokaryoteThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles...
s, in which peptidoglycanPeptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria, forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid residues...
s are the main structural molecules
- The plasmodesmata
Plasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells enabling transport and communication between them. Species that have plasmodesmata include members of the Charophyceae, Charales and Coleochaetales , as well as all embryophytes, better known...
, linking pores in the cell wall that allow each plant cell to communicate with other adjacent cells; this is different from the functionally analogous system of gap junctionA gap junction or nexus is a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells....
s between animal cells.
- Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s, especially chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s that contain chlorophyllChlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
, the pigment that gives plantPlants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants,...
s their green color and allows them to perform photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
- Higher plants, including conifers
The conifers, division Pinophyta, also known as division Coniferophyta or Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. Pinophytes are gymnosperms. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority...
and flowering plantThe flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants...
s (Angiospermae) lack the flagellaeA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
and centrioleA centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
s that are present in animal cells.
Fungal cell
Fungal cells are most similar to animal cells, with the following exceptions:
- A cell wall containing chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods, such as crustaceans Chitin...
- Less definition between cells; the hypha
A hypha is a long, branching filamentous cell of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or more...
e of higher fungi have porous partitions called septaIn anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.- In human anatomy :* Interatrial septum, the wall of tissue that separates the left and right atria of the heart...
, which allow the passage of cytoplasm, organelles, and, sometimes, nuclei. Primitive fungi have few or no septa, so each organism is essentially a giant multinucleate supercell; these fungi are described as coenocytic.
- Only the most primitive fungi, chytrids, have flagella.
Other eukaryotic cells
Eukaryotes are a very diverse group, and their cell structures are equally diverse. Many have cell walls; many do not. Many have chloroplasts, derived from primary, secondary, or even tertiary endosymbiosis; and many do not. Some groups have unique structures, such as the cyanelles of the
glaucophyteThe glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and Viridiplantae they form the Archaeplastida...
s, the haptonema of the
haptophyteThe haptophytes, classed either as the Prymnesiophyta or Haptophyta, are a phylum of algae.The term "Haptophyceae" is sometimes used. This ending implies classification at a lower level...
s, or the ejectisomes of the
cryptomonadThe cryptomonads are a group of algae, most of which have chloroplasts. They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10-50 μm in size and flattened in shape, with an anterior groove or pocket...
s. Other structures, such as
pseudopods, are found in various eukaryote groups in different forms, such as the lobose
amoebozoaThe Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
ns or the reticulose foraminiferans.
Reproduction
Nuclear division is often coordinated with
cell divisionCell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing...
. This generally takes place by
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
, a process that allows each daughter nucleus to receive one copy of each chromosome. In most eukaryotes, there is also a process of sexual reproduction, typically involving an alternation between haploid generations, wherein only one copy of each chromosome is present, and diploid generations, wherein two are present, occurring through nuclear fusion (syngamy) and
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
. There is considerable variation in this pattern, however.
Eukaryotes have a smaller surface area to volume ratio than prokaryotes, and thus have lower metabolic rates and longer generation times. In some multicellular organisms, cells specialized for metabolism will have enlarged surface areas, such as intestinal vili.
Origin and evolution
The origin of the eukaryotic cell was a milestone in the evolution of life, since they include all complex cells and almost all multi-cellular organisms. The timing of this series of events is hard to determine;
KnollAndrew H. Knoll is the Fisher Professor of Natural History and a Professor of Earth and Planetary Sciences at Harvard University. He is best known for his work on Precambrian microfossils and using stable isotopes for stratigraphic correlation, but has longstanding interests in geobiology,...
(2006) suggests they developed approximately 1.6 - 2.1 billion years ago. Some
acritarchAcritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
s are known from at least 1650 million years ago, and the possible alga
GrypaniaGrypania is an early, tube-shaped fossil from the Proterozoic Eon. The organism could have been a giant bacterium or bacterial colony, but because of its size and consistent form, is more likely to have been a eukaryotic alga. The oldest probable Grypania fossils date to about 2,100 million years...
has been found as far back as 2100 million years ago.
Fossils that are clearly related to modern groups start appearing around 1.2 billion years ago, in the form of a red alga, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back to 1.6 to 1.7 billion years ago.
Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of
steraneSteranes are a class of 4-cyclic compounds derived from steroids or sterols via diagenetic and catagenetic degradation and saturation. They are sometimes used as biomarkers for the presence of eukaryotic cells. The sterane structure constitutes the core of all sterols and steroids....
s in
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
n
shaleShale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable...
s indicates that eukaryotes were present 2.7 billion years ago.
rRNA trees constructed during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group (not technically a true
crownA crown group is the smallest monophyletic group, or "clade", to contain the last common ancestor of all members, and all of that ancestor's descendants...
), which was usually divided by the form of the mitochondrial cristae; see
crown eukaryotesCrown eukaryotes are an artificial group of eukaryotic organisms found at the top of molecular phylogenetic trees including both eukaryotes and prokaryotes...
. The few groups that lack 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.
More recent work has painted a different picture. Most eukaryotes are now included in one of the following supergroups, although the relationship between these groups, and the
monophylyIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it consists of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly...
of each group, is not yet clear:. Some groups position in the tree is still unclear, as in
Kamera lensKamera lens is a unicellular, flagellate organism and the only species of its genus Kamera. Though the species is known for centuries, it is poorly known. Its systematic position within the Eukaryota is unsure....
.
OpisthokontThe opisthokonts are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the organisms which were previously grouped in the paraphyletic phylum Choanozoa of the protist "kingdom"...
s |
AnimalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s, fungiA fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants, animals and bacteria...
, choanoflagellateThe choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals...
s, etc. |
| Amoebozoa The Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
|
Most lobose amoeboid Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amoeba" are often used interchangeably even by biologists, and especially refers to a creature moving by using pseudopods. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
s and slime mouldSlime mold is a broad term describing fungi-like organisms that use spores to reproduce. They were formerly classified as fungi, but are no longer considered part of this group....
s |
RhizariaThe Rhizaria are a species-rich supergroup of unicellulareukaryotes. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, and these make up...
|
ForaminiferaThe Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate...
, Radiolaria, and various other amoeboidAmoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amoeba" are often used interchangeably even by biologists, and especially refers to a creature moving by using pseudopods. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
protozoa |
ExcavateThe excavates are a major assemblage of unicellular eukaryotes, often known as Excavata. The phylogenetic category Excavata contains a variety of free-living and symbiotic forms, and includes some important parasites of humans.-Characteristics:...
s |
Various flagellate Flagellates are cells with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Higher plants and fungi do not produce flagellate cells, but the closely related green algae and chytrids do...
protozoa |
ArchaeplastidaThe Archaeplastida are a major line of eukaryotes, comprising the land plants, green and red algae, and a small group called the glaucophytes. All of these organisms have plastids surrounded by two membranes, suggesting they developed directly from endosymbiotic cyanobacteria...
(or Primoplantae) |
Land plantsThe embryophytes are the most familiar group of plants. They are often called land plants because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees, flowers, ferns, mosses, and various other green land...
, green algae, red algae, and glaucophyteThe glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and Viridiplantae they form the Archaeplastida...
s |
ChromalveolateChromalveolata is a eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first proposed in 1981...
s |
HeterokontThe heterokonts or stramenopiles are a major line of eukaryotes presently containing more than 100,000 known species, most of them diatoms. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton...
s, HaptophyteThe haptophytes, classed either as the Prymnesiophyta or Haptophyta, are a phylum of algae.The term "Haptophyceae" is sometimes used. This ending implies classification at a lower level...
s, CryptomonadThe cryptomonads are a group of algae, most of which have chloroplasts. They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10-50 μm in size and flattened in shape, with an anterior groove or pocket...
s, and AlveolateThe alveolates are a major line of protists.-Phyla:There are three phyla, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:...
s. |
A further proposal is that eukaryotes can be classified into two larger clades, the
unikontUnikonts are members of the Unikonta, a taxonomic group proposed by Thomas Cavalier-Smith. It includes amoebozoa and opisthokonts.-Clade:The group includes eukaryotic cells that, for the most part, have a single emergent flagellum, or are amoebae with no flagella. The unikonts include opisthokonts...
s and the
bikontA Bikont is a eukaryotic cell with two flagella, as its name suggests. It is a division of eukaryotes.-Enzymes:Another shared trait of bikonts is the fusion of two genes into a single unit: the genes for thymidylate synthase and dihydrofolate reductase encode a singleprotein with two...
s, that derive from an ancestral uniflagellar organism and a biflagellate, respectively. In this system, the opisthokonts and amoebozoans are considered unikonts, and the rest bikonts. The correct relationships of the organisms in the
ArchaeplastidaThe Archaeplastida are a major line of eukaryotes, comprising the land plants, green and red algae, and a small group called the glaucophytes. All of these organisms have plastids surrounded by two membranes, suggesting they developed directly from endosymbiotic cyanobacteria...
and
ChromalveolateChromalveolata is a eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first proposed in 1981...
s is far from clear, and it seems likely that neither group, as defined above, is monophyletic. Some small protist groups have not been related to any of these supergroups, in particular the characteristics of the anaerobic protist
BreviataBreviata anathema is a single-celled protist. It lacks mitochondria but has numerous differences with other amitochondiate Amoebozoa like Entamoeba and Endolimax. Because B. anathema has genes which are known from mitochondria, it is likely to possess degenerate mitrochondria. The closest...
anathema seems to contradict the unikont/bikont division.
Eukaryotes are more closely related to
ArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
than
BacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, at least in terms of nuclear DNA and genetic machinery, and one controversial idea is to place them with Archaea in the clade
NeomuraNeomura is a speculative clade composed of the two domains of life of Archaea and Eukaryota. The group was first proposed by Thomas Cavalier-Smith and its name means "new walls"; so called because it is thought to have evolved from Bacteria, and one of the major changes was the replacement of...
. However, 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 eubacterium, and the nucleus from an archaeon, from a virus
Viral eukaryogenesis is the hypothesis that the cell nucleus of eukaryotic life forms evolved from a large DNA virus in a form of endosymbiosis within an archaea cell, being a form of symbiogenesis...
, or from a pre-cell.
- Eukaryotes developed from Archaea, and acquired their eubacterial characteristics from the proto-mitochondrion.
- Eukaryotes and Archaea developed separately from a modified eubacterium.
One recent cladogram of Eukarya
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 hypothesisThe hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryote , giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have...
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
syntrophySyntrophy, Cross-feeding, or Cross feeding is the phenomenon that one species lives off the products of another species.For example house dust mites live off human skin flakes, of which a healthy human being produces about 1 gram per day...
between an archaea closely related to
ThermoplasmatalesIn taxonomy, the Thermoplasmatales are an order of the Thermoplasmata. All are acidophiles, growing optimally at pH below 2. Picrophilus is currently the most acidophilic of all known organisms growing at a minimum pH of -0.06. Many of these organisms do not contain a cell wall, although this is...
and an
α-proteobacteriumThe Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
, likely a
symbiosisThe term symbosis commonly describes close and often long-term interactions between different biological species...
driven by sulfur or hydrogen. The mitochondrion and its genome is a remnant of the α-proteobacterial endosymbiont.
External links