A unicellular organism, also known as a single-celled organism, is an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
that consists of a single
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
, unlike a
multicellular organism that consists of multiple cells. Organisms fall into two general categories:
prokaryotic
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
organisms and
eukaryotic
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
organisms. All prokaryotes are unicellular and are classified into
bacteria
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. Typically a few micrometr ...
and
archaea. Many eukaryotes are multicellular, but some are unicellular such as
protozoa, unicellular
algae, and unicellular
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
. Unicellular organisms are thought to be the oldest form of life, with early
protocell
A protocell (or protobiont) is a self-organized, endogenously ordered, spherical collection of lipids proposed as a stepping-stone toward the origin of life. A central question in evolution is how simple protocells first arose and how they could ...
s possibly emerging 3.8–4.0 billion years ago.
Although some prokaryotes live in
colonies
In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...
, they are not specialised cells with differing functions. These organisms live together, and each cell must carry out all life processes to survive. In contrast, even the simplest multicellular organisms have cells that depend on each other to survive.
Most multicellular organisms have a unicellular life-cycle stage.
Gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s, for example, are reproductive unicells for multicellular organisms. Additionally, multicellularity appears to have
evolved independently many times in the history of life.
Some organisms are partially unicellular, like ''
Dictyostelium discoideum
''Dictyostelium discoideum'' is a species of soil-dwelling amoeba belonging to the phylum Amoebozoa, infraphylum Mycetozoa. Commonly referred to as slime mold, ''D. discoideum'' is a eukaryote that transitions from a collection of unicellular ...
''. Additionally, unicellular organisms can be
multinucleate
Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordina ...
, like ''
Caulerpa
''Caulerpa'' is a genus of seaweeds in the family Caulerpaceae (among the green algae). They are unusual because they consist of only one cell with many nuclei, making them among the biggest single cells in the world. A species in the Mediterran ...
'', ''
Plasmodium
''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a blood-feeding insect host which then injects parasites into a ver ...
'', and
Myxogastria
Myxogastria/Myxogastrea (myxogastrids, ICZN) or Myxomycetes ( ICN), is a class of slime molds that contains 5 orders, 14 families, 62 genera, and 888 species. They are colloquially known as the ''plasmodial'' or ''acellular ...
.
Evolutionary hypothesis
Primitive
protocell
A protocell (or protobiont) is a self-organized, endogenously ordered, spherical collection of lipids proposed as a stepping-stone toward the origin of life. A central question in evolution is how simple protocells first arose and how they could ...
s were the precursors to today's unicellular organisms. Although the
origin of life
In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
is largely still a mystery, in the currently prevailing theory, known as the
RNA world hypothesis
The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existenc ...
, early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication.
Compartmentalization was necessary for chemical reactions to be more likely as well as to differentiate reactions with the external environment. For example, an early RNA replicator
ribozyme
Ribozymes (ribonucleic acid enzymes) are RNA molecules that have the ability to catalyze specific biochemical reactions, including RNA splicing in gene expression, similar to the action of protein enzymes. The 1982 discovery of ribozymes demons ...
may have replicated other replicator ribozymes of different RNA sequences if not kept separate.
Such hypothetic cells with an RNA genome instead of the usual DNA genome are called '
ribocells' or 'ribocytes'.
When
amphiphiles like
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids includ ...
s are placed in water, the hydrophobic tails aggregate to form
micelle
A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated coll ...
s and
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 o ...
, with the hydrophilic ends facing outwards.
Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment.
Because of their simplicity and ability to self-assemble in water, it is likely that these simple
membranes predated other forms of early biological molecules.
Prokaryotes
Prokaryotes lack membrane-bound organelles, such as
mitochondria or a
nucleus
Nucleus ( : 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
Nucl ...
. Instead, most prokaryotes have an irregular region that contains DNA, known as the
nucleoid. Most prokaryotes have a single, circular
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
, which is in contrast to eukaryotes, which typically have linear chromosomes. Nutritionally, prokaryotes have the ability to utilize a wide range of organic and inorganic material for use in metabolism, including sulfur, cellulose, ammonia, or nitrite.
Prokaryotes are relatively ubiquitous in the environment and some (known as extremophiles) thrive in extreme environments.
Bacteria
![Stromatolites in Sharkbay](https://upload.wikimedia.org/wikipedia/commons/1/1b/Stromatolites_in_Sharkbay.jpg)
Bacteria are one of the world's oldest forms of life, and are found virtually everywhere in nature.
Many common bacteria have
plasmids, which are short, circular, self-replicating DNA molecules that are separate from the bacterial chromosome. Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance.
Bacteria predominantly reproduce asexually through a process called
binary fission
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that ta ...
. However, about 80 different species can undergo a sexual process referred to as natural
genetic transformation.
Transformation is a bacterial process for transferring DNA from one cell to another, and is apparently an adaptation for
repairing DNA damage in the recipient cell.
In addition, plasmids can be exchanged through the use of a
pilus
A pilus (Latin for 'hair'; plural: ''pili'') is a hair-like appendage found on the surface of many bacteria and archaea. The terms ''pilus'' and '' fimbria'' (Latin for 'fringe'; plural: ''fimbriae'') can be used interchangeably, although some r ...
in a process known as
conjugation
Conjugation or conjugate may refer to:
Linguistics
* Grammatical conjugation, the modification of a verb from its basic form
* Emotive conjugation or Russell's conjugation, the use of loaded language
Mathematics
* Complex conjugation, the chang ...
.
The photosynthetic
cyanobacteria are arguably the most successful bacteria, and changed the early atmosphere of the earth by oxygenating it.
Stromatolite
Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). T ...
s, structures made up of layers of
calcium carbonate and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records.
The existence of stromatolites gives an excellent record as to the development of cyanobacteria, which are represented across the
Archaean (4 billion to 2.5 billion years ago),
Proterozoic (2.5 billion to 540 million years ago), and
Phanerozoic (540 million years ago to present day) eons.
Much of the fossilized stromatolites of the world can be found in
Western Australia
Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...
.
There, some of the oldest stromatolites have been found, some dating back to about 3,430 million years ago.
Clonal
aging occurs naturally in
bacteria
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. Typically a few micrometr ...
, and is apparently due to the accumulation of damage that can happen even in the absence of external stressors.
Archaea
![Echinoderms 600](https://upload.wikimedia.org/wikipedia/commons/1/1f/Echinoderms_600.jpg)
Hydrothermal vents release heat and
hydrogen sulfide, allowing extremophiles to survive using
chemolithotroph
Lithotrophs are a diverse group of organisms using an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (i.e., ATP production) via aerobi ...
ic growth.
Archaea are generally similar in appearance to bacteria, hence their original classification as bacteria, but have significant molecular differences most notably in their membrane structure and ribosomal RNA.
By sequencing the ribosomal RNA, it was found that the Archaea most likely split from bacteria and were the precursors to modern eukaryotes, and are actually more phylogenetically related to eukaryotes.
As their name suggests, Archaea comes from a Greek word ''archaios,'' meaning original, ancient, or primitive.
Some archaea inhabit the most biologically inhospitable environments on earth, and this is believed to in some ways mimic the early, harsh conditions that life was likely exposed to. Examples of these Archaean
extremophile
An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
s are as follows:
*
Thermophile
A thermophile is an organism—a type of extremophile—that thrives at relatively high temperatures, between . Many thermophiles are archaea, though they can be bacteria or fungi. Thermophilic eubacteria are suggested to have been among the earl ...
s, optimum growth temperature of 50 °C-110 °C, including the genera ''
Pyrobaculum
''Pyrobaculum'' is a genus of the Thermoproteaceae.
Description and significance
As its Latin name ''Pyrobaculum'' (the "fire stick") suggests, the archaeon is rod-shaped and isolated from locations with high temperatures. It is Gram-negative ...
'', ''
Pyrodictium'', ''
Pyrococcus'', ''
Thermus aquaticus
''Thermus aquaticus'' is a species of bacteria that can tolerate high temperatures, one of several thermophilic bacteria that belong to the ''Deinococcota'' phylum. It is the source of the heat-resistant enzyme ''Taq'' DNA polymerase, one of th ...
'' and ''
Melanopyrus.''
*
Psychrophile
Psychrophiles or cryophiles (adj. ''psychrophilic'' or ''cryophilic'') are extremophilic organisms that are capable of growth and reproduction in low temperatures, ranging from to . They have an optimal growth temperature at . They are found in ...
s, optimum growth temperature of less than 15 °C, including the genera ''
Methanogenium
''Methanogenium'' is a genus (biology), genus of archaeans in the family Methanomicrobiaceae. The type species is ''Methanogenium cariaci''.
Description and habitat
The species within ''Methanogenium'' are coccus, coccoid in shape and Gram-nega ...
'' and ''
Halorubrum
''Halorubrum'' is a genus in the family Halorubraceae. ''Halorubrum'' species areusually halophilic and can be found in waters with high salt concentration such as the Dead Sea or Lake Zabuye.
Genetic exchange
A population of the haloarchaea '' ...
.''
*
Alkaliphile Alkaliphiles are a class of extremophilic microbes capable of survival in alkaline ( pH roughly 8.5–11) environments, growing optimally around a pH of 10. These bacteria can be further categorized as obligate alkaliphiles (those that require hig ...
s, optimum growth pH of greater than 8, including the genus ''
Natronomonas
''Natronomonas'' (common abbreviation ''Nmn.'').
is a genus of the Halobacteriaceae.See the NCBIbr>webpage on Natronomonas Data extracted from the
Description and significance
''Natronomonas pharaonis'' is an aerobic, extremely haloalkalip ...
''.
*
Acidophile
Acidophiles or acidophilic organisms are those that thrive under highly acidic conditions (usually at pH 5.0 or below). These organisms can be found in different branches of the tree of life, including Archaea, Bacteria,Becker, A.Types of Bacteria ...
s, optimum growth pH of less than 3, including the genera ''
Sulfolobus and
Picrophilus
In taxonomy, ''Picrophilus'' is an archaean genus of the family Picrophilaceae.
''Picrophilus'' is an extremely acidophilic genus within Euryarchaeota. These microbes are the most acidophilic organisms currently known, with the ability to gr ...
''.
*
Piezophiles A piezophile (from Greek "piezo-" for pressure and "-phile" for loving) is an organism with optimal growth under high hydrostatic pressure i.e. an organism that has its maximum rate of growth at a hydrostatic pressure equal to or above 10 MPa (= 99 ...
, (also known as
barophiles), prefer high pressure up to 130 MPa, such as deep ocean environments, including the genera ''
Methanococcus'' and ''
Pyrococcus''.
*
Halophile
The halophiles, named after the Greek word for "salt-loving", are extremophiles that thrive in high salt concentrations. While most halophiles are classified into the domain Archaea, there are also bacterial halophiles and some eukaryotic species, ...
s, grow optimally in high salt concentrations between 0.2 M and 5.2 M
NaCl
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...
, including the genera ''
Haloarcula
''Haloarcula'' (common abbreviation ''Har.'') is a genus of extreme halophilic Archaea in the class of Halobactaria.
Cell Structure
''Haloarcula'' species can be distinguished from other genera in the family Halobacteriaceae by the presence of ...
'', ''
Haloferax
In taxonomy, ''Haloferax'' (common abbreviation: ''Hfx.'') is a genus of the Haloferacaceae.
Genetic exchange
Cells of ''H. mediterranei'' and cells of the related species '' H. volcanii'' can undergo a process of genetic exchange between two ...
'', ''
Halococcus''.
Methanogens are a significant subset of archaea and include many extremophiles, but are also ubiquitous in wetland environments as well as the ruminant and hindgut of animals.
This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms o ...
.
They are the only known organisms capable of producing methane. Under stressful environmental conditions that cause
DNA damage
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
, some species of archaea aggregate and transfer DNA between cells.
The function of this transfer appears to be to replace damaged DNA sequence information in the recipient cell by undamaged sequence information from the donor cell.
Eukaryotes
Eukaryotic cells contain membrane bound organelles, such as mitochondria, a nucleus, and
chloroplasts. Prokaryotic cells probably transitioned into eukaryotic cells between 2.0 and 1.4 billion years ago.
This was an important step in evolution. In contrast to prokaryotes, eukaryotes reproduce by using
mitosis and
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
. Sex appears to be a ubiquitous and ancient, and inherent attribute of
eukaryotic
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
life.
Meiosis, a true sexual process, allows for efficient
recombinational repair of DNA damage
and a greater range of genetic diversity by combining the DNA of the parents followed by
recombination.
Metabolic functions in eukaryotes are more specialized as well by sectioning specific processes into organelles.
The
endosymbiotic theory holds that mitochondria and chloroplasts have bacterial origins. Both organelles contain their own sets of DNA and have bacteria-like ribosomes. It is likely that modern mitochondria were once a species similar to ''
Rickettsia
''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "ricke ...
'', with the parasitic ability to enter a cell.
However, if the bacteria were capable of respiration, it would have been beneficial for the larger cell to allow the parasite to live in return for energy and detoxification of oxygen.
Chloroplasts probably became symbionts through a similar set of events, and are most likely descendants of cyanobacteria. While not all eukaryotes have mitochondria or chloroplasts, mitochondria are found in most eukaryotes, and chloroplasts are found in all plants and algae. Photosynthesis and respiration are essentially the reverse of one another, and the advent of respiration coupled with photosynthesis enabled much greater access to energy than
fermentation alone.
Protozoa
Protozoa are largely defined by their method of locomotion, including
flagella,
cilia, and
pseudopodia.
While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under the kingdom Protozoa:
Euglenozoa,
Amoebozoa, Choanozoa ''sensu'' Cavalier-Smith,
Loukozoa
Loukozoa (+ Ancyromonads) (From Greek ''loukos'': groove) is a proposed taxon used in some classifications of eukaryotes, consisting of the Metamonada and Malawimonadea. Ancyromonads are closely related to this group, as sister of the entire grou ...
,
Percolozoa
The Percolozoa are a group of colourless, non-photosynthetic Excavata, including many that can transform between amoeboid, flagellate, and cyst stages.
Characteristics
Most Percolozoa are found as bacterivores in soil, fresh water and occasio ...
,
Microsporidia
Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or pr ...
and
Sulcozoa. Protozoa, like plants and animals, can be considered heterotrophs or autotrophs.
Autotrophs like ''
Euglena
''Euglena'' is a genus of single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. Species of ''Euglena'' are found in f ...
'' are capable of producing their energy using photosynthesis, while heterotrophic protozoa consume food by either funneling it through a mouth-like gullet or engulfing it with pseudopods, a form of
phagocytosis
Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
.
While protozoa reproduce mainly asexually, some protozoa are capable of sexual reproduction.
Protozoa with sexual capability include the pathogenic species ''
Plasmodium falciparum'', ''
Toxoplasma gondii
''Toxoplasma gondii'' () is an obligate intracellular parasitic protozoan (specifically an apicomplexan) that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but felids, such as d ...
'', ''
Trypanosoma brucei
''Trypanosoma brucei'' is a species of parasitic kinetoplastid belonging to the genus '' Trypanosoma'' that is present in sub-Saharan Africa. Unlike other protozoan parasites that normally infect blood and tissue cells, it is exclusively extrace ...
'', ''
Giardia duodenalis
''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus ''Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The ...
'' and ''
Leishmania'' species.
Ciliophora
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...
, or ciliates, are a group of protists that utilize cilia for locomotion. Examples include ''
Paramecium
''
''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'',
''Stentors'', and ''
Vorticella
''Vorticella'' is a genus of bell-shaped ciliates that have stalks to attach themselves to substrates. The stalks have contractile myonemes, allowing them to pull the cell body against substrates. The formation of the stalk happens after the fr ...
''. Ciliates are widely abundant in almost all environments where water can be found, and the cilia beat rhythmically in order to propel the organism. Many ciliates have
trichocyst
A trichocyst is an organelle found in certain ciliates and dinoflagellate
The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the ...
s, which are spear-like organelles that can be discharged to catch prey, anchor themselves, or for defense.
Ciliates are also capable of sexual reproduction, and utilize two nuclei unique to ciliates: a
macronucleus
A macronucleus (formerly also meganucleus) is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, such as metabolism. During conjugation, ...
for normal metabolic control and a separate
micronucleus
Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...
that undergoes meiosis.
Examples of such ciliates are ''
Paramecium
''
''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'' and ''
Tetrahymena
''Tetrahymena'', a unicellular eukaryote, is a genus of free-living ciliates. The genus Tetrahymena is the most widely studied member of its phylum. It can produce, store and react with different types of hormones. Tetrahymena cells can recog ...
'' that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions.
The Amebozoa utilize pseudopodia and cytoplasmic flow to move in their environment. ''
Entamoeba histolytica
''Entamoeba histolytica'' is an anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 million people worldwid ...
'' is the cause of amebic dysentery. ''
Entamoeba histolytica
''Entamoeba histolytica'' is an anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 million people worldwid ...
'' appears to be capable of
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
.
Unicellular algae
![CSIRO ScienceImage 7632 SEM diatom](https://upload.wikimedia.org/wikipedia/commons/a/a1/CSIRO_ScienceImage_7632_SEM_diatom.jpg)
Unicellular algae are plant-like autotrophs and contain
chlorophyll.
They include groups that have both multicellular and unicellular species:
*
Euglenophyta
Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typic ...
, flagellated, mostly unicellular algae that occur often in fresh water.
In contrast to most other algae, they lack cell walls and can be
mixotroph A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...
ic (both autotrophic and heterotrophic).
An example is ''
Euglena gracilis''.
*
Chlorophyta
Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a ...
(green algae), mostly unicellular algae found in fresh water.
The chlorophyta are of particular importance because they are believed to be most closely related to the evolution of land plants.
*
Diatoms, unicellular algae that have siliceous cell walls.
They are the most abundant form of algae in the ocean, although they can be found in fresh water as well.
They account for about 40% of the world's primary marine production, and produce about 25% of the world's oxygen.
Diatoms are very diverse, and comprise about 100,000 species.
*
Dinoflagellates, unicellular flagellated algae, with some that are armored with
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
. Dinoflagellates can be mixotrophic, and are the algae responsible for
red tide
A harmful algal bloom (HAB) (or excessive algae growth) is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are sometimes ...
.
Some dinoflagellates, like ''
Pyrocystis fusiformis'', are capable of
bioluminescence.
Unicellular fungi
![Kg3](https://upload.wikimedia.org/wikipedia/commons/2/29/Kg3.jpg)
Unicellular fungi include the
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
s. Fungi are found in most habitats, although most are found on land.
Yeasts reproduce through mitosis, and many use a process called
budding
Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...
, where most of the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
is held by the mother cell.
''
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have b ...
'' ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread. ''S. cerevisiae'' is also an important model organism, since it is a eukaryotic organism that's easy to grow. It has been used to research
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
and
neurodegenerative disease
A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...
s as well as to understand the
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...
. Furthermore, research using ''
S. cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have bee ...
'' has played a central role in understanding the mechanism of meiotic
recombination and the adaptive function of
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
.
Candida spp. are responsible for
candidiasis
Candidiasis is a fungal infection due to any type of '' Candida'' (a type of yeast). When it affects the mouth, in some countries it is commonly called thrush. Signs and symptoms include white patches on the tongue or other areas of the mouth ...
, causing infections of the mouth and/or throat (known as thrush) and vagina (commonly called yeast infection).
Macroscopic unicellular organisms
Most unicellular organisms are of
microscopic
The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded as the scale be ...
size and are thus classified as
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s. However, some unicellular protists and bacteria are
macroscopic
The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic.
Overview
When applied to physical phenomena a ...
and visible to the naked eye. Examples include:
* ''
Brefeldia maxima'', a
slime mold
Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mul ...
, examples have been reported up to a centimetre thick with a surface area of over a square metre and weighed up to around 20 kg
*
Xenophyophores, protozoans of the phylum
Foraminifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
, are the largest examples known, with ''
Syringammina fragilissima
''Syringammina'' is a xenophyophore found off the coast of Scotland, near Rockall. It is one of the largest single-celled organisms known, at up to across. It was first described in 1882 by the oceanographer John Murray, after being discovered ...
'' achieving a diameter of up to
* ''
Nummulite
A nummulite is a large lenticular fossil, characterized by its numerous coils, subdivided by septa into chambers. They are the shells of the fossil and present-day marine protozoan ''Nummulites'', a type of foraminiferan. Nummulites commonly vary ...
'', foraminiferans
* ''
Valonia ventricosa
''Valonia ventricosa'', also known as bubble algae, sea grape or sailor's eyeballs is a species of algae found in oceans throughout the world in tropical and subtropical regions, within the phylum Chlorophyta. It is one of the largest known unice ...
'', an
alga
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
of the class
Chlorophyceae, can reach a diameter of
* ''
Acetabularia
''Acetabularia'' is a genus of green algae in the family Polyphysaceae, Typically found in subtropical waters, ''Acetabularia'' is a single-celled organism, but gigantic in size and complex in form, making it an excellent model organism for stu ...
'', algae
* ''
Caulerpa
''Caulerpa'' is a genus of seaweeds in the family Caulerpaceae (among the green algae). They are unusual because they consist of only one cell with many nuclei, making them among the biggest single cells in the world. A species in the Mediterran ...
'', algae,
may grow to 3 metres long
* ''
Gromia sphaerica'', amoeba,
* ''
Thiomargarita namibiensis
''Thiomargarita namibiensis'' is a Gram-negative coccoid bacterium, found in the ocean sediments of the continental shelf of Namibia. It is the second largest bacterium ever discovered, as a rule in diameter, but sometimes attaining . Cells of ...
'' is the largest bacterium, reaching a diameter of up to 0.75 mm
* ''
Epulopiscium fishelsoni
"''Candidatus'' Epulonipiscium" is a genus of Gram-positive bacteria that have a symbiotic relationship with surgeonfish. These bacteria are known for their unusually large size, many ranging from 200–700 μm in length. Until the discovery of ...
'', a bacterium
* ''
Stentor
In Greek mythology, Stentor (Ancient Greek: Στέντωρ; ''gen''.: Στέντορος) was a herald of the Greek forces during the Trojan War.
Mythology
Stentor is mentioned briefly in Homer's ''Iliad'' in which Hera in the guise of Stent ...
'', ciliates nicknamed trumpet animalcules
* ''
Bursaria'', largest colpodean ciliates.
See also
*
Abiogenesis
In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
*
Asexual reproduction
*
Colonial organism
In biology, a colony is composed of two or more conspecific individuals living in close association with, or connected to, one another. This association is usually for mutual benefit such as stronger defense or the ability to attack bigger prey.
...
*
Individuality in biology
*
Largest organisms
The largest organisms now found on Earth can be determined according to various aspects of an organism's size, such as: mass, volume, area, length, height, or even genome size. Some organisms group together to form a superorganism (such as ants ...
*
Modularity in biology
Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...
*
Multicellular organism
*
Sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
*
Superorganism
A superorganism or supraorganism is a group of synergetically interacting organisms of the same species. A community of synergetically interacting organisms of different species is called a holobiont.
Concept
The term superorganism is used m ...
References
{{Evolution
Microorganisms