A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea. Many eukaryotes are multicellular, but some are unicellular such as protozoa, unicellular algae, and unicellular fungi. Unicellular organisms are thought to be the oldest form of life, with early organisms emerging 3.5–3.8 billion years ago.
Although some prokaryotes live in colonies, 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. , for example, are reproductive unicells for multicellular organisms.
Some organisms are partially unicellular, like Dictyostelium discoideum. Additionally, unicellular organisms can be multinucleate, like Caulerpa, Plasmodium, and Myxogastria.
Evolutionary hypothesis
Primitive
were the precursors to today's unicellular organisms. Although the origin of life is largely still a mystery, in the currently prevailing theory, known as the
RNA world, 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 may have replicated other replicator ribozymes of different RNA sequences if not kept separate.
When like are placed in water, the hydrophobic tails aggregate to form and vesicles, with the hydrophilic ends facing outwards.
Prokaryotes
Prokaryotes lack membrane-bound organelles, such as
Mitochondrion or a
Cell nucleus.
Instead, most prokaryotes have an irregular region that contains DNA, known as the
nucleoid.
Most prokaryotes have a single, circular
chromosome, 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
Bacteria are one of the world's oldest forms of life, and are found virtually everywhere in nature.
Many common bacteria have
, 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. 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
DNA repair damage in the recipient cell.
In addition, plasmids can be exchanged through the use of a
pilus in a process known as conjugation.
The photosynthetic cyanobacteria are arguably the most successful bacteria, and changed the early atmosphere of the earth by oxygenating it.
Clonal ageing occurs naturally in bacteria, and is apparently due to the accumulation of damage that can happen even in the absence of external stressors.
Archaea
Hydrothermal vents release heat and
hydrogen sulfide, allowing extremophiles to survive using
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 are as follows:
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, optimum growth temperature of 50 °C-110 °C, including the genera Pyrobaculum, Pyrodictium, Pyrococcus, Thermus aquaticus and Melanopyrus.
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, optimum growth temperature of less than 15 °C, including the genera Methanogenium and Halorubrum.
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, optimum growth pH of greater than 8, including the genus Natronomonas.
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, optimum growth pH of less than 3, including the genera Sulfolobus and Picrophilus.
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Barophile, (also known as Piezophile), prefer high pressure up to 130 MPa, such as deep ocean environments, including the genera Methanococcus and Pyrococcus.
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, grow optimally in high salt concentrations between 0.2 M and 5.2 M Sodium chloride, including the genera Haloarcula, Haloferax, Halococcus.
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.
They are the only known organisms capable of producing methane.
Under stressful environmental conditions that cause DNA damage, 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. Some examples include mitochondria, a nucleus, or the Golgi apparatus. 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. Sex appears to be a ubiquitous and ancient, and inherent attribute of
eukaryote 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 Symbiogenesis 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, with the parasitic ability to enter a cell.
Protozoa
Protozoa are largely defined by their method of locomotion, including
Flagellum,
Cilium, 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,
Loukozoa,
Percolozoa,
Microsporidia and
Sulcozoa.
Protozoa, like plants and animals, can be considered heterotrophs or autotrophs.
Autotrophs like
Euglena 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.
While protozoa reproduce mainly asexually, some protozoa are capable of sexual reproduction.
with sexual capability include the pathogenic species
Plasmodium falciparum,
Toxoplasma gondii,
Trypanosoma brucei,
Giardia duodenalis and
Leishmania species.
Ciliate, or ciliates, are a group of protists that utilize cilia for locomotion. Examples include Paramecium, Stentors, and Vorticella.
The Amebozoa utilize pseudopodia and cytoplasmic flow to move in their environment. Entamoeba histolytica is the cause of amebic dysentery.
Unicellular algae
Unicellular algae are plant-like autotrophs and contain
chlorophyll.
They include groups that have both multicellular and unicellular species:
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Euglenid, flagellated, mostly unicellular algae that occur often in fresh water.
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Chlorophyta (green algae), mostly unicellular algae found in fresh water.
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, unicellular algae that have siliceous cell walls.
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, unicellular flagellated algae, with some that are armored with cellulose.
Unicellular fungi
Unicellular fungi include the
. Fungi are found in most habitats, although most are found on land.
Yeasts reproduce through mitosis, and many use a process called
budding, where most of the
cytoplasm is held by the mother cell.
ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread.
is also an important model organism, since it is a eukaryotic organism that is easy to grow. It has been used to research
cancer and neurodegenerative diseases as well as to understand the
cell cycle.
Furthermore, research using
S. cerevisiae has played a central role in understanding the mechanism of meiotic recombination and the adaptive function of
meiosis. Candida spp. are responsible for
candidiasis, 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 size and are thus classified as
. However, some unicellular protists and bacteria are macroscopic and visible to the naked eye.
[ Max Planck Society Research News Release Accessed 21 May 2009] Examples include:
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Brefeldia maxima, a slime mold, 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
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, protozoans of the phylum Foraminifera, are the largest examples known, with Syringammina fragilissima achieving a diameter of up to
[ Researchers Identify Mysterious Life Forms in the Desert. Accessed 2011-10-24.]
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Nummulite, foraminiferans
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Valonia ventricosa, an alga of the class Chlorophyceae, can reach a diameter of
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Acetabularia, algae
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Caulerpa, algae,
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Gromia sphaerica, amoeba,
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Thiomargarita magnifica is the largest bacterium, reaching a length of up to 20 mm
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Epulopiscium fishelsoni, a bacterium
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Stentor, ciliates nicknamed trumpet animalcules
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Bursaria, largest colpodean ciliates.
See also
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Abiogenesis
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Asexual reproduction
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Colonial organism
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Individuality in biology
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Largest organisms
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Modularity in biology
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Multicellular organism
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Sexual reproduction
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Superorganism
