Unicellular organism

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Unicellular organism
Ventricaria ventricosa.JPG
Valonia ventricosa , a species of alga with a diameter that ranges typically from 1 to 4 centimetres (0.4 to 1.6 in) is among the largest unicellular species

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 protocells possibly emerging 3.8–4.8 billion years ago. [1] [2]

Contents

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. Gametes, for example, are reproductive unicells for multicellular organisms. [3] Additionally, multicellularity appears to have evolved independently many times in the history of life.

Some organisms are partially unicellular, like Dictyostelium discoideum . Additionally, unicellular organisms can be multinucleate, like Caulerpa , Plasmodium , and Myxogastria.

Evolutionary hypothesis

Primitive protocells 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 hypothesis, early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication. [4]

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. [5] Such hypothetic cells with an RNA genome instead of the usual DNA genome are called 'ribocells' or 'ribocytes'. [4]

When amphiphiles like lipids are placed in water, the hydrophobic tails aggregate to form micelles and vesicles, with the hydrophilic ends facing outwards. [2] [5] Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment. [5] 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. [2]

Prokaryotes

Prokaryotes lack membrane-bound organelles, such as mitochondria or a nucleus. [6] Instead, most prokaryotes have an irregular region that contains DNA, known as the nucleoid. [7] Most prokaryotes have a single, circular chromosome, which is in contrast to eukaryotes, which typically have linear chromosomes. [8] 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. [9] Prokaryotes are relatively ubiquitous in the environment and some (known as extremophiles) thrive in extreme environments.

Bacteria

Modern stromatolites in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm. Stromatolites in Sharkbay.jpg
Modern stromatolites in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm.

Bacteria are one of the world's oldest forms of life, and are found virtually everywhere in nature. [9] Many common bacteria have plasmids, which are short, circular, self-replicating DNA molecules that are separate from the bacterial chromosome. [11] Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance. [12] 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. [13] 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. [14] In addition, plasmids can be exchanged through the use of a pilus in a process known as conjugation. [12]

The photosynthetic cyanobacteria are arguably the most successful bacteria, and changed the early atmosphere of the earth by oxygenating it. [15] Stromatolites, structures made up of layers of calcium carbonate and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records. [15] [16] 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. [16] Much of the fossilized stromatolites of the world can be found in Western Australia. [16] There, some of the oldest stromatolites have been found, some dating back to about 3,430 million years ago. [16]

Clonal aging occurs naturally in bacteria, and is apparently due to the accumulation of damage that can happen even in the absence of external stressors. [17]

Archaea

A bottom-dwelling community found deep in the European Arctic. Echinoderms 600.jpg
A bottom-dwelling community found deep in the European Arctic.

Hydrothermal vents release heat and hydrogen sulfide, allowing extremophiles to survive using chemolithotrophic growth. [19] 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. [20] [21] 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. [21] As their name suggests, Archaea comes from a Greek word archaios, meaning original, ancient, or primitive. [22]

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[ citation needed ]. Examples of these Archaean extremophiles are as follows:

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. [27] This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of adenosine triphosphate. [27] They are the only known organisms capable of producing methane. [28] Under stressful environmental conditions that cause DNA damage, some species of archaea aggregate and transfer DNA between cells. [29] 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. [30]

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. [31] 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 eukaryotic life. [32] Meiosis, a true sexual process, allows for efficient recombinational repair of DNA damage [14] and a greater range of genetic diversity by combining the DNA of the parents followed by recombination. [31] Metabolic functions in eukaryotes are more specialized as well by sectioning specific processes into organelles.[ citation needed ]

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 , with the parasitic ability to enter a cell. [33] 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. [33] Chloroplasts probably became symbionts through a similar set of events, and are most likely descendants of cyanobacteria. [34] 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.[ citation needed ]

Protozoa

Paramecium tetraurelia, a ciliate, with oral groove visible Paramecia tetraurelia.jpeg
Paramecium tetraurelia, a ciliate, with oral groove visible

Protozoa are largely defined by their method of locomotion, including flagella, cilia, and pseudopodia. [35] 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, Percolozoa, Microsporidia and Sulcozoa. [36] [37] Protozoa, like plants and animals, can be considered heterotrophs or autotrophs. [33] 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. [33] While protozoa reproduce mainly asexually, some protozoa are capable of sexual reproduction. [33] Protozoa with sexual capability include the pathogenic species Plasmodium falciparum , Toxoplasma gondii , Trypanosoma brucei , Giardia duodenalis and Leishmania species. [14]

Ciliophora, or ciliates, are a group of protists that utilize cilia for locomotion. Examples include Paramecium , Stentors, and Vorticella . [38] Ciliates are widely abundant in almost all environments where water can be found, and the cilia beat rhythmically in order to propel the organism. [39] Many ciliates have trichocysts, which are spear-like organelles that can be discharged to catch prey, anchor themselves, or for defense. [40] [41] Ciliates are also capable of sexual reproduction, and utilize two nuclei unique to ciliates: a macronucleus for normal metabolic control and a separate micronucleus that undergoes meiosis. [40] Examples of such ciliates are Paramecium and Tetrahymena that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions.[ citation needed ]

The Amebozoa utilize pseudopodia and cytoplasmic flow to move in their environment. Entamoeba histolytica is the cause of amebic dysentery. [42] Entamoeba histolytica appears to be capable of meiosis. [43]

Unicellular algae

A scanning electron microscope image of a diatom CSIRO ScienceImage 7632 SEM diatom.jpg
A scanning electron microscope image of a diatom

Unicellular algae are plant-like autotrophs and contain chlorophyll. [44] They include groups that have both multicellular and unicellular species:

Unicellular fungi

Transmission electron microscope image of budding Ogataea polymorpha Kg3.jpg
Transmission electron microscope image of budding Ogataea polymorpha

Unicellular fungi include the yeasts. Fungi are found in most habitats, although most are found on land. [50] Yeasts reproduce through mitosis, and many use a process called budding, where most of the cytoplasm is held by the mother cell. [50] Saccharomyces cerevisiae ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread. [51] S. cerevisiae 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. [52] [53] 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). [54]

Macroscopic unicellular organisms

Most unicellular organisms are of microscopic size and are thus classified as microorganisms. However, some unicellular protists and bacteria are macroscopic and visible to the naked eye. [55] Examples include:

See also

Related Research Articles

<span class="mw-page-title-main">Cell (biology)</span> Basic unit of many life forms

The cell is the basic structural and functional unit of all forms of life. Every cell consists of cytoplasm enclosed within a membrane; many cells contain organelles, each with a specific function. The term comes from the Latin word cellula meaning 'small room'. Most cells are only visible under a microscope. Cells emerged on Earth about 4 billion years ago. All cells are capable of replication, protein synthesis, and motility.

<span class="mw-page-title-main">Microorganism</span> Microscopic living organism

A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells.

In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name organelle comes from the idea that these structures are parts of cells, as organs are to the body, hence organelle, the suffix -elle being a diminutive. Organelles are either separately enclosed within their own lipid bilayers or are spatially distinct functional units without a surrounding lipid bilayer. Although most organelles are functional units within cells, some function units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.

<span class="mw-page-title-main">Kingdom (biology)</span> Taxonomic rank

In biology, a kingdom is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla.

<span class="mw-page-title-main">Symbiogenesis</span> Evolutionary theory holding that eukaryotic organelles evolved through symbiosis with prokaryotes

Symbiogenesis is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly other organelles of eukaryotic cells are descended from formerly free-living prokaryotes taken one inside the other in endosymbiosis. Mitochondria appear to be phylogenetically related to Rickettsiales bacteria, while chloroplasts are thought to be related to cyanobacteria.

<span class="mw-page-title-main">Domain (biology)</span> Taxonomic rank

In biological taxonomy, a domain, also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of taxonomy devised by Carl Woese, Otto Kandler and Mark Wheelis in 1990.

<span class="mw-page-title-main">Three-domain system</span> Hypothesis for classification of life

The three-domain system is a taxonomic classification system that groups all cellular life into three domains, namely Archaea, Bacteria and Eukarya, introduced by Carl Woese, Otto Kandler and Mark Wheelis in 1990. The key difference from earlier classifications such as the two-empire system and the five-kingdom classification is the splitting of Archaea from Bacteria as completely different organisms. It has been challenged by the two-domain system that divides organisms into Bacteria and Archaea only, as Eukaryotes are considered as a clade of Archaea.

<span class="mw-page-title-main">Multicellular organism</span> Organism that consists of more than one cell

A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

<span class="mw-page-title-main">Prokaryote</span> Unicellular organism lacking a membrane-bound nucleus

A prokaryote is a single-cell organism whose cell lacks a nucleus and other membrane-bound organelles. The word prokaryote comes from the Ancient Greek πρό 'before' and κάρυον 'nut, kernel'. In the two-empire system arising from the work of Édouard Chatton, prokaryotes were classified within the empire Prokaryota. But in the three-domain system, based upon molecular analysis, prokaryotes are divided into two domains: Bacteria and Archaea. Organisms with nuclei are placed in a third domain, Eukaryota.

<span class="mw-page-title-main">Protist</span> Eukaryotes other than animals, plants or fungi

A protist or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a polyphyletic grouping of several independent clades that evolved from the last eukaryotic common ancestor.

<span class="mw-page-title-main">Protozoa</span> Single-celled eukaryotic organisms that feed on organic matter

Protozoa are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically, protozoans were regarded as "one-celled animals".

<span class="mw-page-title-main">Archaea</span> Domain of single-celled organisms

Archaea is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotic. Archaea were initially classified as bacteria, receiving the name archaebacteria, but this term has fallen out of use.

<span class="mw-page-title-main">Eukaryote</span> Domain of life whose cells have nuclei

The eukaryotes constitute the domain of Eukarya or Eukaryota, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes: the Bacteria and the Archaea. Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass is much larger than that of prokaryotes.

Fission, in biology, is the division of a single entity into two or more parts and the regeneration of those parts to separate entities resembling the original. The object experiencing fission is usually a cell, but the term may also refer to how organisms, bodies, populations, or species split into discrete parts. The fission may be binary fission, in which a single organism produces two parts, or multiple fission, in which a single entity produces multiple parts.

<span class="mw-page-title-main">Marine microorganisms</span> Any life form too small for the naked human eye to see that lives in a marine environment

Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism is any microscopic living organism or virus, which is invisibly small to the unaided human eye without magnification. Microorganisms are very diverse. They can be single-celled or multicellular and include bacteria, archaea, viruses, and most protozoa, as well as some fungi, algae, and animals, such as rotifers and copepods. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses as microorganisms, but others consider these as non-living.

<span class="mw-page-title-main">Lokiarchaeota</span> Phylum of archaea

Lokiarchaeota is a proposed phylum of the Archaea. The phylum includes all members of the group previously named Deep Sea Archaeal Group, also known as Marine Benthic Group B. Lokiarchaeota is part of the superphylum Asgard containing the phyla: Lokiarchaeota, Thorarchaeota, Odinarchaeota, Heimdallarchaeota, and Helarchaeota. A phylogenetic analysis disclosed a monophyletic grouping of the Lokiarchaeota with the eukaryotes. The analysis revealed several genes with cell membrane-related functions. The presence of such genes support the hypothesis of an archaeal host for the emergence of the eukaryotes; the eocyte-like scenarios.

The initial version of a classification system of life by British zoologist Thomas Cavalier-Smith appeared in 1978. This initial system continued to be modified in subsequent versions that were published until he died in 2021. As with classifications of others, such as Carl Linnaeus, Ernst Haeckel, Robert Whittaker, and Carl Woese, Cavalier-Smith's classification attempts to incorporate the latest developments in taxonomy., Cavalier-Smith used his classifications to convey his opinions about the evolutionary relationships among various organisms, principally microbial. His classifications complemented his ideas communicated in scientific publications, talks, and diagrams. Different iterations might have a wider or narrow scope, include different groupings, provide greater or lesser detail, and place groups in different arrangements as his thinking changed. His classifications has been a major influence in the modern taxonomy, particularly of protists.

<span class="mw-page-title-main">Eukaryogenesis</span> Process of forming the first eukaryotic cell

Eukaryogenesis, the process which created the eukaryotic cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved symbiogenesis, in which archaea and bacteria came together to create the first eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex, a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor (LECA), gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.

<span class="mw-page-title-main">Marine prokaryotes</span> Marine bacteria and marine archaea

Marine prokaryotes are marine bacteria and marine archaea. They are defined by their habitat as prokaryotes that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. All cellular life forms can be divided into prokaryotes and eukaryotes. Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, whereas prokaryotes are the organisms that do not have a nucleus enclosed within a membrane. The three-domain system of classifying life adds another division: the prokaryotes are divided into two domains of life, the microscopic bacteria and the microscopic archaea, while everything else, the eukaryotes, become the third domain.

<span class="mw-page-title-main">Marine protists</span> Protists that live in saltwater or brackish water

Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as marine single-celled prokaryotes and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics because they are paraphyletic.

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