Exocytosis ( / ˌ ɛ k s oʊ s aɪ ˈ t oʊ s ɪ s / ) is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins ) out of the cell ( exo- + cytosis ). As an active transport mechanism, exocytosis requires the use of energy to transport material. Exocytosis and its counterpart, endocytosis , are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic portion of the cell membrane by passive means. Exocytosis is the process by which a large amount of molecules are released; thus it is a form of bulk transport. Exocytosis occurs via secretory portals at the cell plasma membrane called porosomes . Porosomes are permanent cup-shaped lipoprotein structures at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.
43-595: See text for subclasses. 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 undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of their life cycle ) and are variously used in swimming, crawling, attachment, feeding, and sensation. Ciliates are an important group of protists , common almost anywhere there
86-411: A chain of new organisms); and palintomy (multiple fissions, usually within a cyst ). Fission may occur spontaneously, as part of the vegetative cell cycle . Alternatively, it may proceed as a result of self-fertilization ( autogamy ), or it may follow conjugation , a sexual phenomenon in which ciliates of compatible mating types exchange genetic material. While conjugation is sometimes described as
129-487: A cilium. These are arranged into rows called kineties , which run from the anterior to posterior of the cell. The body and oral kinetids make up the infraciliature , an organization unique to the ciliates and important in their classification, and include various fibrils and microtubules involved in coordinating the cilia. In some forms there are also body polykinetids, for instance, among the spirotrichs where they generally form bristles called cirri . The infraciliature
172-533: A close relationship between the ciliates, Apicomplexa , and dinoflagellates . These superficially dissimilar groups make up the alveolates . Most ciliates are heterotrophs , feeding on smaller organisms, such as bacteria and algae , and detritus swept into the oral groove (mouth) by modified oral cilia. This usually includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids , groups of many cilia together with associated structures. The food
215-464: A complex that brings the two fusion membranes together. For instance, in synapses, the SNARE complex is formed by syntaxin-1 and SNAP25 at the plasma membrane and VAMP2 at the vesicle membrane. Exocytosis in neuronal chemical synapses is Ca triggered and serves interneuronal signalling. The calcium sensors that trigger exocytosis might interact either with the SNARE complex or with the phospholipids of
258-448: A form of reproduction, it is not directly connected with reproductive processes, and does not directly result in an increase in the number of individual ciliates or their progeny. Ciliate conjugation is a sexual phenomenon that results in genetic recombination and nuclear reorganization within the cell. During conjugation, two ciliates of a compatible mating type form a bridge between their cytoplasms . The micronuclei undergo meiosis ,
301-533: A leech cocoon from the Triassic period , about 200 million years ago. According to the 2016 phylogenetic analysis, Mesodiniea is consistently found as the sister group to all other ciliates. Additionally, two big sub-groups are distinguished inside subphylum Intramacronucleata : SAL ( Spirotrichea + Armophorea + Litostomatea ) and CONthreeP or Ventrata ( Colpodea + Oligohymenophorea + Nassophorea + Phyllopharyngea + Plagiopylea + Prostomatea ). The class Protocruziea
344-407: A new macronucleus is generated from the post-conjugal micronucleus. Food vacuoles are formed through phagocytosis and typically follow a particular path through the cell as their contents are digested and broken down by lysosomes so the substances the vacuole contains are then small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by
387-416: A single gene . In Tetrahymena , the micronucleus has 10 chromosomes (five per haploid genome), while the macronucleus has over 20,000 chromosomes. In addition, the micronuclear genes are interrupted by numerous "internal eliminated sequences" (IESs). During development of the macronucleus, IESs are deleted and the remaining gene segments, macronuclear destined sequences (MDSs), are spliced together to give
430-463: A specific sorting signal on the vesicles, a clathrin coat, as well as an increase in intracellular calcium. In multicellular organisms, this mechanism initiates many forms of intercellular communication such as synaptic transmission, hormone secretion by neuroendocrine cells, and immune cells' secretion. In neurons and endocrine cells, the SNARE-proteins and SM-proteins catalyze the fusion by forming
473-446: A synaptic vesicle but before exocytosis, such that the influx of calcium ions is all that is needed to trigger nearly instantaneous neurotransmitter release. In other cell types, whose secretion is constitutive (i.e. continuous, calcium ion independent, non-triggered) there is no priming. Transient vesicle fusion is driven by SNARE proteins, resulting in release of vesicle contents into the extracellular space (or in case of neurons in
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#1732858841566516-555: A vesicle over a moderately small distance. For example, vesicles that transport proteins from the Golgi apparatus to the cell surface area, will be likely to use motor proteins and a cytoskeletal track to get closer to their target. Before tethering would have been appropriate, many of the proteins used for the active transport would have been instead set for passive transport, because the Golgi apparatus does not require ATP to transport proteins. Both
559-417: Is a large coiled-coil protein, also involved in synaptic exocytosis, marking the 'hotspots' fusion points of the secretory carriers fusion. Exocyst is an octameric protein complex. In mammals, exocyst components localize in both plasma membrane, and Golgi apparatus and the exocyst proteins are colocalized at the fusion point of the post-Golgi vesicles. The membrane fusion of the constitutive exocytosis, probably,
602-593: Is also a mechanism by which cells are able to insert membrane proteins (such as ion channels and cell surface receptors ), lipids , and other components into the cell membrane. Vesicles containing these membrane components fully fuse with and become part of the outer cell membrane. The term was proposed by De Duve in 1963. In eukaryotes , there are two types of exocytosis: 1) Ca triggered non-constitutive (i.e., regulated exocytosis) and 2) non-Ca triggered constitutive (i.e., non-regulated). Ca triggered non-constitutive exocytosis requires an external signal,
645-631: Is found as the sister group to Ventrata / CONthreeP . The class Cariacotrichea was excluded from the analysis, but it was originally established as part of Intramacronucleata . The odontostomatids were identified in 2018 as its own class Odontostomatea , related to Armophorea . Mesodiniea Karyorelictea Heterotrichea Odontostomatea Armophorea Litostomatea Spirotrichea Cariacotrichea Protocruziea Discotrichida Colpodea Nassophorea Phyllopharyngea Oligohymenophorea Prostomatea Plagiopylea Several different classification schemes have been proposed for
688-450: Is guided by long RNAs derived from the parental macronucleus. More than 95% of micronuclear DNA is eliminated during spirotrich macronuclear development. ln clonal populations of Paramecium , aging occurs over successive generations leading to a gradual loss of vitality, unless the cell line is revitalized by conjugation or autogamy . In Paramecium tetraurelia , the clonally aging line loses vitality and expires after about 200 fissions, if
731-463: Is known to cause disease in humans. Ciliates reproduce asexually , by various kinds of fission . During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis (except among the Karyorelictean ciliates, whose macronuclei do not divide). The cell then divides in two, and each new cell obtains a copy of the micronucleus and the macronucleus. Typically,
774-437: Is mediated by SNAP29 and Syntaxin19 at the plasma membrane and YKT6 or VAMP3 at the vesicle membrane. Vesicular exocytosis in prokaryote gram negative bacteria is a third mechanism and latest finding in exocytosis. The periplasm is pinched off as bacterial outer membrane vesicles (OMVs) for translocating microbial biochemical signals into eukaryotic host cells or other microbes located nearby, accomplishing control of
817-600: Is moved by the cilia through the mouth pore into the gullet, which forms food vacuoles. Many species are also mixotrophic , combining phagotrophy and phototrophy through kleptoplasty or symbiosis with photosynthetic microbes. The ciliate Halteria has been observed to feed on chloroviruses . Feeding techniques vary considerably, however. Some ciliates are mouthless and feed by absorption ( osmotrophy ), while others are predatory and feed on other protozoa and in particular on other ciliates. Some ciliates parasitize animals , although only one species, Balantidium coli ,
860-424: Is one of the main components of the cell cortex . Others are the alveoli , small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are also generally present. The presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate
903-478: Is referred to as "isogamontic" conjugation. In some groups, partners are different in size and shape. This is referred to as "anisogamontic" conjugation. In sessile peritrichs , for instance, one sexual partner (the microconjugant) is small and mobile, while the other (macroconjugant) is large and sessile . In Paramecium caudatum , the stages of conjugation are as follows (see diagram at right): Ciliates contain two types of nuclei: somatic " macronucleus " and
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#1732858841566946-522: Is water—in lakes, ponds, oceans, rivers, and soils, including anoxic and oxygen-depleted habitats. About 4,500 unique free-living species have been described, and the potential number of extant species is estimated at 27,000–40,000. Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites . Ciliate species range in size from as little as 10 μm in some colpodeans to as much as 4 mm in length in some geleiids , and include some of
989-526: The cell membrane , where they dock and fuse at porosomes and their contents (i.e., water-soluble molecules) are secreted into the extracellular environment. This secretion is possible because the vesicle transiently fuses with the plasma membrane. In the context of neurotransmission , neurotransmitters are typically released from synaptic vesicles into the synaptic cleft via exocytosis; however, neurotransmitters can also be released via reverse transport through membrane transport proteins . Exocytosis
1032-614: The germline " micronucleus ". Only the DNA in the micronucleus is passed on during sexual reproduction (conjugation). On the other hand, only the DNA in the macronucleus is actively expressed and results in the phenotype of the organism. Macronuclear DNA is derived from micronuclear DNA by amazingly extensive DNA rearrangement and amplification. The macronucleus begins as a copy of the micronucleus. The micronuclear chromosomes are fragmented into many smaller pieces and amplified to give many copies. The resulting macronuclear chromosomes often contain only
1075-441: The phenotype of the organism). The latter is generated from the micronucleus by amplification of the genome and heavy editing. The micronucleus passes its genetic material to offspring, but does not express its genes. The macronucleus provides the small nuclear RNA for vegetative growth. Division of the macronucleus occurs in most ciliate species, apart from those in class Karyorelictea, whose macronuclei are replaced every time
1118-426: The actin- and the microtubule-base are implicated in these processes, along with several motor proteins . Once the vesicles reach their targets, they come into contact with tethering factors that can restrain them. It is useful to distinguish between the initial, loose tethering of vesicles to their objective from the more stable, packing interactions. Tethering involves links over distances of more than about half
1161-415: The calcium triggered kinetics of exocytosis in a specific fashion. Constitutive exocytosis is performed by all cells and serves the release of components of the extracellular matrix or delivery of newly synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle . There is no clear consensus about the machinery and molecular processes that drive
1204-583: The cause of aging in P. tetraurelia . Until recently, the oldest ciliate fossils known were tintinnids from the Ordovician period . In 2007, Li et al. published a description of fossil ciliates from the Doushantuo Formation , about 580 million years ago, in the Ediacaran period . These included two types of tintinnids and a possible ancestral suctorian. A fossil Vorticella has been discovered inside
1247-530: The cell divides. Macronuclear division is accomplished by amitosis , and the segregation of the chromosomes occurs by a process whose mechanism is unknown. After a certain number of generations (200–350, in Paramecium aurelia , and as many as 1,500 in Tetrahymena ) the cell shows signs of aging, and the macronuclei must be regenerated from the micronuclei. Usually, this occurs following conjugation , after which
1290-423: The cell is divided transversally, with the anterior half of the ciliate (the proter ) forming one new organism, and the posterior half (the opisthe ) forming another. However, other types of fission occur in some ciliate groups. These include budding (the emergence of small ciliated offspring, or "swarmers", from the body of a mature parent); strobilation (multiple divisions along the cell body, producing
1333-466: The cell line is not rejuvenated by conjugation or self-fertilization. The basis for clonal aging was clarified by the transplantation experiments of Aufderheide in 1986 who demonstrated that the macronucleus, rather than the cytoplasm, is responsible for clonal aging. Additional experiments by Smith-Sonneborn, Holmes and Holmes, and Gilley and Blackburn demonstrated that, during clonal aging, DNA damage increases dramatically. Thus, DNA damage appears to be
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1376-673: The ciliate phylum known to be pathogenic to humans is Balantidium coli , which causes the disease balantidiasis . It is not pathogenic to the domestic pig, the primary reservoir of this pathogen. Alveolate Too Many Requests If you report this error to the Wikimedia System Administrators, please include the details below. Request from 172.68.168.132 via cp1112 cp1112, Varnish XID 389756149 Upstream caches: cp1112 int Error: 429, Too Many Requests at Fri, 29 Nov 2024 05:40:41 GMT Exocytosis In exocytosis, membrane-bound secretory vesicles are carried to
1419-429: The ciliates. The following scheme is based on a molecular phylogenetic analysis of up to four genes from 152 species representing 110 families: Some old classifications included Opalinidae in the ciliates. The fundamental difference between multiciliate flagellates (e.g., hemimastigids , Stephanopogon , Multicilia , opalines ) and ciliates is the presence of macronuclei in ciliates alone. The only member of
1462-503: The diameter of a vesicle from a given membrane surface (>25 nm). Tethering interactions are likely to be involved in concentrating synaptic vesicles at the synapse . Secretory vesicles transiently dock and fuse at the porosome at the cell plasma membrane, via a tight t-/v-SNARE ring complex. In neuronal exocytosis, the term priming has been used to include all of the molecular rearrangements and ATP-dependent protein and lipid modifications that take place after initial docking of
1505-498: The formation, budding, translocation and fusion of the post-Golgi vesicles to the plasma membrane. The fusion involves membrane tethering (recognition) and membrane fusion. It is still unclear if the machinery between the constitutive and regulated secretion is different. The machinery required for constitutive exocytosis has not been studied as much as the mechanism of regulated exocytosis. Two tethering complexes are associated with constitutive exocytosis in mammals, ELKS and Exocyst. ELKS
1548-439: The fusing membranes. Synaptotagmin has been recognized as the major sensor for Ca triggered exocytosis in animals. However, synaptotagmin proteins are absent in plants and unicellular eukaryotes. Other potential calcium sensors for exocytosis are EF-hand proteins (Ex: Calmodulin) and C2-domain (Ex: Ferlins, E-synaptotagmin, Doc2b) containing proteins. It is unclear how the different calcium sensors can cooperate together and mediate
1591-551: The macronuclei disappear, and haploid micronuclei are exchanged over the bridge. In some ciliates (peritrichs, chonotrichs and some suctorians ), conjugating cells become permanently fused, and one conjugant is absorbed by the other. In most ciliate groups, however, the cells separate after conjugation, and both form new macronuclei from their micronuclei. Conjugation and autogamy are always followed by fission. In many ciliates, such as Paramecium , conjugating partners (gamonts) are similar or indistinguishable in size and shape. This
1634-414: The most morphologically complex protozoans. In most systems of taxonomy , " Ciliophora " is ranked as a phylum under any of several kingdoms , including Chromista , Protista or Protozoa . In some older systems of classification, such as the influential taxonomic works of Alfred Kahl , ciliated protozoa are placed within the class " Ciliata " (a term which can also refer to a genus of fish ). In
1677-581: The operational gene. Tetrahymena has about 6,000 IESs and about 15% of micronuclear DNA is eliminated during this process. The process is guided by small RNAs and epigenetic chromatin marks. In spirotrich ciliates (such as Oxytricha ), the process is even more complex due to "gene scrambling": the MDSs in the micronucleus are often in different order and orientation from that in the macronuclear gene, and so in addition to deletion, DNA inversion and translocation are required for "unscrambling". This process
1720-409: The secreting microbe on its environment - including invasion of host, endotoxemia, competing with other microbes for nutrition, etc. This finding of membrane vesicle trafficking occurring at the host–pathogen interface also dispels the myth that exocytosis is purely a eukaryotic cell phenomenon. Five steps are involved in exocytosis: Certain vesicle-trafficking steps require the transportation of
1763-590: The synaptic cleft). The merging of the donor and the acceptor membranes accomplishes three tasks: Retrieval of synaptic vesicles occurs by endocytosis . Most synaptic vesicles are recycled without a full fusion into the membrane ( kiss-and-run fusion ) via porosome . Non-constitutive exocytosis and subsequent endocytosis are highly energy expending processes, and thus, are dependent on mitochondria . Examination of cells following secretion using electron microscopy demonstrate increased presence of partially empty vesicles following secretion. This suggested that during
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1806-632: The taxonomic scheme endorsed by the International Society of Protistologists , which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata . Unlike most other eukaryotes , ciliates have two different sorts of nuclei : a tiny, diploid micronucleus (the "generative nucleus", which carries the germline of the cell), and a large, ampliploid macronucleus (the "vegetative nucleus", which takes care of general cell regulation, expressing
1849-555: The time it reaches the cytoproct ( anal pore ) is discharged by exocytosis . Most ciliates also have one or more prominent contractile vacuoles , which collect water and expel it from the cell to maintain osmotic pressure , or in some function to maintain ionic balance. In some genera, such as Paramecium , these have a distinctive star shape, with each point being a collecting tube. Mostly, body cilia are arranged in mono- and dikinetids , which respectively include one and two kinetosomes (basal bodies), each of which may support
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