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Exocytosis () is a form of active transport in which a cell transports molecules (such as proteins) out of the cell (''exo-'' + ''cytosis'') by expelling them in an energy-using process. 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 plasma or cell membrane by passive means. In exocytosis, secretory vesicles carry their contents across the cell membrane and into the extracellular space. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane. However, the mechanism of the secretion of intravesicular contents out of the cell is very different from the incorporation in the cell membrane of ion channels, signaling molecules, or receptors. While for membrane recycling and the incorporation in the cell membrane of ion channels, signaling molecules, or receptors complete membrane merger is required, for cell secretion there is transient vesicle fusion with the cell membrane in a process called exocytosis, dumping its contents out of the cell's environment. Examination of cells following secretion using electron microscopy demonstrate increased presence of partially empty vesicles following secretion. This suggested that during the secretory process, only a portion of the vesicular content is able to exit the cell. This could only be possible if the vesicle were to temporarily establish continuity with the cell plasma membrane, expel a portion of its contents, then detach, reseal, and withdraw into the cytosol (endocytose). In this way, the secretory vesicle could be reused for subsequent rounds of exo-endocytosis, until completely empty of its contents. ==Types== In multicellular organisms there are two types of exocytosis: 1) Ca2+ triggered non-constitutive (i.e., regulated exocytosis) and 2) non-Ca2+ triggered constitutive. Exocytosis in neuronal chemical synapses is Ca2+ triggered and serves interneuronal signalling. ''Constitutive exocytosis'' is performed by all cells and serves the release of components of the extracellular matrix, or just delivery of newly synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle. ''Regulated exocytosis'', on the other hand, requires an external signal, a specific sorting signal on the vesicles, a clathrin coat, as well as an increase in intracellular calcium. Exocytosis is the opposite of endocytosis. 3) ''Vesicular exocytosis'' in prokaryote gram negative microbes is the latest finding in exocytosis. Herein, periplasm of gram negative microbes is pinched off as bacterial outer membrane vesicles (OMVs) for translocating microbial biochemical signals into eukaryotic host cells〔YashRoy R C (1993) Eelectron microscope studies of surface pili and vesicles of ''Salmonella'' 3,10:r:- organisms. ''Indian Journal of Animal Sciences'', vol. 63, pp. 99-102.https://www.researchgate.net/publication/230817087_Electron_microscope_studies_of_surface_pilli_and_vesicles_of_Salmonella_310r-_organisms?ev=prf_pub〕 or other microbes located nearby,〔Kadurugamuwa J L and Beveridge T J (1996) Bacteriolytic effect of membrane vesicles from ''Pseudomonas aeruginosa'' on other bacterial including pathogens: conceptually new antibiotics. ''Journal of Bacteriology'', vol. 178(10), pp. 2767-2774.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC178010/〕 accomplishing control of 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 breaks the myth that exo-cytosis is purely a eukaryotic cell phenomenon.〔YashRoy R.C.(1998). Discovery of vesicular exocytosis in procaryotes and its role in Salmonella invasion. Current Science, vol 75(10), pp 1062-1066.http://www.currentscience.ac.in/cs/Downloads/article_id_075_10_1062_1066_0.pdf〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「exocytosis」の詳細全文を読む スポンサード リンク
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