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In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of
endocytosis Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. E ...
in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell membrane, resulting in their containment within a small vesicle inside the cell. These pinocytotic vesicles then typically fuse with
early endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane can f ...
s to hydrolyze (break down) the particles. Pinocytosis is variably subdivided into categories depending on the molecular mechanism and the fate of the internalized molecules.


Function

In humans, this process occurs primarily for absorption of fat droplets. In endocytosis the cell plasma membrane extends and folds around desired extracellular material, forming a pouch that pinches off creating an internalized vesicle. The invaginated pinocytosis vesicles are much smaller than those generated by phagocytosis. The vesicles eventually fuse with the lysosome, whereupon the vesicle contents are digested. Pinocytosis involves a considerable investment of cellular energy in the form of ATP.


Pinocytosis and ATP

Pinocytosis is used primarily for clearing extracellular fluids (ECF) and as part of immune surveillance. In contrast to phagocytosis, it generates very small amounts of ATP from the wastes of alternative substances such as lipids (fat). Unlike
receptor-mediated endocytosis Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses – by the inward budding of the plasma membrane (invagination). Thi ...
, pinocytosis is nonspecific in the substances that it transport: the cell takes in surrounding fluids, including all solutes present.


Etymology and pronunciation

The word ''pinocytosis'' () uses combining forms of ''pino-'' + '' cyto-'' + ''
-osis OSIS or ''variant'' may refer to: ;OSIS * Open Source Information System * Open Scripture Information Standard * ''Organisasi Siswa Intra Sekolah'', Indonesian for Intra-school students organization ;-osis The suffix ''wikt:-osis, -osis'' is used ...
'', all New Latin from Greek, reflecting ''píno'', to drink, and cytosis. The term was proposed by W. H. Lewis in 1931.


Non-specific, adsorptive pinocytosis

Non-specific, adsorptive pinocytosis is a form of
endocytosis Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. E ...
, a process in which small particles are taken in by a cell by splitting off small vesicles from the cell membrane.
Cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
ic proteins bind to the negative cell surface and are taken up via the clathrin-mediated system, thus the uptake is intermediate between
receptor-mediated endocytosis Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses – by the inward budding of the plasma membrane (invagination). Thi ...
and non-specific, non-adsorptive pinocytosis. The clathrin-coated pits occupy about 2% of the surface area of the cell and only last about a minute, with an estimated 2500 leaving the average cell surface each minute. The clathrin coats are lost almost immediately, and the membrane is subsequently recycled to the cell surface.


Macropinocytosis

Macropinocytosis is a clathrin-independent endocytic mechanism that can be activated in practically all animal cells, resulting in uptake. In most cell types, it does not occur continuously but rather is induced for a limited time in response to cell-surface receptor activation by specific cargoes, including growth factors, ligands of integrins, and apoptotic cell remnants. These ligands activate a complex signaling pathway, resulting in a change in actin dynamics and the formation of cell-surface protrusions of filopodia and lamellopodia, commonly called ''ruffles''. When ruffles collapse back onto the membrane, large fluid-filled endocytic vesicles form called macropinosomes, which can transiently increase the bulk fluid uptake of a cell by up to tenfold. Macropinocytosis is a solely degradative pathway: macropinosomes acidify and then fuse with late endosomes or endolysosomes, without recycling their cargo back to the plasma membrane. Some bacteria and viruses have evolved to induce macropinocytosis as a mechanism for entering host cells. Some of these can stop the degradation processes in order to survive inside the macropinosome, which may transform into smaller and long-lasting vacuoles containing the viruses or bacteria (some of which may replicate inside), or simply escape through the wall of the macropinosome when inside. For example, the gut pathogen ''Salmonella'' typhimurium injects toxins into the host cell in order to induce macropinocytosis as a form of uptake, inhibits the degradation of the macropinosome, and forms a salmonella-containing vacuole, or SCV, wherein it can replicate.


See also

* Phagocytosis *
Receptor-mediated endocytosis Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses – by the inward budding of the plasma membrane (invagination). Thi ...
* Caveolae


References

*Campbell, Reece, Mitchell: "Biology", Sixth Edition, Copyright 2002 P. 151 *Marshall, Ben, Incredible Biological Advancements of the 20th Century, Copyright 2001 p. 899 *Alrt, Pablo, Global Society Harvard study, copyright 2003 p. 189 *Brooker, Robert: "Biology", Second Edition, Copyright 2011 p. 116 *Cherrr, Malik, The Only Edition, Copyright 2012, p. 256 *Abbas, Abul, et al. "Basic Immunology: Functions and Disorders of the Immune System." 5th ed. Elsevier, 2016. p. 69 {{Membrane transport Cellular processes