Endocytosis is a process where cells absorb material (
moleculessuch as proteins) from the outside by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large polarmolecules, and thus cannot pass through the hydrophobic plasma membraneor cell membrane. The function of endocytosis is the opposite of exocytosis.
The absorption of material from the outside environment of the cell is commonly divided into two processes:
Phagocytosis(literally, cell-eating) is the process by which cells ingest large objects, such as cells which have undergone apoptosis, bacteria, or viruses. The membrane folds around the object, and the object is sealed off into a large vacuoleknown as a phagosome.
Pinocytosis(literally, cell-drinking). This process is concerned with the uptake of solutesand single molecules such as proteins.
*Receptor-mediated endocytosis is a more specific active event where the cytoplasm membrane folds inward to form coated pits. These inward budding vesicles bud to form cytoplasmic vesicles.
There are three types of endocytosis: namely, macropinocytosis, caveolar endocytosis, and
*Macropinocytosis is the invagination of the
cell membraneto form a pocket, which then pinches off into the cell to form a vesiclefilled with extracellular fluid (and molecules within it). The filling of the pocket occurs in a non-specific manner. The vesicle then travels into the cytosoland fuses with other vesicles such as endosomesand lysosomes.
*Caveolae consists of the protein caveolin-1 with a bilayer enriched in
cholesteroland glycolipids. Caveolae are flask-shape pits in the membrane that resemble the shape of a cave (hence the name caveolae). Uptake of extracellular moleculesare also believed to be specifically mediated via receptors in caveolae.
*Clathrin-mediated endocytosis is the specific uptake of large extracellular molecules such as proteins, membrane localized receptors and
ion-channels. These receptors are associated with the cytosolic protein clathrin, which initiates the formation of a vesicle by forming a crystalline coat on the inner surface of the cell's membrane.
The major route for endocytosis in most cells, and the best-understood, is that mediated by the molecule
clathrin. This large protein assists in the formation of a coated pit on the inner surface of the plasma membraneof the cell. This pit then buds into the cell to form a coated vesicle in the cytoplasm of the cell. In so doing, it brings into the cell not only a small area of the surface of the cell but also a small volume of fluid from outside the cell.
Vesicles selectively concentrate and exclude certain proteins during formation and are not representative of the membrane as a whole.
AP2 adaptorsare multisubunit complexes that perform this function at the plasma membrane. The best-understood receptors that are found concentrated in coated vesicles of mammalian cells are the LDL receptor(which removes LDLfrom circulating blood), the transferrin receptor (which brings ferric ions bound by transferrininto the cell) and certain hormone receptors (such as that for EGF).
At any one moment, about 25% of the plasma membrane of a fibroblast is made up of coated pits. As a coated pit has a life of about a minute before it buds into the cell, a fibroblast takes up its surface by this route about once every 50 minutes. Coated vesicles formed from the plasma membrane have a diameter of about 100nm and a lifetime measured in a few seconds. Once the coat has been shed, the remaining vesicle fuses with
endosomesand proceeds down the endocytic pathway. The actual budding-in process, whereby a pit is converted to a vesicle, is carried out by clathrin assisted by a set of cytoplasmic proteins, which includes dynaminand adaptors such as adaptin.
Coated pits and vesicles were first seen in thin sections of tissue in the electron microscope by Thomas Roth and Keith Porter in 1964. The importance of them for the clearance of LDL from blood was discovered by R. G Anderson,
Michael S. Brownand Joseph L. Goldsteinin 1976. Coated vesicles were first purified by Barbara Pearse, who discovered the clathrin coat molecule, also in 1976.
* [http://www.biologyreference.com/Dn-Ep/Endocytosis.html Endocytosis at biologyreference.com]
* [http://www.endocytosis.org Endocytosis - researching endocytic mechanisms at endocytosis.org]
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