Editing Endocytosis (section)

== Principal components ==
The endocytic pathway of mammalian cells consists of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface (as in early endosomes and recycling endosomes), or sort them to degradation (as in late endosomes and lysosomes). The principal components of the endocytic pathway are:<ref name="endo00" />
* '''Early [[endosome]]s''' are the first compartment of the endocytic pathway. Early endosomes are often located in the periphery of the cell, and receive most types of vesicles coming from the cell surface. They have a characteristic tubulo-vesicular structure (vesicles up to 1&nbsp;μm in diameter with connected tubules of approx. 50&nbsp;nm diameter) and a mildly acidic pH. They are principally sorting organelles where many endocytosed ligands dissociate from their [[Receptor (biochemistry)|receptors]] in the acid pH of the compartment, and from which many of the receptors recycle to the cell surface (via tubules).<ref name="endo1">{{cite journal | vauthors = Mellman I | title = Endocytosis and molecular sorting | journal = Annual Review of Cell and Developmental Biology | volume = 12 | pages = 575–625 | year = 1996 | pmid = 8970738 | doi = 10.1146/annurev.cellbio.12.1.575 }}</ref><ref name="endo2">{{cite journal | vauthors = Mukherjee S, Ghosh RN, Maxfield FR | title = Endocytosis | journal = Physiological Reviews | volume = 77 | issue = 3 | pages = 759–803 | date = July 1997 | pmid = 9234965 | doi = 10.1152/physrev.1997.77.3.759 }}</ref> It is also the site of sorting into transcytotic pathway to later compartments (like late endosomes or lysosomes) via transvesicular compartments (like multivesicular bodies (MVB) or endosomal carrier vesicles (ECVs)).
* '''Late endosomes''' receive endocytosed material en route to [[lysosome]]s, usually from early endosomes in the endocytic pathway, from trans-Golgi network (TGN) in the biosynthetic pathway, and from [[phagosome]]s in the phagocytic pathway.<ref name="endo3">{{cite journal | vauthors = Stoorvogel W, Strous GJ, Geuze HJ, Oorschot V, Schwartz AL | title = Late endosomes derive from early endosomes by maturation | journal = Cell | volume = 65 | issue = 3 | pages = 417–427 | date = May 1991 | pmid = 1850321 | doi = 10.1016/0092-8674(91)90459-C | s2cid = 31539542 }}</ref> Late endosomes often contain proteins characteristic of nucleosomes, mitochondria and mRNAs including lysosomal membrane glycoproteins and acid hydrolases. They are acidic (approx. pH 5.5), and are part of the trafficking pathway of [[mannose-6-phosphate]] receptors. Late endosomes are thought to mediate a final set of sorting events prior the delivery of material to lysosomes.
* '''[[Lysosome]]s''' are the last compartment of the endocytic pathway.  Their chief function is to break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds.  These are then returned to the cytoplasm as new cell-building materials. To accomplish this, lysosomes use some 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum, modified in the [[Golgi apparatus]] and function in an acidic environment.<ref>{{cite journal | vauthors = Weissmann G | title = Lysosome | journal = The New England Journal of Medicine | volume = 273 | issue = 20 | pages = 1084–90 contd | date = November 1965 | pmid = 5319614 | doi = 10.1056/NEJM196511112732006 }}</ref> The approximate pH of a lysosome is 4.8 and by [[electron microscopy]] (EM) usually appear as large [[vacuoles]] (1-2&nbsp;μm in diameter) containing electron dense material. They have a high content of lysosomal membrane proteins and active lysosomal hydrolases, but no mannose-6-phosphate receptor. They are generally regarded as the principal hydrolytic compartment of the cell.<ref name="endo4">{{cite journal | vauthors = Gruenberg J, Maxfield FR | title = Membrane transport in the endocytic pathway | journal = Current Opinion in Cell Biology | volume = 7 | issue = 4 | pages = 552–563 | date = August 1995 | pmid = 7495576 | doi = 10.1016/0955-0674(95)80013-1 }}</ref><ref name="endo5">{{cite journal | vauthors = Luzio JP, Rous BA, Bright NA, Pryor PR, Mullock BM, Piper RC | title = Lysosome-endosome fusion and lysosome biogenesis | journal = Journal of Cell Science | volume = 113 | issue = 9 | pages = 1515–1524 | date = May 2000 | pmid = 10751143 | doi = 10.1242/jcs.113.9.1515 | doi-access = free }}{{Dead link|date=February 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

It was recently found that an [[eisosome]] serves as a portal of endocytosis in yeast.<ref name="pmid16496001">{{cite journal | vauthors = Walther TC, Brickner JH, Aguilar PS, Bernales S, Pantoja C, Walter P | title = Eisosomes mark static sites of endocytosis | journal = Nature | volume = 439 | issue = 7079 | pages = 998–1003 | date = February 2006 | pmid = 16496001 | doi = 10.1038/nature04472 | s2cid = 2838121 | bibcode = 2006Natur.439..998W }}</ref>