View source for Endosome

An endosome is a membrane-bounded compartment or [[Organelles|organelle]]. It consists of a set of connected membrane tubes and larger [[Vesicles|vesicles]]&nbsp;<ref>Alberts, Bray, Hopkin et al. (2003) Essential Cell Biology, 2nd edition, New York: Garland Science. p.499</ref>.<br>Its key function is to sort endocytosed material, which is either recycled back to the [[Plasma membrane|plasma membrane]], or sorted to [[Lysosome|lysosomes]] for degradation <ref>Alberts, Bray, Hopkin et al. (2003) Essential Cell Biology, 2nd edition, New York: Garland Science. p.499</ref>.<br> 

There are two types of endosome involved in the endocytic pathway. Early endosomes (or endocytic vesicles) bud off from the plasma membrane during receptor-mediated [[Endocytosis|endocytosis]]. Late endosomes have an acidic internal [[PH|pH]]&nbsp;and sort proteins to lysosomes <ref>Lodish, Berk, Kaiser et al. (2007) Molecular Cell Biology, 6th edition, New York: WH Freeman. p.373</ref>.

In [[Receptor-mediated endocytosis|receptor-mediated endocytosis]], invagination of the plasma membrane causes [[Macromolecules|macromolecules]], bound to receptor proteins, to enter the [[Cell|cell]]. The invaginated membrane pinches off, forming a vesicle containing this extracellular material (which is soluble and bound to [[Receptor|receptors]]). An early endosome receives this vesicle and acts as a 'sorting station' <ref>Lodish, Berk, Kaiser et al. (2007) Molecular Cell Biology, 6th edition, New York: WH Freeman. p.373</ref>.&nbsp;It recycles some of the membrane proteins back to the plasma membrane, other membrane proteins are transported to a late endosome. Thus further sorting occurs in late endosomes. The membrane and internal contents of late endosomes are delivered to lysosomes for degradation. This step forms the end of the endocytic pathway.

The internal environment of an endosome is acidic (pH 5-6). This is maintained by an ATP-driven [[Proton pump|H+ pump]] in&nbsp;the endosomal membrane. This acidic environment is crucial to the endosome's role of sorting as it causes many receptors to release their bound cargo, thus enabling the recycling of membrane proteins <ref>Alberts, Bray, Hopkin et al. (2003) Essential Cell Biology, 2nd edition, New York: Garland Science. p.526</ref>. 

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