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An endosome is a membrane-bounded compartment or organelle. It consists of a set of connected membrane tubes and larger vesicles. (ECB)
Its key function is to sort ingested material by the process of endocytosis, which is either recycled back to the plasma membrane, or sorted to lysosomes for degradation. (ECB)
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. Late endosomes (have an acidic internal pH) sort proteins to lysosomes. (MCB)

In receptor-mediated endocytosis, invagination of the plasma membrane causes macromolecules, bound to receptor proteins, to enter the cell. The invaginated membrane pinches off, forming a vesicle containing this extracellular material (which is soluble and bound to receptors). An early endosome receives this vesicle and acts as a 'sorting station'. (MCB) It recycles some of the membrane proteins back to the plasma membrane, and 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 H+ pump in 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.

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