In biology, an endosome's a membrane-bound compartment inside cells, roughly 500 nm in diameter when fully mature. Function Many endocytotic vesicles, derived from the plasma membrane, are either transported to a pre-existing endosome and fuse with it or are acidified via proton pump to become an endosome. Some endocytosed material passes through endosomes on its way to lysosomes. Endosomes are, in part, responsible for the sorting of endocytosed material before transport to lysosomes. This allo… (More on Endosomes)