Endosomes Take Up Soluble Macromolecules from the Cell Exterior

Although transport proteins in the plasma membrane mediate the movement of ions and small molecules across the lipid bilayer, proteins and some other soluble macromole-cules in the extracellular milieu are internalized by endocy-tosis. In this process, a segment of the plasma membrane invaginates into a "coated pit," whose cytosolic face is lined by a specific set of proteins including clathrin. The pit pinches from the membrane into a small membrane-bounded vesicle that contains extracellular material and is delivered to an early endosome, a sorting station of membrane-limited tubules and vesicles (Figure 5-20a, b). From this compartment, some membrane proteins are recycled back to the plasma membrane; other membrane proteins are transported to a late endosome where further sorting takes place. The endo-cytic pathway ends when a late endosome delivers its membrane and internal contents to lysosomes for degradation. The entire endocytic pathway is described in some detail in Chapter 17.

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