Exocytosis: Membrane proteins are fine for channeling the movement of ions and small molecules, but for transporting large molecules, a different strategy is required. When cells need to send large molecules (like proteins) outside their plasma membrane borders, they turn to exocytosis. Sizable cargo is loaded into spherical membrane vesicles. These vesicles move toward the plasma membrane and fuse with it, exposing the vesicle interior to the outside of the cell and releasing its contents.
Endocytosis: Sometimes cells have cause to import large molecules. For this challenging task, the solution is endocytosis, which is essentially exocytosis in reverse. Molecules to be imported contact the exterior surface of the plasma membrane, triggering the membrane to fold inward, enveloping them. The infolded membrane pinches off into a vesicle containing the imported molecules, which can be further transported to their eventual destination within the cell.