There are two basic mechanisms for the uptake of substances into the cell. Small molecules such as amino acids, mono-saccharides, and most ions are transported directly across the plasma membrane into the cytoplasm. Macromolecules like peptides and proteins are taken up by theprocess of recep-tor-mediated endocytosis (Goldstein et al., 1985); they are specifically recognized and bound with high affinity to re-ceptors on the cell surface. These are then clustered into clathrin-coated patches of the plasma membrane called coated pits, which invaginate and pinch off as coated vesicles. The cytoplasmic clathrin coat is rapidly removed, and the vesicles fuse with an acidic endosomal compartment. Although inside the cell, the ligands are still separated from the cytoplasm by a membrane. In most cases, they are delivered to lysosomes and hydrolyzed before their degradationproducts are released into the cytoplasm.

The endosome is a pivotal sorting compartment: distinct vesicles depart from this compartment either toward prely-sosomes and lysosomes (the degradative pathway) or back toward the plasma membrane (the recycling pathway). In polarized cells, which have separate apical and basolateral plasma membrane domains, there are recycling routes from endosomes toeither one of them and transcytotic routes to the