Schematic diagram of the core endocytic and exocytic pathways in mammalian cells. Red arrows indicate the inward endocytic pathway that, for example, internalizes ligand-bound receptors and delivers them to lysosomes for breakdown. Black arrows indicate the outward pathway that delivers membrane and proteins from the endoplasmic reticulum (ER) to the plasma membrane or to the extracellular environment (by means of secretory vesicles). Fluorescently labeled ligands enable the fate of the internalized receptor-bound ligand to be tracked in the cell. In the case illustrated here, the EGF-receptor complex is directed to the lysosomes for breakdown, which is part of the mechanism for downregulating the signal, while the transferrin-receptor complex sheds its iron in an early endosome and is then recycled to the cell surface via recycling endosomes to capture more iron from the extracellular environment.