Final answer:
During receptor-mediated endocytosis, receptors trapped in an indented pit that is coated on the cytoplasmic side with clathrin form vesicles to internalize substances into the cell.
Step-by-step explanation:
During receptor-mediated endocytosis, the portion of the receptor molecule that lies inside the plasma membrane is trapped in an indented pit that is coated on the cytoplasmic side with a protein called clathrin. This specialized endocytosis process relies on the affinity of receptors for specific extracellular substances.
Upon binding their ligands, the receptors aggregate in the differentiated regions of the cell membrane known as coated pits. These coated pits then invaginate and pinch off, forming a coated vesicle which brings the extracellular contents into the cell. After delivering their contents to the necessary cellular destinations, the vesicle membranes are recycled back to the plasma membrane. Clathrin is the main protein involved on the surface of the invaginated coated pit and is important for the formation of the clathrin-coated vesicles that characterizes this process.