receptor-mediated endocytosis

Meaning: A cell takes in a specific ligand when it binds a receptor on the outer face of the plasma membrane. Bound receptors cluster in a pit coated on the cytoplasmic side with clathrin, which helps the membrane curve inward. The pit pinches off as a vesicle, which sheds its coat and fuses with an endosome. There the lower pH often releases the ligand, and the receptors are frequently recycled back to the surface. The process is selective and saturable, and it requires energy.

On the MCAT: The classic passage is LDL uptake. LDL particles bind LDL receptors, are internalized, and deliver cholesterol to the cell. If the receptors are defective or missing, LDL stays in the blood and plasma cholesterol rises. Expect questions on saturation (uptake levels off once the receptors are occupied, like Vmax for an enzyme), on blocking the receptor or clathrin and predicting reduced uptake, and on comparing this process with other bulk transport. Transferrin-bound iron uptake and some viral entry also appear as examples.

Common confusion: Students lump it with pinocytosis, which takes in extracellular fluid nonspecifically. Receptor-mediated uptake picks out particular ligands and concentrates them, so it is selective. It is also not facilitated diffusion: the cargo enters inside a vesicle, and the process requires energy.