Plasma membrane impedes passage of most substances but many molecules enter or leave at rapid rates. Facilitated transport is passive transport of specific solutes down their concentration gradient, facilitated by a carrier protein. During endocytosis, cells take in substances by vesicle formation as plasma membrane pinches off. In phagocytosis, cells engulf large particles forming an endocytic vesicle. When the endocytic vesicle fuses with a lysosome, digestion occurs.
Pinocytosis occurs when vesicles form around a liquid or very small particles.
Receptor-mediated endocytosis occurs when specific macromolecules bind to plasma membrane receptors. This allows cells to receive specific molecules and then sort them within the cell. This location is a coated pit with a layer of fibrous protein called clathrin, on cytoplasmic side. When cholesterol enters a cell, membrane and receptors are returned to the plasma membrane. The plasma membrane is outer living boundary of a cell.