ENDOCYTOSIS

Another process by which substances move across the plasma membrane is endocytosis. Endocytosis (Gr. endon, within) involves bulk movement of materials across the plasma membrane, rather than movement of individual molecules. The three forms of endocytosis are pinocytosis, phagocytosis, and receptor-mediated endocytosis. Pinocytosis (“cell drinking,” from Gr. pinein, to drink cyto, cell) is nonspecific uptake of small droplets of extracellular fluid. Any small solid dissolved in the fluid is also taken into the cell. Pinocytosis occurs when a small portion of the plasma membrane indents (invaginates). The open end of the invagination seals itself off, forming a small vesicle. This tiny vesicle detaches from the plasma membrane and moves into the cytoplasm. Phagocytosis (“cell eating,” from Gr. phagein, to eat cyto, cell) is similar to pinocytosis, except that the cell takes in solid material rather than liquid. Commonly, an organelle called a lysosome combines with the vesicle to form a phagolysosome (“digestion vacuole”), and lysosomal digestive enzymes break down the vesicle’s contents.
Receptor-mediated endocytosis involves a specific receptor protein on the plasma membrane that “recognizes” an extracellular molecule and binds with it. This reaction somehow stimulates the membrane to indent and create a vesicle containing the selected molecule. A variety of important molecules (such as cholesterol) are brought into cells in this manner.