Receptor-mediated endocytosis involves a mechanism by which cells eat and digest large molecules. Certain types of molecules combine with receptor proteins that are embedded inside of the plasma membrane. After being enclosed inside, the plasma membrane forms a vesicle around the molecule, which is internalized by the cell. Another name given to this process is clathrin-dependent endocytosis.
Molecules that bind to receptors are called ligands. Receptors are located within pits of a protein-coated area of the plasma membrane. As the ligand is attached to the membrane, it is enclosed, and the coating detaches. The vesicle is then called an endosome, which further divides into two separate vesicles.
One vesicle contains receptors, the other holds the molecule or particle. Returning to the plasma membrane, receptors are recycled while the remaining vesicle binds with a lysosome. Contents are broken down by the lysosome and incorporated into the cytosol of the cell.
Low-density lipoproteins (LDL) are transported into cells by receptor-mediated endocytosis. Active in transporting lipids from the liver to other areas of the body, LDL makes it possible to move cholesterol and other lipids in the water-based blood and lymph. Otherwise, clumps of fat would float and disturb normal blood function.
Doctors commonly order a laboratory test referred to as the blood lipid profile, which measures the concentration of fats, cholesterol and lipoproteins responsible for carrying them. High blood cholesterol is a risk factor for cardiovascular disease. Even though it is required by cells, cholesterol that remains in the bloodstream can lead to plaque formation on arterial walls.
Iron is carried by the glycoprotein transferrin via receptor-mediated endocytosis. Manufactured and stored in the liver, transferrin plays a key role in iron transport from the small intestine, where it is absorbed. Transferrin receptors stay attached to the ligand in this case, which is slightly different from LDL metabolism.
In addition to receptor-mediated endocytosis, several other types of endocytotic mechanisms function within biological organisms. These mechanisms include pinocytosis and phagocytosis. Phagocytosis, which literally means "cell eating," is the process during which the cell ingests particles such as bacteria and food. It is used by protists and white blood cells.
With phagocytosis, a particle attached to the cell's surface is enclosed within folds of the plasma membrane. Folds form a vacuole, a type of membranous sac. Digesting the particle, a lysosome combines with the vacuole.
Pinocytosis literally means "cell drinking." Tiny droplets of fluid are taken into the cell. Droplets become trapped within pits of the plasma membrane, forming small vesicles that resemble tiny, individual bubbles. Fluid inside of vesicles are dissolved into the cytosol.