Endocytic cycle

The whole cycle of endocytosis plus exocytosis is known as the endocytic cycle. Most animal cells engage portions of their surface plasma membranes in a process called endocytosis. The main route of endocytosis is the coated pit, which buds into a cell to form a cytoplasmic vesicle—clathrin-coated vesicle. The membrane so internalised is processed in a series of intracellular organelles, which include endosomes and lysosomes. Some of this membrane is returned to the cell surface by a process called exocytosis.

The endocytic cycle is crucial for the survival of individual cells and multicellular organisms.

Low-density lipoprotein (LDL) originates in the liver and is transported around the body by the blood stream. From there it is taken up by other cells, such as fibroblasts, and degraded: this provides a source of cholesterol for the growth of these other cells. LDL in the blood binds to LDL receptors on the surface of fibroblasts; these receptors concentrate in coated pits (they are about 200x as concentrated here as along the rest of the cell’s plasma membrane) and are internalised when the pit becomes a coated vesicle. The itinerary of the LDL receptor inside the cell is complex, but it spends little time there. Within a fraction of a minute, it has released its LDL cargo and is returned to the cell surface by exocytosis. It is now ready for another round of LDL clearance.

High levels of LDL in the blood are observed in atherosclerosis and associated with the disease; the endocytic cycle reduces LDL through consuming it. This may or may not be useful in regulating increased levels of LDL, although it may have limitations, or not increase at all in the presence of extra LDL molecules.

Transferrin is a plasma protein that is able to combine with iron ions: It is the vehicle with which iron is carried around the body. Free ferric ions are toxic; but cells need iron to build many of their proteins including and hemoglobin. Ferric ions are carried in the blood tightly bound to transferrin as ferritransferrin. Dividing cells, which need the iron, gain it by binding the ferritransferrin to transferrin receptors on their surfaces. These receptors also have a high affinity for coated pits. Like the LDL receptor, the transferrin receptor is internalised into a coated vesicle. The iron is released inside the cell and the receptor returned to the cell surface. The route this receptor takes inside the cell appears to be different from that taken by the LDL receptor, because it takes about 10 minutes before it is exocytosed.

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