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endocytic cycle
Most animal cells take up 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 — a 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 whole cycle of endocytosis plus exocytosis is known as the endocytic cycle.
The endocytic cycle is crucial for the survival of individual cells and multicellular organisms.
==LDL metabolism==
Low-density lipoprotein (LDL) originates in the liver and is transported around an animal 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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