After docking, there is a secondary influx of calcium at the active zone, which causes the fusion of the vesicle to the membrane. This forms a temporary ion channel. As mentioned, this pore opens because of the binding of synaptophysin and physophilin. It requires ATP energy in order to do this. If calcium doesn’t influx, no fusion pore can happen. Synaptotagmin binds to calcium; if it does not bind to calcium, it acts like a clamp in order to inhibit the fusion of the vesicle. It activates fusion when calcium is present by pulling the vesicle closer to the presynaptic membrane. After the pore happens, it dilates quickly so that exocytosis can occur. This excess membrane made by fusion does not keep enlarging the membrane. Instead, it forms a pit that is coated with clathrin. This clathrin-coted pit eventually pinches off the vesicle again so that an empty synaptic vesicle is made again.
RECEPTOR-MEDIATED ENDOCYTOSIS Endocytosis involves the essential opposite of exocytosis. Substances are brought into the cell, forming a vesicle. There are two types of endocytosis: pinocytosis, which is also referred to as cell-drinking, and phagocytosis, which is also referred to as cell-eating. There is also receptor-mediated endocytosis, and caveolae, which will be discussed. Clathrin-mediated endocytosis is the same thing as receptor-mediated endocytosis. It makes small vesicles about 100 nanometers across, coated with clathrin. They are formed from clathrin-coated pits that start the entire process of endocytosis. There are different receptors that participate in this type of endocytosis. Caveolae are not related to clathrin. There is a cholesterol-binding protein called caveolin that participates in making small pits in the membrane. They are seen in smooth muscle cells, adipocytes, endothelial cells, type 1 pneumocytes, and fibroblasts in high concentrations. There are receptors in these caveolae that mediate their formation. Caveolae participate in potocytosis, which is the uptake of molecules that are released into the cytosol rather than into other organelles.
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