Endocytosis and exocytosis are two major forms of vesicular transport. Endocytosis brings molecules and other substances into the cell. In exocytosis, synthesized molecules and other substances leave the cell. Endocytosis is associated with the formation and budding of vesicles from the plasma membrane; exocytosis is associated with the fusion of vesicles originating from intracellular organelles with the plasma membrane and is a primary secretory modality.
formed by virtually every cell in the organism, and it is constitutive; i.e., it involves a continuous dynamic formation of small vesicles at the cell surface (Fig. 2.9a). Recent studies indicate that mechanoenzymes such as GTPase (dynamin) are involved in pinocytotic vesicle scission (the process of pinching off from the plasma membrane). Pinocytotic vesicles are visible with the TEM, and they have a smooth surface. These smooth pinocytotic vesicles are especially numerous in the endothelium of blood vessels (Fig. 2.9b) and in smooth muscle cells. Pinocytosis does not require clathrin and therefore may be referred to as clathrin-inclependent endocytosis.
• Receptor-mediated endocytosis allows entry of specific molecules into the cell. In this mechanism, receptors for specific molecules, called cargo receptors, accumulate in
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