Exocytosis is the process by which a vesicle moves from the cytoplasm to the plasma membrane, where it discharges its contents to the extracellular space

A variety of molecules produced by the cell for export are initially delivered from the site of their formation to the Golgi apparatus. The next step involves sorting and packaging the secretory product into transport vesicles that are destined to fuse with the plasma membrane in a process known as exocytosis. The molecules that travel this route are often chemically modified (e.g., glycosylated, sulfated) as they pass through different cellular compartments. The membrane that is added to the plasma membrane by exocytosis is recovered into the cytoplasmic compartment by an endocytotic process. There are two general pathways of exocytosis:

• In the constitutive pathway, substances designated for export are continuously delivered in transport vesicles to the plasma membrane. Proteins that leave the cell by this process are secreted immediately after their synthesis and exit from the Golgi apparatus, as seen in the secretion of immunoglobulins by plasma cells and of tropocollagen by fibroblasts. This pathway is present to some degree in all cells. The TEM reveals that these cells lack secretory granules.

• In the regidated secretory pathway, specialized cells, such as endocrine and exocrine cells and neurons, concentrate secretory proteins and transiently store them in secretory vesicles within the cytoplasm (Fig. 2.12). In this case, a regulatory event (hormonal or neural stimulus) must be activated for secretion to occur, as in the release of zymogen granules by chief cells of the gastric mucosa and by acinar cells of the pancreas. The signaling stimulus causes a transient influx of Ca2+ into the cytoplasm, which in turn stimulates secretory vesicles to fuse with the plasma membrane and discharge their contents (Fig. 2.13).

In addition to excretory pathways, proteins can be transported between the Golgi apparatus and other organelles along endosomal pathways. These pathways are used for delivery of organelle-specific proteins, such as lysosomal structural proteins, into the appropriate organelles.

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