what is exocytosis

Exocytosis is a form of active transport and bulk transport in which a cell transports molecules out of the cell. It is the process by which the contents of a secretory granule are released across the plasma membrane. Exocytosis occurs via secretory portals at the cell plasma membrane called porosomes. There are three exocytosis pathways that deliver vesicles to the plasma membrane and the extracellular space. The process requires energy to transport material. Exocytosis can be constitutive (all cells) or regulated (specialized cells such as neurons, endocrine and exocrine cells) . Regulated exocytosis is usually, but not always, triggered by an increase in the cytosolic free Ca2+ concentration. Five steps are involved in exocytosis: vesicle trafficking, tethering, docking, priming, and fusion.