In this picture, you can see that exocyosis is a type of active transport in which a cell transports the molecules such as protein out of the cell by expelling them in an energy using process. Exocytosis and its counterparts, endocytosis, are used by all the cells because most chemical substances that are important to them are large polar molecules that can not pass through the hybrophobic protein of the cell membrane by passive means. In exocytosis, membrane bound secretory vesicles are carried to the cell membrane, and their contents, water soluble molecules such as proteins, are secerted into the extracellular environment. The secretion is possible because the vesicles transiently fuse with the outer cell membrane. Exocytosis is also a mechanism by which the cells are able to insert membrane protein (such as ion channels and cell surface receptors), lipids, and other components into the cell membrane Vesicles containing theses membrane components fully fuse with and become part of the outer cell membrane.
Exocytosis is the cellular process in which intracellular vesicles in the cytoplasm fuse with the plasma membrane and release their contents into the extracellular space. There are five steps in the exocytosis. the five steps are: Vesicle Trafficking, Vesicle Tethering, Vesicle Docking, Vesicle Priming, and Vesicle Fusing. Certain vesicle trafficking steps require the transportation of a vesicle over a moderately small distance. For example, vesicles that transport protein from the Golgi Apparatus to the cell surface area, will be likelt to use moto protiens and a cytoskeleton track to get closer to their target. Before tethering would have been appropriate, many of the proteins used for the active transport would have been instead set for passive transport, because the golgi apparatus does not require ATP to transport proteins. Vesicle Tethering is useful to distinguish between the initial, loose tethering of vesicle to their objective from the most stable, packing interactions. Tethering involves links over distances of more than half of the diameter of a vesicle from a given membrane surface. Tethering interactions are likely to involved in concentrating synaptic vesicles at the synapse. Tethered vesicles are alos involved in the regular cell's transportation process. Vesicle Docking are secretory transiently dock at the cell plasma membrane, preceding the formation of a tight complex, leading to the priming and the establishment of continutiy between the opposing bilayer. Vesicles Priming is usedto include all of the molecuualr erarrangments and ATP dependent protien and lipid modifications that take place after initial docking of a synaptic vesicle but before exocytosis, such as the influx of calcium ions is all that is needed to trigger nearly all release of the nutrotransmition. In other cell types there may be no priming. Vesicle Fusion is driven by SNARE protien, resulting in the release of the vesicle contents into the extracellular space. The merging of the donor and the acceptor membranes accomplishes 3 tasks. (Task 1) The surface of the plasma membrane increases (by the surface of the fused vesicle). This is important for the regulation of the cell size, during the cell growth. (Task 2) The substancses within the vesicles are released into the exterior. These might be waste products or toxic, or signaling molecules like hormones or nutrotransmeter duryng synaptic transition. (Task 3) Proteins embedden within the vesicle membrane are now part of the plasma membrane. The side of the protein that was facing the inside of the vesicle, now faces the outside of the cell.