<P> In exocytosis, membrane - bound secretory vesicles are carried to the cell membrane, and their contents (i.e., water - soluble molecules) are secreted into the extracellular environment . This secretion is possible because the vesicle transiently fuses with the outer cell membrane . In the context of neurotransmission, neurotransmitters are typically released from synaptic vesicles into the synaptic cleft via exocytosis; however, neurotransmitters can also be released via reverse transport through membrane transport proteins . </P> <P> Exocytosis is also a mechanism by which cells are able to insert membrane proteins (such as ion channels and cell surface receptors), lipids, and other components into the cell membrane . Vesicles containing these membrane components fully fuse with and become part of the outer cell membrane . </P> <P> The term was proposed by De Duve in 1963 . </P> <P> In eukaryotes there are two types of exocytosis: 1) Ca triggered non-constitutive (i.e., regulated exocytosis) and 2) non-Ca triggered constitutive (i.e., non-regulated). Ca triggered non-constitutive exocytosis requires an external signal, a specific sorting signal on the vesicles, a clathrin coat, as well as an increase in intracellular calcium . Exocytosis in neuronal chemical synapses is Ca triggered and serves interneuronal signalling . Constitutive exocytosis is performed by all cells and serves the release of components of the extracellular matrix or delivery of newly synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle . </P>

Does exocytosis concentrate a specific target molecule within vesicles