<P> Exocytosis (/ ˌɛksoʊsaɪˈtoʊsɪs /) is a form of active transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell (exo - + cytosis) by expelling them through an energy - dependent process . Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic portion of the cell membrane by passive means . </P> <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>

Packages protein for transport out of the cell