Exocytosis is a type of active transport in which the cell membrane fuses with secretory vesicles and expels their content (water-soluble molecules, hormones, neurotransmitters, etc.) into the extracellular space. Exocytosis can be regulated or constitutive. Regulated exocytosis occurs in response of a specific signal while constitutive exocytosis secretes molecules without the need of a signal. Ca2+ is an external signal that triggers regulated exocytosis which is particularly important in neuron synapses.
