exocytosis
/ EK-soh-sy-TOH-sis /
Just as a cell can swallow big things by wrapping them in membrane, it can also spit big things out the same way in reverse. When a cell needs to release something too large to slip through a protein door — a digestive enzyme, a hormone, a neurotransmitter, a building block for its wall — it packages the cargo into a little membrane bubble, floats that bubble to the surface, and merges it with the outer membrane so the contents spill outside. This is exocytosis.
Mechanically, exocytosis is endocytosis run backward. A vesicle (a small membrane sac) carrying cargo drifts to the plasma membrane, the two membranes touch and fuse, and the vesicle turns inside-out, dumping its load into the outside world. A neat side effect: the vesicle's membrane becomes part of the cell surface, which is how a cell adds new membrane and how it balances the membrane it loses during endocytosis. Like endocytosis, it needs energy and is tightly controlled.
Exocytosis is how cells talk, build, and defend. Nerve cells release their chemical messengers across a synapse by exocytosis; gland cells secrete insulin, mucus, tears, and milk this way; cells export the proteins they make for use outside, and add fresh material to their own surface. Many of these releases are triggered on cue — a rise in calcium inside the cell is the usual signal that says now, release.
When one nerve cell signals the next, a vesicle full of messenger chemicals fuses with the cell membrane and empties into the gap — exocytosis turning an electrical pulse into a chemical message.
A vesicle fuses with the surface and spills its cargo outside.
Exocytosis does more than secrete cargo: each fusing vesicle also hands its membrane to the cell surface, which is how cells grow their membrane and offset what endocytosis takes away.