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A vesicle is a small compartment enclosed by a lipid bilayer. Its vesicle lumen can carry soluble cargo, while its lipid bilayer carries lipids and membrane proteins. In a transport pathway, budding separates a carrier from one compartment and membrane fusion joins it to another. Orientation matters: fusion of a secretory vesicle with the cell membrane connects its vesicle lumen to the extracellular space while incorporating its membrane proteins into the surface. ^[ref]

A vesicle is therefore more than a freely diffusing package. Recognition, movement and membrane fusion require distinct interactions. In a reconstitution experiment, complementary SNARE proteins placed in separate liposomes brought opposing lipid bilayers together and promoted fusion; the experiment isolated one part of transport from the rest of a living cell. ^[ref]

The vesicle lumen is the enclosed interior of a vesicle, separated from the cytosol by its lipid bilayer. It can contain dissolved cargo, ions and portions of membrane proteins facing inward. In a synaptic vesicle, for example, the neurotransmitter is stored in the vesicle lumen before exocytosis.

Membrane topology makes this interior useful to track. When a vesicle buds from the endoplasmic reticulum, its vesicle lumen derives from the interior of the endoplasmic reticulum; fusion with the Golgi apparatus delivers the cargo into the next lumen without exposing it to the cytosol. When a secretory vesicle undergoes exocytosis, the vesicle lumen opens to the extracellular space. Thus an enclosed compartment can transport material through a biological cell while keeping it on the same side of a cell membrane throughout.^[ref]