= Vesicle lumen
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.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 13, The Molecular Mechanisms of Membrane Transport and the Maintenance of Compartmental Diversity: conservation of membrane orientation during budding and fusion]{ref}
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