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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]

Transfer of cargo in membrane-bounded vesicles. Budding selects material and produces a carrier, while targeting and vesicle fusion deliver it to another compartment, allowing transport without exposing the cargo directly to the surrounding cytoplasm.

A v-SNARE is a SNARE protein assigned to a transport vesicle in a specified fusion event. Complementary interaction with a t-SNARE on the target produces a membrane-bridging SNARE complex. Synaptobrevin is a familiar example in neuronal secretion. The label describes which membrane initially carries the component; after membrane fusion, both sets of components occupy the same membrane and must be reorganized for reuse. ^[ref]

Wolfgang Helfrich developed a continuum description of how lipid bilayers store elastic energy when bent. His 1973 theory distinguishes curvature, stretching and tilt, and relates the shapes of closed vesicles to curvature elasticity. In modern notation its bending contribution iswhere $H$ is mean curvature, $K$ is Gaussian curvature, $C_0$ is the preferred sum of principal curvatures associated with spontaneous curvature, and $\kappa ,\bar{\kappa }$ are elastic coefficients. The model explains how a flexible membrane can prefer a shape even without a rigid supporting skeleton.^[ref]