vesicle
/ VESS-ih-kul /
Inside a cell, things constantly need to get from one place to another — a freshly made protein from the factory to the export dock, a swallowed nutrient to the recycling center. The cell cannot just let these float loose in its crowded interior. Instead it ships them in tiny sealed bubbles, like little delivery parcels made of membrane. Each such bubble is a vesicle.
A vesicle is a small sac wrapped in the same kind of phospholipid membrane as the cell's outer skin, holding cargo safely sealed inside. Because membranes can pinch off from one place and fuse into another, vesicles are the cell's mail system: a vesicle buds off a membrane here, drifts (often hauled along by motor proteins on tracks), and fuses with a membrane there, delivering its contents without ever spilling them into the open cell. The same bubble can carry proteins, fats, waste, or signaling chemicals.
Vesicles tie the whole transport story together. Endocytosis brings cargo in inside an inward-budding vesicle; exocytosis sends cargo out by fusing a vesicle with the surface; and a constant inner traffic of vesicles moves goods between the cell's compartments. Synaptic vesicles store the chemicals a nerve releases; lysosomes and many organelles are vesicle-like sacs. Whenever something travels through a cell wrapped in its own little membrane bag, a vesicle is doing the carrying.
A protein bound for export leaves the cell's packaging center inside a vesicle, which then rafts to the surface and fuses with it, releasing the protein outside — a parcel handed off from one membrane to another.
A vesicle is a membrane parcel that carries cargo safely across the cell.
A vesicle is wrapped in the same phospholipid membrane as the cell surface, which is exactly why it can seamlessly bud off one membrane and fuse into another — they speak the same material language.