Final answer:
Phospholipid molecules assemble to form spherical vesicles called liposomes, consisting of a phospholipid bilayer similar to cell membranes.
Step-by-step explanation:
Phospholipid molecules spontaneously assemble to form the walls of fluid-filled spherical vesicles, which are called liposomes. These structures are comprised of a phospholipid bilayer, where the amphipathic nature of phospholipids, with their hydrophilic heads and hydrophobic tails, allows them to arrange in a way that the heads face the aqueous environment and the tails avoid water, effectively creating a barrier. This structure is crucial for many physiological functions and is similar to cell membranes, which are also composed of phospholipid bilayers.