Final answer:
Lipids self-assemble in water because of their amphipathic properties, forming structures like micelles and bilayers to shield their hydrophobic parts from water, which leads to the creation of barriers like cell membranes.
Step-by-step explanation:
Lipids self-assemble in water due to their amphipathic nature; this means that they have regions that are both hydrophobic (water-fearing) and hydrophilic (water-loving). In aqueous environments, lipid molecules spontaneously orient themselves into structures like micelles, bilayers, and liposomes, creating a barrier that separates the water and other substances on either side. Micelles form with hydrophobic tails in the center, shielding them from water, while the hydrophilic heads face the external aqueous environment. Similarly, lipid bilayers arrange themselves with hydrophilic heads at both the exterior surfaces and hydrophobic tails sequestered inside, which is the foundational structure of cell membranes ensuring energetic stability and separation of cellular contents from the external environment.