An amphipathic molecule contains both a water-compatible polar or charged region and a nonpolar region. Because the two parts interact differently with water, many amphipathic molecules organize spontaneously into larger structures.

Consider a molecule with a polar head group and a nonpolar hydrocarbon tail. Dispersed individually in water, the polar head can interact favorably with the solvent while the tail exposes nonpolar surface to water. Aggregation can reduce that exposed nonpolar area while keeping the polar groups in contact with water.

Two common self-assembled structures are:

• a micelle, in which tails point inward and polar heads face the surrounding water;
• a bilayer, in which two sheets of molecules arrange tail-to-tail, leaving polar heads exposed to water on both sides.

The preferred structure depends on molecular geometry and concentration. Molecules with one relatively bulky head and one tail often form micelles readily, whereas phospholipids with two hydrocarbon tails commonly favor bilayers.

Self-assembly is dynamic

A bilayer or micelle is not a rigid covalent object. Individual molecules are held together mainly by the hydrophobic effect together with van der Waals and electrostatic interactions. Molecules can diffuse laterally, exchange with solution and reorganize while the overall assembly persists.

This distinction is important: self-assembly means that the organized structure is thermodynamically favored under the stated conditions without requiring a covalent bond between every neighboring component.

Why closed compartments can form

An exposed edge of a bilayer would leave hydrophobic tails in contact with water. Bending and closing the sheet can eliminate much of that unfavorable edge. Bilayers can therefore form sealed vesicles that separate one aqueous region from another.

This physical principle is the foundation of biological membranes. The full behavior of a cell membrane also depends on proteins, sterols, carbohydrates, ion gradients and active processes, but the ability to create an aqueous compartment begins with amphipathic molecular self-assembly.