Two chemical regions, one organized structure
An amphipathic molecule has both a water-interacting region and a hydrophobic region. In water, many membrane phospholipids assemble with their polar heads exposed to water and their nonpolar tails sheltered together.
A bilayer provides water-facing surfaces on both sides while burying the tails in its interior. This arrangement follows the energetics of interactions with water; it does not require neighboring molecules to become one covalent polymer.
Example: a phospholipid vesicle
Consider a small bilayer sheet suspended in water. Its exposed edges put hydrophobic tails in contact with water. Closing into a vesicle can eliminate those edges: heads face the surrounding water and the water-filled interior, while tails remain between the two surfaces.
The resulting compartment can separate solutions, but a bare bilayer is not equally permeable to every solute. Small nonpolar molecules cross more readily than ions, which generally need transport proteins to cross rapidly.
Predict the structure carefully
Amphipathic does not automatically mean bilayer-forming. Molecular geometry and conditions matter; some amphipathic molecules favor micelles. First identify the polar and nonpolar regions, then consider how they can pack while limiting tail exposure to water.
Related question
Apply this knowledge
Use the concept guide to understand the reasoning, then return to the complete question and worked answer.
What Are the Monomers of Lipids?Sources
These references support the core concepts and interpretation boundaries explained above.