micelle

Meaning: A micelle is a spherical aggregate of amphipathic molecules in water. The polar or charged heads face the solvent and the nonpolar tails cluster in the interior. Micelles form above a critical micelle concentration. The driving force is the hydrophobic effect: burying the tails releases ordered water molecules, so the entropy of the water increases. Single-tailed molecules such as soaps (fatty acid salts), detergents and bile salts typically form micelles, while bulkier two-tailed phospholipids usually form bilayers or liposomes.

On the MCAT: Soap or detergent passages: grease dissolves in the hydrophobic core while the charged surface keeps the particle dispersed in water. Digestion passages: bile salts emulsify dietary fat, and mixed micelles carry fatty acids and monoglycerides to the intestinal lining for absorption. You may also be asked which part faces outward, or what happens in a nonpolar solvent, where a reverse micelle puts the heads inside.

Common confusion: Micelle versus bilayer. A micelle is a single-layer sphere with a hydrophobic core and no water-filled interior; a liposome is a bilayer enclosing an aqueous space. Tail number and shape usually decide which forms: one tail favors a micelle, two tails favor a bilayer.