An artificial liposome built purely from phospholipids, containing no proteins, is bathed in four solutes at equal concentration. Which solute enters the liposome fastest by crossing directly through the bilayer?

Core Concept

Small nonpolar molecules like O2 dissolve into the hydrophobic core formed by fatty acid tails and slip across unassisted. Charged species must shed their hydration shells to enter that low-dielectric interior, an energetically punishing step, and large polar solutes face the same desolvation cost plus size barriers. Protein-free permeability therefore falls off sharply from small gases toward ions and macromolecules.