Practice question

Aqueous compartments surround cells, so membrane lipids must present hydrophilic surface to water while hiding hydrophobic interior away from solvent. Phospholipid headgroups such as phosphocholine, phosphoethanolamine, phosphoserine, and phosphoinositol are zwitterionic or anionic, heavily hydrated, forming hydrogen bonds with water and creating interfacial region of high polarity and dielectric constant. This polar curtain, two molecules thick around bilayer perimeter, repels hydrophobic compounds and prevents uncontrolled leakage of cellular metabolites, establishing hydrophilic barrier essential for compartmentalization and selective permeability. Hydrophobic effect drives spontaneous bilayer assembly: burying acyl chains minimizes ordered water cages surrounding hydrocarbons, increasing entropy and lowering free energy. Van der Waals interactions provide additional stabilization within core. Cholesterol's amphipathic nature contributes modestly, but protein channels actually provide aqueous pathways that bypass hydrophobic core for ions. Fatty acid tails form continuous hydrocarbon interior that is barrier to polar solutes. Energetic cost of exposing acyl chains explains spontaneous vesicle closure and rapid self-healing after injury.