Mica Piro Chapter 7: Membrane Structure & Function Selective permeability: the plasma membrane allows some substances to cross it more easily than others Cellular Membranes are Fluid Mosaics of Lipids and Proteins (7.1) The ability of phospholipids to form membranes is inherent in their molecular structure?a phospholipid is an amphipathic molecule (has both a hydrophilic region and a hydrophobic region) A phospholipid bilayer can exist as a stable boundary between 2 aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to the water Most membrane proteins are amphipathic