ABSTRACT
Lipid polymorphism is one area of membrane research which depends most on freeze-fracture electron microscopy. All biological lipids can be grouped in one of the two main structural forms: those which form bilayers and those which do not. X-ray diffraction and nuclear magnetic resonance spectroscopy can be used effectively to distinguish between these two groups, however, neither technique can distinguish some structural differences within each group. The common nonbilayer phases formed by lipids in aqueous dispersions are micelles, inverted micelles, inverted hexagonal or hexagonal II, and cubic. Micelles are generally too small to be seen by freeze-fracture electron microscopic techniques. Inverted micelles are larger due to the water pocket enclosed by the lipid and can be seen by electron microscopy. Freeze-fracture replicas of intact membranes of chloroplast and retinal rod outer segment disks however do not show the presence of any nonbilayer lipid phases.
