title: Lipid polymorphism and the roles of lipids in membranes.
abstract: The reasons for lipid diversity in membranes are not understood. Here we review evidence supporting the proposal that factors related to the polymorphic capabilities of lipids provide a rationale for lipid diversity. In particular, the ability of lipids to adopt different polymorphic phases appears to be related to a generalized shape property, where lipids with a cylindrical geometry preferentially adopt the bilayer phase whereas 'cone' shaped lipids adopt the hexagonal HII phase. Lipid diversity may then be considered to satisfy three demands. The first is obviously a need for bilayer forming lipids to provide the basic permeability barrier, whereas the second concerns a need for non-bilayer lipids and associated structures for fusion and related membrane contact phenomena to proceed. A third, and less obvious demand satisfied by nonbilayer lipids concerns the ability of lipids of different shapes to modulate the order in the hydrocarbon region when constrained to a bilayer organization. These possibilities are summarized in a metamorphic mosaic model of membranes.
mesh_terms: D005614:Freeze Fracturing; D008051:Lipid Bilayers; D008081:Liposomes; D008854:Microscopy, Electron; D008954:Models, Biological; D008958:Models, Molecular; D008968:Molecular Conformation; D013329:Structure-Activity Relationship
chemical_list: D008051:Lipid Bilayers; D008081:Liposomes