Comparative Geometry of Cytomembranes and Water-Lipid Systems

Cells studied in electron microscopy show in their fine structure a series of definite morphological patterns : equidistant parallel membranes, hexagonal arrays of tubes, cubic lattices etc. All these structures come from the folding and from the topological rehandling of bilayered fluid membranes. Phospholipids are essential component of membranes and are at the origin of the bilayered and fluid structure of the cell membrane. These phospholipids, when purified and in presence of water and oily components, form liquid crystalline phases, with similar geometrical structures (equidistant parallel surfaces, hexagonally packed cylinders, cubic arrays etc.) but there are several differences separating them from their biological counterpart. The proportion of water is much higher in cell membrane systems. The distribution of oily components often has a distinct topology. The periodicities observed in these patterns are reduced by a factor of about ten, when prepared in vitro, relative to those observed in cells. It is shown that these differences originate from the asymmetry of cell membranes and that several factors may be involved in saddle-shaped membranes.