Periodic minimal surface organizations of the lipid bilayer at the lung surface and in cubic cytomembrane assemblies.

The existence of infinite periodic lipid bilayer structures in biological systems was first demonstrated in cell membrane assemblies. Such periodicity is only possible in symmetric bilayers, and their occurrence is discussed here in relation to the asymmetry of cell membranes in vivo. A periodic membrane conformation in the prolamellar body of plants corresponds to a dormant state without photosynthesis. A similar reversible formation of a dormant state has also been observed in the mitochondria of the amoeba Chaos. In these cases the energy production has become insufficient to maintain the membrane asymmetry. Formation of membranes that are symmetric over the bilayer is proposed to be a principal mechanism behind formation of cubic membrane systems. Another type of bicontinuous minimal surface structure is considered to form the alveolar lining of mammals at normal breathing conditions. The CLP surface corresponds to such a tetragonal surface phase. It is also a symmetric bilayer and in a state of zero energy expenditure. Structural alternatives of the bilayer conformation in this latter system are also discussed here.