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cells is the cell membrane, which allows cells to maintain an internal 

 milieu different from the external environment in the composition 

 and concentration of compounds. The main structural component of 

 all membranes -- with rare minor variations — is the lipid bilayer. It 

 is as fundamental a structure in cells as the DNA double helix, in that 

 it constitutes the basic permeability barrier which delimits the cells 

 and controls their interaction with the environment. 



In prebiotic experiments, some alternatives to lipid bilayers as 

 permeability barriers have been proposed, including coacervates, 

 microspheres of proteinoid, or other heterogeneous polymers, 

 micelles, and surfaces of minerals. All of these show some ability to 

 selectively concentrate or retain some of the molecules characteristic 

 of living cells. However, none of them is as efficient as lipid bilayers, 

 and in addition it is difficult to account for how these could evolve 

 into the lipid bilayer membranes of modern cells. 



In the absence of an understanding of exactly how membranes 

 or other structures became coupled to genetic mechanisms, it is best 

 to remain undogmatic about which structures were most important, 

 or about whether genetic mechanisms developed before or after the 

 structural components. 



It is useful at this point to list some properties of lipid bilayers 

 that may be relevant to the development of replicating systems on 

 the prebiotic Earth. These properties fall into subcategories that 

 include physical, chemical, and supramolecular aspects of lipid 

 organization in aqueous environments. Only one class of lipids, the 

 phospholipids (fig. V-6), is generally involved in the formation of 

 membranes. It is a striking property of most phospholipids that in an 

 aqueous environment they form stable bilayer structures that typi- 

 cally close to form vesicular membranes. Since phospholipids have 

 been synthesized under plausible prebiotic conditions and have been 

 demonstrated to form vesicles, it is assumed that lipid bilayer vesicles 

 were present on the prebiotic Earth. 



The next question concerns how such structures might contrib- 

 ute to prebiotic evolution. The chemical properties of lipid bilayers 

 include a highly charged surface and a nonpolar interior of the 

 bilayer. These two properties represent an almost totally unexplored 

 area for research in prebiotic evolution, and suggest a number of new 



