101 



and to provide a barrier for the electrochemical gradients produced 

 by the pigment-enzyme systems. It is reasonable to assume that pig- 

 ment molecules, formed under prebiotic conditions, would partition 

 into the nonpolar phase of lipid bilayer membranes and offer primi- 

 tive light energy-trapping functions. 



A significant physical property of the lipid bilayer is its relative 

 permeability to various ionic and molecular species. In contemporary 

 cells, the bilayer is understood to be the major barrier to free diffu- 

 sion of water-soluble substances. The special permeability properties 

 of cell membranes must be attributed to their other major constitu- 

 ent, the proteinaceous channels and enzymes that permit or drive 

 ion and chemical transport. Recently, small peptides (e.g., valinomy- 

 cin) have been shown to form specific ionic channels in a lipid 

 bilayer. 



Energy Transduction by Early Membranes 



We can now go on to discuss possible contributions of mem- 

 brane structure to evolution of energy transduction systems on the 

 prebiotic Earth. The energy necessary to generate and maintain the 

 organization of early life forms has usually been assumed to have 

 come from chemical reactions of compounds synthesized by pre- 

 biotic processes. There is, however, another source of energy, which 

 has largely been overlooked. Given the existence of lipid bilayer- 

 enclosed vesicles, concentration gradients across the lipid bilayer 

 would arise if the environment should change after the vesicles have 

 formed. Chemical reactions favored by the milieu inside the vesicles 

 would also set up concentration gradients. Concentration gradients 

 of charged solutes could generate diffusion potentials and the flux of 

 one charged species across the lipid bilayer could be coupled to the 

 flux of any other charged species. If amino acids are polymerized 

 inside a vesicle, this would give rise to an amino acid gradient, as 

 would the breakdown of the polymer. In a similar fashion, many 

 chemical reactions would change the acid/base nature of the vesicle 

 interior. These could couple chemical reactions in the vesicle interior 

 to transport processes. For example, many organic weak acids and 

 bases permeate lipid bilayers easily in their undissociated form and 

 would accumulate or be depleted inside vesicles if pH gradients 

 existed. 



