Turning Membranes into Machines
ENCAPSULATION was a slow, progressive process, punctuated by many evolutionary acquisitions. By necessity, the earliest of these acquisitions concerned mostly means of ensuring vital exchanges with the environment. Soon, however, the scope widened. Once phospholipid bilayers were formed, this new fabric turned out to be much more than a convenient boundary. It presented burgeoning life with numerous opportunities for useful innovation. A whole new class of proteins emerged, fitted with one or more hydrophobic sequences that allowed insertion within membranes. Thus immobilized, the proteins could participate in a variety of novel functions that were sufficiently advantageous to favor the evolutionary selection of the mutant protocells that made the proteins. By far the most important development of this sort was the putting together of a machinery coupling downhill electron transfer reversibly to proton extrusion. Emergence of this machinery was a truly revolutionary advance in the ability of life to derive energy from environmental sources.
Imagine the following scenario. It may not have happened as depicted, but the scenario is plausible and tells in a simple fashion how emerging life may have hit upon the invention that completely transformed its future--made this future possible, in fact.
Owing to some mutational event, a protocell acquires an electron-carrying molecule constructed so as to fit within the fabric of the protocell's membrane. What makes this carrier useful, and favors its selection, is that it can serve as a bridge for electrons across the membrane, between an internal donor and an outside acceptor