6.2: The origin of biological membranes

The hydrophobic “tails” of modern lipids range from 16 to 20 carbons in length. The earliest membranes, however, were likely to have been composed of molecules with shorter hydrophobic chains. Based on the properties of lipids, we can map out a plausible scenario for the appearance of membranes. Lipids with very short hydrophobic chains, from 2

to 4 carbons in length, dissolve in water (can you explain why?) As the lengths of the hydrophobic chains increases, the molecules begin to self-assemble into micelles. By the time the hydrophobic chains reach ~10 carbons in length, it becomes more difficult to fit the hydrocarbon chains into the interior of a micelle without making larger and larger spaces between the hydrophilic heads. Water molecules can begin to move through these spaces and interact with the hydrocarbon tails. At this point, the hydrocarbon-chain lipid molecules begin to associate with one another into semi-stable bilayers (→). One interesting feature of bilayers is that the length of the hydrocarbon chain is no longer structurally limiting, in contrast to the situation in micelles. One problem, though, are the edges of the bilayer, where the hydrocarbon region of the lipid would come in contact with water, a thermodynamically unfavorable situation. This problem is avoided by linking edges of the bilayer to one another, forming a closed balloon-like structure. Such bilayers can capture regions of solvent, that is water and the solutes dissolved within it.

Bilayer stability increases further as hydrophobic chain length increases. At the same time, membrane permeability decreases. It is a reasonable assumption that the earliest biological systems used shorter chain lipids to build their "proto-membranes" and that these membranes were relatively leaky.^[236] The appearance of more complex lipids, capable of forming more impermeable membranes, likely depended upon the appearance of mechanisms, presumably protein-based, that enabled hydrophilic molecules to pass through the membranes. The interdependence of these changes is known as co-evolution. Co-evolutionary processes were apparently common enough to make the establishment of living systems possible at least under the conditions that existed on the early Earth.

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