Biological Membranes: The Structure Of Biological Membranes

Membranes play an integral function in trapping and securing metabolic products within the borders of a cell within an aqueous environment. Without a selectively permeable border surrounding sites of anabolic function, potential useful products of this metabolism would simply diffuse away in the aqueous environment contained within and surrounding the cell. However, securing metabolites within the cell also comes with a price of not being able to acquire potentially useful compounds from the surrounding environment. Some very small gases and polar uncharged compounds are able to simply diffuse across this membrane, moving to the site of lower concentration on either side of the membrane. However, larger uncharged and charged polar molecules,

The propensity of these hydrophobic tails to self-associate to exclude interactions with water, via the energy provided by Van der Waals forces and preferential hydrogen bonding between hydrophobic tails, and the propensity of the hydrophilic heads to interact with the aqueous environment are the basis for the thermodynamic stability and self-assembly of the bilayers of biological membranes. However, many of these bilayers are asymmetrical in nature, with the inner leaflet having fewer amphipathic molecules or a different lipid composition than the outer leaflet, attributing the circular curvature of most biological membranes. For the lipid component of these membranes, phospholipids, with varying head groups such as phosphatidylcholine and phosphatidylserine, are the primary component of these bilayers, with sphingolipids, such as sphingomyelin, and sterols, such as cholesterol, typically found in lesser concentrations, but both still play an integral role in biological membranes (Nicolson, 2014). Small gaseous molecules, like O2, hydrophobic molecules, like benzene, and small polar but uncharged molecules, like ethanol, are essentially able to dissolve in the phospholipid bilayer and thus cross the diffuse across the cell

Primordial cells would have had a similar concept and function to this compartmentalization, though perhaps not utilizing the same components as current cells. Their membrane would have most likely consisted of amphiphilic molecules like fatty acids or possibly polyprenyl phosphates, similar to modern day archaea. However, having a pure lipid bilayer would result in inadequate exchange of charged ions and large polar molecule between the environment and the cell, especially without the use of transporter proteins. Propositions have been proposed that cell membranes and membrane proteins have co-evolved, in that, cell membranes have moved from porous to ion-tight, just as membrane proteins have moved from amphiphilic pore forming proteins to very hydrophobic integral membrane proteins. A proposed schematic of this membrane-protein co-development involving the increasing complexity of F and V-type ATPases and sodium ion transporters with membranes porous to both protons and sodium ions, becoming more ion-tight can be observed in figure 1-5 (Mulkidjanian et al.