Transport of small molecules across the bilayer

A lipid bilayer has to be always aetive as many molecules and ions are continuously being transported across the layer. The layer is not too thin on a molecular scale, almost 5 nm thick, which is much wider than the scale of most molecules (not proteins though). The radius of a water molecule is only 0.14 nm and that of is 0.15 nm. The thickness of the layer is small on a macroscopic-length scale and molecular details are important. That is, hydrodynamic arguments are not useful or meaningful. In fact, transport of molecules across phospholipid bilayers has given rise to a host of anomalies which are yet to be understood at a molecular level. We list a few in the following. 

Some of the above apparent anomalies ean be understood in the following fashion. If Across is the time required by a solute molecule to cross the width, then we ean define a diflhision constant for any given solute by 

We shall eall this the lipid bilayer diflhrsion (LBD) series. In the above, Dnp denotes the difiusion coefficient of a non-polar molecule of the same size as water. The above series is of importance because many of the species involved are of similar sizes. 

Water molecules and other small non-charged polar moleeules eertainly would find a lower stabilizing atmosphere than the cations but higher than the non-polar solutes of the same size. The non-polar solutes should face the lowest free-energy barrier, followed by water molecules, when they are of the same size (sueh as water and K ions). The anions should faee the largest barrier. 

Water surrounding lipid bilayers its role as a lubricant 

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