Permeation of protons, potassium ions, and small polar molecules through phospholipid bilayers as a function of membrane thickness - PMC Biophys J . 1996 Jan;70(1):339–348. doi: 10.1016/S0006-3495(96)79575-9 # Permeation of protons, potassium ions, and small polar molecules through phospholipid bilayers as a function of membrane thickness. 1, A G Volkov 1, A N Van Hoek 1, T H Haines 1, D W Deamer 1 - Author information - Copyright and License information 1Department of Chemistry and Biochemistry, University of California, Santa Cruz 95064, USA. stefan@chemistry.ucsc.edu PMCID: PMC1224932 PMID: 8770210 ## Abstract Two mechanisms have been proposed to account for solute permeation of lipid bilayers. Partitioning into the hydrophobic phase of the bilayer, followed by diffusion, is accepted by many for the permeation of water and other small neutral solutes, but transient pores have also been proposed to account for both water and ionic solute permeation. These two mechanisms make distinctively different predictions about the permeability coefficient as a function of bilayer thickness. Whereas the solubility-diffusion mechanism predicts only a modest variation related to b

s can cross the plasma membrane is passive diffusion . During passive diffusion , a molecule simply dissolves in the phospholipid bilayer , diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane. No membrane proteins are involved and the direction of transport is determined simply by the relative concentrations of the molecule inside and outside of the cell. The net flow of molecules is always down their concentration gradient—from a compartment with a high concentration to one with a lower concentration of the molecule. Passive diffusion is thus a nonselective process by which any molecule able to dissolve in the phospholipid bilayer is able to cross the plasma membrane and equilibrate between the inside and outside of the cell. Importantly, only small, relatively hydrophobic molecules are able to diffuse across a phospholipid bilayer at significant rates ( Figure 12.15 ). Thus, gases (such as O 2 and CO 2 ), hydrophobic molecules (such as benzene), and small polar but uncharged molecules (such as H 2 O and ethanol) are able to diffuse across the plasma membrane. Other biological molecules, however, are unable to dissolve in the hydrophobic interior of the phospholipid bilayer. Consequently, larger uncharged polar molecules such as glucose are unable to cross the plasma membrane by passive diffusion , as are charged molecules of any size (including small ions such as H + , Na + , K + , and Cl - ). The passage of these molecules across the membrane instead requires the activity of specific transport and channel proteins , which therefore control the traffic of most biological molecules into and out of the cell. Figure 12.15 Permeability of phospholipid bilayers. Gases, hydrophobic molecules, and small polar uncharged molecules can diffuse through phospholipid bilayers. Larger polar molecules and charged molecules cannot. Facilitated Diffusion and Carrier Proteins Facilitated diffusion , like passive diffusion , invol