Question

At physiological pH, cell membranes are much more permeable to ethanol (C₂H₅OH) than to acetic acid (C₂H₄O₂). A) Why is this so? B) Why does the permeability to one of these (which one?) increase if the pH is dropped?

Answer 1

Ans is here because simulations show that ethanol is able to pass through the bilayer much more easily than acetic acid. This can be explained by the hydrophobic nature of the carbon tail of ethanol, making passing through the hydrophobic chain regions of lipid bilayers easier.

If we drop the pH there will be incresed permeability of acetic acid because Acetic acid is transported across the membrane from the feed phase in its unionized molecular form and not in its ionic form that is acetate. This required that the pH of the feed phase is lower than the pKa value of acetic acid which is 4.76. So when pH will be dropped to less than 4.76 it will lead to increased peremeability and Ethanol does not have a pH. As pH by its very definition is the log of the hydrogen ion concentration in aqueous solution.