Which of these molecules is most likely to pass through a pure lipid bilayer?

Question

BlackTom AI · Accepted Answer

To determine which molecule would most readily cross a pure lipid bilayer, we should consider two main factors: lipophilicity (ability to dissolve in the lipid core) and ionization state at physiological conditions. 
Option 1: Aspirin. Aspirin is acetylsalicylic acid, which is a relatively small molecule with a lipophilic acetyl group that reduces its overall polarity compared to its acidic form. In its uncharged (neutral) form, aspirin is more lipophilic and thus more likely to diffuse through a lipid bilayer than charged species. 
Option 2: Salicylic acid. Salicylic acid itself has a carboxyl group that is deprotonated at physiological pH, rendering it negatively charged and highly polar. The charged state dramatically lowers its ability to partition into and traverse the hydrophobic core of a lipid bilayer. Therefore, salicylic acid would cross the membrane less readily than a neutral form. 
Option 3: Acetylsalicylate. This is the deprotonated conjugate base form (the acetylated ester can still lose a proton from the phenolic or other acidic sites, but in general the species present in physiological conditions tends to be charged), making it even less capable of crossing the lipid bilayer than the neutral aspirin. Its charged nature reduces permeability through the membrane. 
Option 4: More that one of these. Given the above rationale, only aspirin (in its uncharged form) is favored for passive diffusion through a pure lipid bilayer, while the other two acidic forms are predominantly ionized and poorly permeable. Therefore, the statement 'More that one of these' would not be correct. 
In summary, the neutral, relatively lipophilic aspirin is the best candidate among the options to pass through a pure lipid bilayer, whereas salicylic acid and acetylsalicylate, typically charged at physiological pH, are less likely to do so.

251A-LIFESCI-7A-LEC-1 AoL 1 - Group Phase Makeup

Which of these molecules is most likely to pass through a pure lipid bilayer?

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