When does something become too polar to cross the cell membrane? (Spoilers from OG and Scored FL)

[Azaelea]

According to the AAMC, serotonin is too polar to cross the cell membrane, as discussed in OG B/B question 17. Here's the structure for reference:

But the AAMC says this molecule can cross the cell membrane (as discussed in the scored FL, B/B question 21):

So I'm a bit confused. When exactly does a molecule become too polar to cross the membrane? Is there a good rule of thumb to follow?

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[aldol16]

Unfortunately, there is no good rule of thumb, but you can tell that the second molecule is less polar than the first for several reasons. First, it has an extended pi system that is generally non-polar. Second, it's not a particularly good H-bonding molecule. And third, it's got no really polar functional groups except for the maleic anhydride-like derivative. Conversely, the first molecule has two very polar groups - the amine and the hydroxyl, both of which can hydrogen bond.

 

[Mr. otcoD]

Does polarity take precedence over size? because the 2nd molecule is almost 4 times the size of serotonin

 

[Azaelea]

Unfortunately, there is no good rule of thumb, but you can tell that the second molecule is less polar than the first for several reasons. First, it has an extended pi system that is generally non-polar. Second, it's not a particularly good H-bonding molecule. And third, it's got no really polar functional groups except for the maleic anhydride-like derivative. Conversely, the first molecule has two very polar groups - the amine and the hydroxyl, both of which can hydrogen bond.

Thanks for the response! 🙂 I'll make sure to look more carefully at the key features you mentioned in the future.