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It is often stated that small molecules or nonpolar molecules can diffuse through the plasma membrane because they can pass through the middle nonpolar bit, but why don't the polar sides block these nonpolar molecules. Estrogen is nonpolar and can diffuse across the membrane right? Why don't the polar heads of the phospholipids block it? Or look at H+. H+ can't diffuse across the membrane because it's charged (it's not like nonpolar molecule have a repulsive force against it, neutral objects don't repel charged ones as far as I am aware, I don't get why we say polar and nonpolar repel each other, as I understand they just stick to themselves better than each other). Regardless, H+ a small charged molecule would be able to get past the hydrophilic heads right? Estrogen wouldn't be. Where am I going wrong here? Thanks!

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  • $\begingroup$ You might get the answers you're looking for on the chemistry stackexchange $\endgroup$ Commented Sep 22, 2020 at 9:27

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Polar and nonpolar molecules don't actually "repel", it's that polar molecules attract each other much more than nonpolar molecules attract anything. Therefore, to take a polar molecule from the (polar) water on one side of the membrane and bury it in the nonpolar region in the middle of the membrane is difficult because it means breaking its relatively strong interactions with the polar water molecules without forming new strong interactions to compensate.

In the case of a nonpolar molecule passing through the membrane, it is already in water when it approaches the membrane, and transferring it past the polar head groups isn't much "worse" (there's likely a slight effect of the often charged head groups attracting ions that bridge them to one another, but this would be small). The large effect of it being nonpolar, however, is to make it less unfavorable to move it into the middle layer of the membrane where there aren't polar groups for it to interact with.

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