Why is hydrophobic core of plasma membrane a barrier to ions

[junkeeroo]

Hi All,

My textbook tells me,

"Because of its hydrophobic core, the plasma membrane is a physical barrier to free diffusion," i.e., ions and larger molecules will not pass through the lipid bilayer.

I'm having a hard time wrapping my head around the hydrophobic aspect that impedes ions. How does the hydrophobic aspect of the fatty acid tails in the lipid bilayer stop ions from diffusing through?

J

[JackBean]

ions are ionic compounds i.e. highly hydrophylic, i.e. not at all hydrophobic. And because similar goes with similar, hydrophilic ions do not like hydrophobic fatty acids

[jonmoulton]

This is what Jack said but in different terms.

An ion forms electrostatic associations with molecules of water. This makes the combination of the ion and the water more stable than either in isolation. Removing the ion from its network of water associations costs energy. To pass through a hydrophobic barrier, the ion would need to shed its coat of associated water. Systems tend to stay in their more stable states (unless there is sufficient energy about to lift them out of their stable states). At room temperature, there is not enough available energy to strip away the water and pop the ion across the lipid barrier. This is another way of saying that the ion is hydrophilic.

[JackBean]

yeah, that's true

I would just add that sometimes the membrane can be leaky. E.g. the mitochondrial membrane, which is barrier for proton gradient, if that gradient is too high or membrane damaged, the protons can pass through the membrane even without transport

[junkeeroo]

thanks, gentlemen. that helps a great deal.

just wanted to tac this on regarding water (a highly polar molecule) being able to pass through the lipid bilayer:

even though water is highly polar, it passes via osmosis through the lipid bilayer (though it’s expected that it would be repelled by the hydrophobic lipid tails) because, as one hypothesis explains, random movements of the membrane lipids open small gaps between their wiggling tails, allowing water to slip and slide its way through the membrane by moving from gap to gap

cheers!

great site.

[JackBean]

first, it's not so much polar (it doesn't have charge as do ions), second, it's pretty much, third, there are water transporters

[JackBean]

and as I wrote before, even H+ can pass when is the gradient too high