Stupid Question but I can't help it...

[Bernoull]

This is something that has bugged me for too long.. Membrane permeability to lipids. I fully understand the Fluid Mosaic Model, bilayer, phospholipids, amphipathic etc etc etc. I also know 2 important factors for membrane permeability are size and charge and both need to be considered.. small things can pass through despite polarity (H2O) and nonpolar stuff can get in despite size (cholesterol) etc etc

Wat I don't get is how lipids make it past the hydrophilic layer, especially for large lipids, do they use specific channels or wat...

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[Dr Gerrard]

This has actually always bugged me as well. I eventually just gave up and just memorized what happens, but this would be very interesting to know.

My guess is that hydrophobic molecules just have an easier time getting through, since the hydrophilic part of the membrane is not as thick as the hydrophobic part. They might just squeeze through the gaps b/w the hydrophilic heads.

 

[kwokkit]

No such thing as a stupid question, just questions that everyone else thinks will make them look stupid so nobody ends up asking them. I never understood the why of how steroids could bypass through the hydrophilic layer. They do not use cell membrane receptors or channels as far as I know.

Picture a steroid in a solution with a cell nearby. My non-scientific thinking is that since steroids are nonpolar, but water and the phospholipid heads polar, steroids want to get away from both so badly that it gets 'squeezed' by the aqueous solution and the phospholipid heads until it gets into the hydrophobic tails of the bilayer. The problem with this theory is that it predicts that steroids would be stuck in the middle part of the bilayer, so I usually just say it gets pushed all the way through to the interior of the cell, haha..

 

[Bernoull]

This has actually always bugged me as well. I eventually just gave up and just memorized what happens, but this would be very interesting to know.

My guess is that hydrophobic molecules just have an easier time getting through, since the hydrophilic part of the membrane is not as thick as the hydrophobic part. They might just squeeze through the gaps b/w the hydrophilic heads.

I thought about the relative thickness of the hydrophobic VS hydrophilic portions of the membrane too, that would be the only thing that makes sense to me now. I'm thinking of fatty acids (FAs), are they water or lipid soluble... well, it depends on chain length, short FAs r H20 soluble bcos their nonpolar tails aren't long enuff to "dominate" the polar COOH heads, but longer FAs are H2O insoluble, bcos their tails "dominate" their polar head, so it may be that the membrane bilayer character is dominated by the nonpolar FA tails making it nonpolar overall.

Steroid is definitely nonpolar and requires a protein transport molecule to move thru the blood, if the membrane was sufficiently polar, it should need a protein channel as well (given its size) so the only possibilities are for it to

a) squeeze through the polar heads like it say (but i'm not sure if there size allows for this, dunno..)
b) ease right through bcos the membrane is more nonpolar than it is polar.

My 2cents anyway, where are da biochemists when u need one????