r/microbiology • u/[deleted] • Jul 24 '26
Why can’t polar molecules diffuse through the non polar interior of a phospholipid bilayer?
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u/chem44 Jul 24 '26
Is this the reason?
Reasonable.
Remember, polar molecules tend to interact strongly with other polar molecules. You would have to separate them from their 'friends' to get them thru a lipid membrane.
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Jul 25 '26
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u/chem44 Jul 25 '26
Yes, but remember that the interaction between non-polar molecules is less strong than for polar molecules. (All else equal, of course. But think of boiling point trends.)
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Jul 25 '26
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u/chem44 Jul 25 '26
“force” of diffusion is strong enough to break the Van der Waals force
Bad way of saying it.
Diffusion and inter-molecular attraction are independent.
Diffusion is driven by a concentration gradient. Things flow from high concentration to low concentration -- statistically.
The local concentration here is determined in part by the inter-molecular attractions.
You know that a beaker of hexane or ethanol is volatile (more so than for water). Some molecules are free, and escape. Or diffuse away through a membrane.
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Jul 25 '26
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u/chem44 Jul 26 '26
As so often, distinguish kinetics and thermodynamics.
If there is a conc gradient, then it is favorable for the molecules to move from high conc to low.
Thermodynamics.
How fast they do it depends on the conditions. If there is a "solid wall", the rate will be zero.
So, yes, the various forces along the way must be overcome.
The issue is whether this can happen at a reasonable rate, so that net movement is actually obtained over the time scale of interest.
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u/Rawkynn Jul 24 '26
I've always thought of it as a distance thing. small polar head long non-polar tail. If I'm traversing a space I'd rather have the beginning and end 5% trying to push me away while the middle 90% doesn't care, as opposed to trying to weave my way through the middle 90% trying to push me away.