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membrane permeability - let's make it clear!

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membrane permeability - let's make it clear!

Postby kk » Sun Nov 30, 2008 8:01 pm

It is a such contradictory literature on permeability of biological membranes, I would like to see clearly now. What substances can FREELY travel through biological membranes (with NO help of transporters, whatsoever)?

It is also not clear whether all biological membranes act the same way in point of permeability (prokaryotic vs. eukaryotic; membranes rich in saturated fatty acid chains vs. rich in unsaturated ones; etc.)?

In other words, is there any difference in the fine structure among membranes that could affect permeability (aside from having different transporters embedded)?

And just more and more unclear questions regarding this topic :p

How come that some literature says "steroids diffuse into cells" and bind to their receptors? Aren't ALL those hydrophobic molecules carried by proteins in the blood stream and inside the cytosol? How could they just diffuse into cells: does the carrier release them upon contacting the target cells' plasma membranes and after diffusion though the membrane would another carrier bind them inside the cell? Aren't all those steroids taken in by receptor-mediated endocytosis (an ACTIVE transport mechanism, which is SPECIFICLY targets certain cells, as opposed to diffusion that is passive and non-specific).

Other sources say (no reference) glycerol and even ions can freely pass the membrane. Explain me that, I beg you! These are large, charged substances. On the other hand: why urea is an exception? Or is it only the case in the kidney tubules?

What happens is you expose the plasma membrane to long neutral fats? Do those just diffuse into the cell?? Where would they go in there?

I even found articles talking about membranes that do not allow free passage of CO2 (a small, "non-polar" symmetrical molecule)!!

I need to calm down now 8)
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Re: membrane permeability - let's make it clear!

Postby MrMistery » Mon Dec 01, 2008 12:27 am

from what I know, all membranes are freely permeable to oxygen, CO2 and other gaseous substances. Membranes are also somewhat permeable to water, though most water is transported through acquaporins.
And yes, I would say membranes that are more fluid are more permeable to small molecules than membranes with a more ordered structure.
Steroids are bound to protein in the blood. Here your textbook is referring to the steroid hormones of course. they are bound noncovalently to protein in the blood. But as every noncovalent interaction is actually an equilibrium between the bound and unbound state(think freshman year chemistry). True, the equilibrium is shifted to favor the bound state, but there is always a small quantity of unbound hormone that can diffuse inside the cell to bind a nuclear receptor.
No, they are not. You are thinking cholesterol, which is transported into the cell by receptor mediated endocytosis. The main difference is this: when you are talking cholesterol, you need a large quantity of it because the cell needs to include it in its membrane. Therefore cholesterol is transported in the blood large lipoprotein "balls", which can only be taken up by receptor mediated endocytosis. Steroid hormones, however, are just a source of information, and a cell only needs a few molecules in order for them to bind their receptor and turn on transcription.

Glycerol and ions can only pass through channels. I know the glycerol channel is related to water channels by the way. Just as a fun fact.
Urea is transported through a designated carrier. It's not an exception to anything, as far as I know.

If you expose the plasma membrane to long neutral fats I would say they would diffuse into the cell. What would happen there depends on the molecule I guess. If you mean some molecule like an alcane, then nothing. If you mean an ester of a fatty acid, then it will be included in a lipid droplet or used for energy production by beta-oxidation(or other types of fatty acid oxidation).
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
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