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Sea_Guide7219 OP t1_jedd84l wrote

Reply to comment by CrateDane in Is osmotic pressure involved in the circulation of ions through a cell membrane ? by Sea_Guide7219

Yes, thank you for you answer, but it seems to me that the problem here is rather to know if the "contribution" of the concentration gradient is caused by a "force", which would be an "osmotic force".

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CrateDane t1_jeel08l wrote

It's not like a force on an individual ion because there's a lower concentration on the other side. It doesn't know or care about that. It's just thermodynamically favorable if there's an equal concentration. That can be exploited to do work, though, like when the proton gradient across the inner mitochondrial membrane makes the ATP synthase spin and generate ATP.

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Sea_Guide7219 OP t1_jeeyf7z wrote

Thank you.

I still really suspect that the idea of an "osmotic force" causing ions to follow their concentration gradient is not rigorous.

I've found a textbook (fundamentals of biochemistry) which gives two definitions :

  1. osmosis is the net movement of solvent across a semipermeable membrane from a region of high concentration, to a region of low concentration.

  2. osmotic pressure is the pressure one have to apply to equalize the flow of the solvent through the membrane in both direction.

So, it seems to me, again, that it's not rigorous to say that the movement of ions following their concentration gradient is due to "a force", and that this force may be called "osmotic force". Indeed, as far as I know the only thing we define as an "osmotic force" is "osmotic pressure", and it's the pressure needed to "cancel osmosis" if I may say so. So it's exactly the opposit of ion's following their concentration gradient.

I don't know if I that makes sense...

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