Is osmotic pressure involved in the circulation of ions through a cell membrane ?

Submitted by Sea_Guide7219 t3_126dz7v in askscience

Hi,

I have a neurobiology course, and I have a doubt about something my teacher wrote in the syllabus of the course.

It says that ions can circulate through the membrane of a neuron via receptors, and that there is two major factors influencing the circulation of ions : 1) an electrical force, due to the eletric potential difference between the outside and the inside of the cell. 2) a "chemical force" (sic) due to the difference in ions concentration, "also called osmotic force".

I have 3 questions :

  1. is the concept of "chemical force" a valid one ?
  2. most importantly, I'm note very qualified in chemistry, but it seems to me that osmotic pressure is by definition related to the circulation of a solvent through a semi-permeable membrane... here the ions are the solute if I'm not wrong... so how can osmotic pressure have something to do with this ?
  3. the explanations of my teacher, are they "okay" simplification, or are they simply false ?

Looking forward to reading you,

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CrateDane t1_jea97c2 wrote

Your teacher/professor is correct in that both the concentration gradient and the electric field contribute. The Gibbs free energy change of moving an ion across the membrane follows this equation:

ΔG = RTln(c_inside/c_outside) + zFV_membrane

So the first part would be the energy involved in moving with/against the concentration gradient, the second the energy involved in moving with/against the electric field.

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Sea_Guide7219 OP t1_jedd84l wrote

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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DudoVene t1_jea9u6c wrote

chemical force: think about it in a terms of "chemical species" force. as for partial pressure in a mix of gazes, you can see the global pressure (like surrounding air) and the specific distribution of different species in the same volume.

yes osmotic pressure is a very important force in the circulation of ions AND water through membrane. maybe life itself (survey of one cell like for unicellular) rely on this mechanism. disturb the membrane integrity (with detegents like soap, reagents like alcohol, and specific drugs binding ions transporters like botulin toxin) and you kill the cell. so the cell life requests a very fine tuning of inside ions concentrations to maintain proper osmotic pressure inside (and so the water concentration itself) and this mechanism requires itself some energy (to be used by active ion membrane carrier). not only for nerve cells, but any cell in the organism.

hope it helps !

edit : life is a complex system. our actual knowledge must be take with humility and scientific explanations TENDS to describe it with a lot of accuracy. dont mind on your teacher for such simplification. it is a way to teach, not a way to lie. I have learnt in very different ways what could be a "protein" all along school classes but only in university I ultimately learn what was behind. and all past lessons looks "fantasy" to me now. dont be afraid to learn new things with baby steps !

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antiquemule t1_jearmhj wrote

Just to add a bit of detail to the correct answers already posted:

  1. Yes, chemical force is a valid concept. See chemical potential in Wikipedia.

  2. Osmotic pressure does not have to involve circulation of solvent, any more than air pressure has to involve flow of air.

  3. The explanations are correct. They are not even a simplification.

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OpietMushroom t1_jeb4760 wrote

Others have mentioned that the teacher is correct, and that "chemical force" is an actual term to describe physical properties of things. I just wanted to touch on the subject of membranes in cells. Substances can enter our cells either through active or passive means. Ions diffusing by osmosis is a form of passive transport, which means no energy in the form of ATP is required for transport. Ions are able to pass the membrane freely because of their size and chemical properties your teacher discussed. Larger molecules, such as lipids or proteins, require active transport methods that use special proteins in the membrane that use up ATP.

This is why our nerves work so quickly, the cascading signal of ions happens almost instantly! It is a chain of differing electrical potentials caused by ion gradients.

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CrateDane t1_jebavu9 wrote

>Ions are able to pass the membrane freely because of their size and chemical properties your teacher discussed.

Oh no. Apart from a small amount of leakage, ions definitely do not freely cross the membrane. Having control over when and how ions cross membranes is critical to life, not just by enabling neuronal signaling in animals but in being harnessed for ATP synthesis across all domains of life.

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Sea_Guide7219 OP t1_jebavkc wrote

Thank you all for your answers.

I have found this in a textbook, "fundamentals of biochemistry", Voet et al., 2016 :

"When a solution is separated from pure water by a semipermeable membrane that permits the passage of water molecules but not solutes, water moves into the solution due to its tendency to equalize its concentration on both sides of the membrane. Osmosis is the net movement of solvent across the membrane from a region of high concentration (here, pure water) to a region of relatively low concentration (water containing dissolved solute). The osmotic pressure of a solution is the pressure that must be applied to the solution to equalize the flow of water across the membrane in both directions."

So, I do not want to split hairs, but I gather (yet I might off course be totally wrong) that the "circulation" of ions following their concentration gradient is simply the net movement which tends to equilibrate the concentration gradients on both sides of the membrane, not because "a force" is acting on the particles, but simply because more particles go from A to B than from B to A.

What do you think about it ?

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DudoVene t1_jedqphd wrote

I think you get the point but the movment of ions IS a force, meaning you can calculate it or (better example in biochemistry ) translates it in another force. this way, the transfert of H+ ions throught the mitochondria membrane is converted in heat, and mostly in the regeneration of ADP to ATP. another example should be the transfert of seve in tree from roots to leaf : the osmotic pressure is able to elevate a volume of water versus the gravity force of the weight of the water. keep in mind if zero force is involved, any system should be considered at equilibrium. equilibrium in life means death.

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