Can someone explain how osmosis works?

[mrh125]

i understand how diffusion works (high to low concentration), but I never really understood osmosis really well. like I had a practice question, which asked:
58. If a cell wall separates a hypertonic, interstitial fluid from cellular fluid, will there be an osmotic effect?
A) Yes; the cell fluid will become less concentrated.
B) Yes; the cell fluid will become more concentrated.
C) No; osmosis does not apply to biological fluids. D) No; the concentrations are the same on both
sides of the wall.

B is the right answer. Why is this? I always remember thinking of osmosis as the opposite of diffusion, so it'd be from a low to high concentration so that's why cellular fluid would travel from the cell to the hyper tonic interstitial fluid?

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[Bomikepa]

yea i was thinking along the same lines as you were. Is it possible that the answer is wrong?
or does the cell wall have something to do with making osmosis work differently. We usually reference a " semi-permeable" membrane when we talk about osmosis. Usually cell walls are much stronger than that so idk if the cell wall aspect has any play in the answer.

 

[Jkh389]

Hi,

I think your best idea is to think of osmotic 'pulling' effect. Think of the idea of the potential of a system to pull. Because the solution is hypertonic, it means it has more solutes in it. Water tends to move from a hypotonic solution to a hypertonic solution. Therefore the hypotonic solution will become more concentrated as water will move into the hypertonic interstitial fluid.

 

[sps27]

Hi,

I think your best idea is to think of osmotic 'pulling' effect. Think of the idea of the potential of a system to pull. Because the solution is hypertonic, it means it has more solutes in it. Water tends to move from a hypertonic solution to a hypertonic solution. Therefore the hypotonic solution will become more concentrated as water will move into the hypertonic interstitial fluid.

Yes this is correct. A good idea is to remember that water follows the solute. Where ever the solute is more, water will follow it.

Related to this I have a really stupid question. A brain fart if you will. I don't know why. I just cannot explain to myself mathematically i.e., how does dissolving a solute ex., NaCl in say 1 liter of water (which has 55.5 moles of pure water) lowers the concentration of pure water? Conceptually and Intuitively I understand it well. Just dissolve NaCl in water and drink it and your senses should tell you how much pure water is in there......ha ha. TBR says, "when we add a solute molecule (or molecules) we are occupying a volume of space that was once occupied by a water molecule. The more solute molecules we add to our beaker of pure water, the more water molecules we will displace and the lower will be the concentration of pure water". . So my doubt is, displacing a water molecule does not mean that the molecule is lost. The amount of water molecules i.e., 55.5 moles remain the same. The only change is, there are now some moles of solute as well. The volume of the beaker (say 1 liter) is not going to change when a solute is added. So the question is, why do we say the concentration of pure water goes down when the # of water molecules essentially remain the same in the beaker. They don't disappear. They are merely displaced.

So the way I made myself understand is, when a solute is added to a volume of pure water, some water molecules will interact with that solute. They may form for ex., a circle around the solute. Those water molecules are essentially not pure anymore. The more the solute in the beaker, the more the no of molecules of water interacting. Pure water will consist of those molecules which are not interacting with the solute. That number will go down because of adding a solute to the solution. And if that number goes down, concentration of pure water goes down because the volume essentially remains the same. So it is not really displacing a space once occupied by a water molecule, it is rather # of molecules interacting with solute v/s those not interacting with solute(pure water). Is my understanding correct?

 

[PreMedRookie]

I wouldn't think of it as the opposite as diffusion. You are still going from high to low concentration, except it is water that is moving instead of the solutes.

The water is most concentrated where it is most pure (A hypotonic solution)
Water is least concentrated in an area with the most solutes (A hypertonic solution)

So in this case, water will move from inside the cell to the outside of the cell. This ends up making the inside more concentrated because there is less water in the cell now.

 

[PreMedRookie]

Yes this is correct. A good idea is to remember that water follows the solute. Where ever the solute is more, water will follow it.

Related to this I have a really stupid question. A brain fart if you will. I don't know why. I just cannot explain to myself mathematically i.e., how does dissolving a solute ex., NaCl in say 1 liter of water (which has 55.5 moles of pure water) lowers the concentration of pure water? Conceptually and Intuitively I understand it well. Just dissolve NaCl in water and drink it and your senses should tell you how much pure water is in there......ha ha. TBR says, "when we add a solute molecule (or molecules) we are occupying a volume of space that was once occupied by a water molecule. The more solute molecules we add to our beaker of pure water, the more water molecules we will displace and the lower will be the concentration of pure water". . So my doubt is, displacing a water molecule does not mean that the molecule is lost. The amount of water molecules i.e., 55.5 moles remain the same. The only change is, there are now some moles of solute as well. The volume of the beaker (say 1 liter) is not going to change when a solute is added. So the question is, why do we say the concentration of pure water goes down when the # of water molecules essentially remain the same in the beaker. They don't disappear. They are merely displaced.

So the way I made myself understand is, when a solute is added to a volume of pure water, some water molecules will interact with that solute. They may form for ex., a circle around the solute. Those water molecules are essentially not pure anymore. The more the solute in the beaker, the more the no of molecules of water interacting. Pure water will consist of those molecules which are not interacting with the solute. That number will go down because of adding a solute to the solution. And if that number goes down, concentration of pure water goes down because the volume essentially remains the same. So it is not really displacing a space once occupied by a water molecule, it is rather # of molecules interacting with solute v/s those not interacting with solute(pure water). Is my understanding correct?

Think of it like pure water is 100% water. Once you add solutes it is 98 or some other number % water. Thus, it is less concentrated.

 

[mrh125]

I wouldn't think of it as the opposite as diffusion. You are still going from high to low concentration, except it is water that is moving instead of the solutes.

The water is most concentrated where it is most pure (A hypotonic solution)
Water is least concentrated in an area with the most solutes (A hypertonic solution)

So in this case, water will move from inside the cell to the outside of the cell. This ends up making the inside more concentrated because there is less water in the cell now.

Thanks that makes so much more sense right now . I put what you said on a flashcard.

 

[PreMedRookie]

Thanks that makes so much more sense right now . I put what you said on a flashcard.

Aw thanks, that makes me feel so happy haha.