Pressure and tonicity

[eudovcic]

I looked through some books and online but could not find a satisfying explanation.

Anyways, I just want to make sure I am thinking about this the right way. In regards to osmotic pressure and hydrostatic pressure, if a cell (membrane only permeable to water) is placed in a hypotonic solution would the hydrostatic pressure in the cell be greater than the osmotic pressure of the environment, forcing H2O outward until osmotic P = hydrostatic P? Or is the flow of H2O caused only by osmotic pressure within the cell being lower than the osmotic pressure of the environment?

Or is it simply hydrostatic P in the cell is greater than the osmotic pressure in the cell therefore water will tend to be forced outward toward the higher osmotic pressure of the environment?

Since P=pgh, decreasing the volume of the cell would increase the density (as does increasing solute concentration) and so hydrostatic P in the cell increases while the solute concentration of the environment is decreasing causing osmotic P to decrease. If my thinking holds true then the initial hydrostatic P of the cell should be lower than the environments initial hydrostatic pressure. For the hypotonic situation then we would have initial conditions as low hydrostatic P and low osmotic P in the cell and high hydrostatic P and high osmotic P in the environment. The only way for me to intuitively think about this where it makes sense is: (high osmotic P - low osmotic P) > (high hydrostatic P - low hydrostatic P) so that the osmotic P "pulling" water outside the cell is higher than the hydrostatic P "pushing" water into the cell.

Am I right to even think of both in this situation? I know for blood vessels osmotic pressure is relatively constant and fluid exchange is mediated by varying hydrostatic pressure. But thats easier for me to conceptualize as osmotic P is not changing.

Sorry that this is a bit long, but every time I think I understand it a question pops up that presents an answer with completely opposite logic to what I used to solve it lol.

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[monkeyvokes]

if a cell is placed in a hypotonic solution, there are fewer nonpenetrating particles in the solution than in the cell. Consequently, water will enter the cell to go where there are more particles. Water goes to wherever there is more solutes.

 

[eudovcic]

See my confusion? haha

EK states "Most bacteria are hypertonic to their environment. This means that the aqueous solution of their cytosol contains more particles than the aqueous solution surrounding them."

I meant to phrase my question it in a way where the cell is hypotonic to the solution.

Anyway, I understand that water will go to where there are more solutes but, for the sake of understanding and not memorizing, I would like to be able to relate osmotic P and hydrostatic P in a hypothetical situation in a similar way that they are explained for blood vessels.

I know water will flow from low osmotic P to high osmotic P, but where does the hydrostatic P come into play? You can't just ignore it can you? I may be overthinking it, but it's better to be sure

 

[monkeyvokes]

hydrostatic pressure in our blood vessels is due to gravity (conceptualized as a column of fluid) and the pressure supplied by our beating heart, as i understand it. hydrostatic pressure pushes fluid out, while osmotic pressure pulls fluid in (all of the proteins and particles in our blood attract water).
How does hydrostatic pressure relate to an isolated cell? I'm not exactly sure. If a cell was deep in the ocean, the water above it may be too much causing the cell to burst from high pressure.

Let me know if you find a resolution to your question..
thanks

 

[eudovcic]

"This illustration shows water molecules (blue) passing freely in both directions through the semipermeable membrane, while the larger solute molecules remain trapped in the left compartment, diluting the water and reducing its escaping tendency from this cell, compared to the water in the right side. This results in a net osmotic flow of water from the right side which continues until the increased hydrostatic pressure on the left side raises the escaping tendency of the diluted water to that of the pure water at 1 atm, at which point osmotic equilibrium is achieved."

I think I did overthink it for the purposes of the mcat. So if I understand correct, in a hypertonic solution the cell will have greater hydrostatic P than osmotic P thereby pushing H2O outward. This will increase the osmotic P of the cell until it equal the hydrostatic P and filling stops. For hypotonic solutions, the osmotic P in the cell is greater than the hydrostatic P so H2O moves into the cell, increasing hydrostatic P until it equals the osmotic P.


Though I would like to clear up one thing. So in my original post I said that P=pgh and since the H2O is leaving the cell (in a hypertonic solution) that the denisty (p) is increasing therefore the hydrostatic pressure in the cell is also increasing. I failed to consider that while p=m/V that the loss of water molecules would also decrease the m term and density would not change due to volume change. However, if the solute concentration is increasing with the loss of water then density would increase.

In EK they say "as the cell fills with water and the hydrostatic pressure builds, it eventually equals the osmotic pressure, and filling stops."

So if I think about hydrostatic P in terms of P=pgh then would hydrostatic P not be decreasing as the volume of water increases inside the cell (indicating decreasing solute concentration)? Or does the increase in the "h" term overcome the decrease in density?

 

[mehc012]

"This illustration shows water molecules (blue) passing freely in both directions through the semipermeable membrane, while the larger solute molecules remain trapped in the left compartment, diluting the water and reducing its escaping tendency from this cell, compared to the water in the right side. This results in a net osmotic flow of water from the right side which continues until the increased hydrostatic pressure on the left side raises the escaping tendency of the diluted water to that of the pure water at 1 atm, at which point osmotic equilibrium is achieved."

I think I did overthink it for the purposes of the mcat. So if I understand correct, in a hypertonic solution the cell will have greater hydrostatic P than osmotic P thereby pushing H2O outward. This will increase the osmotic P of the cell until it equal the hydrostatic P and filling stops. For hypotonic solutions, the osmotic P in the cell is greater than the hydrostatic P so H2O moves into the cell, increasing hydrostatic P until it equals the osmotic P.


Though I would like to clear up one thing. So in my original post I said that P=pgh and since the H2O is leaving the cell (in a hypertonic solution) that the denisty (p) is increasing therefore the hydrostatic pressure in the cell is also increasing. I failed to consider that while p=m/V that the loss of water molecules would also decrease the m term and density would not change due to volume change. However, if the solute concentration is increasing with the loss of water then density would increase.

In EK they say "as the cell fills with water and the hydrostatic pressure builds, it eventually equals the osmotic pressure, and filling stops."

So if I think about hydrostatic P in terms of P=pgh then would hydrostatic P not be decreasing as the volume of water increases inside the cell (indicating decreasing solute concentration)? Or does the increase in the "h" term overcome the decrease in density?

You're phrasing it backwards. Or maybe not...to be honest you combine "pushing the water outwards" and "the cell will stop filling" in the same sentence and so I am very unclear on what you are intending to describe. Edit: the more I read it, the more I realize that you have it right, but the phrasing is *very* confusing. I'll leave mine up but grey out everything that doesn't address your actual question!

The osmotic pressure is the force caused by the water entering the cell along the concentration gradient.
The hydrostatic pressure, in this case, is the pressure INSIDE the cell due to the volume of the cell and the tension of the membrane. P = ρgh is not particularly useful for this situation...it only describes 1 contributor to the blood pressure in the blood vessel, not the entirety of it, and certainly not the pressure inside a cell.
Furthermore, the density difference between the inside and the outside are not the main issue here. Osmotic pressure is not "the pressure difference due to density variation", it is simply "the pressure difference which would drive water back across the membrane at a rate equal (but in the opposite direction) to the rate caused by the concentration gradient.

Think of it like a not-very-flexible balloon being blown up:
You are blowing air in with a certain amount of pressure
(the water is being driven into the cell by the osmotic pressure)
and the pressure inside of the balloon is slowly increasing as the internal volume increases and the tension of the walls pushes harder
(the hydrostatic pressure inside the cell increases as the internal volume increases)
Eventually, 1 of 2 things will occur: the internal pressure of the balloon will equal the pressure you are blowing into it with, and it will stop filling
(the hydrostatic pressure will = the osmotic pressure and the cell will come into an equilibrium with the surroundings)
OR, if the incoming pressure is too high, you will overcome the strength of the walls and pop the balloon
(if the osmotic pressure is too great, the cell will rupture before it reaches an equilibrium)