Osmotic Pressure

Mikhail · Mar 25, 2005

Hello,

I'm a little bit confused with osmotic pressure. I wanna make sure if I understand this correctly. When a cell is put in distill water, the water will flow to the cell (from hypotonic to hypertonic). My question is: where is osmotic pressure high, inside the cell or outside? Based on the formula of osmotic pressure, it should be inside the cell (I mean this formula: osmotic pressure = R* Molarity* Temperature). Do I think correctly?
Thanks

Mikhail

Pontifex Maximus · Mar 25, 2005

I believe osmotic pressure is high outside of the cell, because the water is trying to push in. Don't quote me on it.

stoleyerscrubz · Mar 25, 2005

Just remember the gradient flows from high pressure to low pressure as all systems want to get to a lower energy state if possible.

Mikhail · Mar 25, 2005

stoleyerscrubz said:
Just remember the gradient flows from high pressure to low pressure as all systems want to get to a lower energy state if possible.

This is a concept of entropy, as I undestand. But when there is a semipermeable membrane, solute cannot cross it, only solvent does. Then, how would this information apply to osmotic pressure?

Thanks a lot for your answers.

Mikhail

QofQuimica · Mar 25, 2005

Osmotic pressure is the pressure that must be applied to your cell membrane to PREVENT the water from coming inside when you place the cell in the hypotonic solution. It's a way to measure how great the difference in concentration is on each side of the membrane; if you have a really concentrated cell in a really dilute solution, you'll need a lot more pressure to stop that water from coming in versus if the two concentrations are closer to each other. If you put the cell into a hypertonic solution, you have the reverse situation. Now, the osmotic pressure is equivalent to the pressure necessary to stop the water from leaving the cell to go into the more concentrated solution surrounding it.

You can use the osmotic pressure equation to calculate what the osmotic pressure is for a given solution, such as 20 g of NaCl dissolved in 500 mL of water at STP. So if your cell is hypertonic to your solution, as in your example, then yes, the osmotic pressure is greater inside the cell versus outside. The hypertonic solution will always have a higher osmotic pressure because it has a higher molarity, and the temperature and gas constant are both constant. Don't forget to convert your temperature to Kelvins, not Celsius, and remember that salts dissociate into ions, so that 1 mole of NaCl is actually 2 moles (approximately) of ions.

senseigmg · Mar 25, 2005

Cozmosis said:
I believe osmotic pressure is high outside of the cell, because the water is trying to push in. Don't quote me on it.

I agree,
from what I've learned (and just googled), osmotic pressure is the pressure of solvents flowing INTO a solution. In your case, it would be the water outside of the cell.

Mikhail · Mar 25, 2005

QofQuimica said:
Osmotic pressure is the pressure that must be applied to your cell membrane to PREVENT the water from coming inside when you place the cell in the hypotonic solution. It's a way to measure how great the difference in concentration is on each side of the membrane; if you have a really concentrated cell in a really dilute solution, you'll need a lot more pressure to stop that water from coming in versus if the two concentrations are closer to each other. If you put the cell into a hypertonic solution, you have the reverse situation. Now, the osmotic pressure is equivalent to the pressure necessary to stop the water from leaving the cell to go into the more concentrated solution surrounding it.

You can use the osmotic pressure equation to calculate what the osmotic pressure is for a given solution, such as 20 g of NaCl dissolved in 500 mL of water at STP. So if your cell is hypertonic to your solution, as in your example, then yes, the osmotic pressure is greater inside the cell versus outside. The hypertonic solution will always have a higher osmotic pressure because it has a higher molarity, and the temperature and gas constant are both constant. Don't forget to convert your temperature to Kelvins, not Celsius, and remember that salts dissociate into ions, so that 1 mole of NaCl is actually 2 moles (approximately) of ions.

QofQuimica, thanks a lot for your answer. It explains everything.

stoleyerscrubz · Mar 25, 2005

This concept applies to physics, biology, chemistry...

if you know that water is flowing into the cell then it is because the osmotic pressure inside is greater than the outside.

there was a thread on this about osmotic pressure and hydrostatic pressure and I don't want to get into that can of worms again.

Mikhail said:
This is a concept of entropy, as I undestand. But when there is a semipermeable membrane, solute cannot cross it, only solvent does. Then, how would this information apply to osmotic pressure?

Thanks a lot for your answers.

Mikhail

confuse · Mar 25, 2005

Hello,
The easiest way to remember hydrostatic pressure and osmotic pressure is that one is a "pushing force" the other is a "pulling force" In this case, since water is moving into the cell, the higher hydrostatic force outside the cell pushes the water in and the higher osmotic pressure inside the cell pulls the water in.

Osmotic Pressure

Mikhail

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Pontifex Maximus

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stoleyerscrubz

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Mikhail

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QofQuimica

Seriously, dude, I think you're overreacting....

senseigmg

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stoleyerscrubz

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confuse

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