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Will someone please explain the difference between ...

Tokspor

Full Member
Jul 3, 2009
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water potential, osmotic potential, and osmotic pressure? also, is there such a thing as water pressure?

everywhere i look on the internet I get conflicting results. Is this concept poorly defined?

Osmotic pressure is defined as MRT (or concentration*gas constant*temperature). Just remember that water will go from low solute concentrations to high solute concentrations. Or, analogously using the equation above, water will go from low MRT to high MRT.

You can remember this rule easily since it's a rare instance within everything you've been studying where something is going from low to high. That's where I think it is confusing.

Water pressure exists, but it is usually referred to as hydrostatic pressure. This time, water goes from high hydrostatic pressure to low hydrostatic pressure. Now we're back to the usual high to low order of things.

Now, hydrostatic pressure means lots of water, which also means not that much solute. So, this means that high hydrostatic pressure (lots of water) corresponds to low osmotic pressure (not that much solute). Water will want to escape from these conditions to a place to low hydrostatic pressure and high osmotic pressure.

For osmotic potential and water potential, water will flow the same way as it did in terms of osmotic pressure and hydrostatic pressure respectively.
 
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thebillsfan

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so if something has a high osmotic pressure, is has a high osmotic potential, and if something has a high hydrostatic pressure, it has a high water potential?

and when I say high water potential, I mean zero, because if there's a solute in it, water potential is NEGATIVE. right?

thanks tokspor
 
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Tokspor

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Jul 3, 2009
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so if something has a high osmotic pressure, is has a high osmotic potential, and if something has a high hydrostatic pressure, it has a high water potential?

and when I say high water potential, I mean zero, because if there's a solute in it, water potential is NEGATIVE. right?

thanks tokspor

Yes, the first one is right. About the second question, are you getting the negative number because of -MRTi (concentration*gas constant*temperature*van't Hoff factor)? It seems from this that yes, the highest possible water potential is zero, and for this number to become more negative means to have more solute concentration M. So the more negative this gets, the lower the water potential but the higher the osmotic potential. You can also just use MRT, where the greater this number gets, the same thing happens: the lower the water potential and the higher the osmotic potential.
 
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dyspareunia

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Mar 6, 2014
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Hi!

I was just reading over this section in EK and im slightly confused what is the difference between water and osmotic potential in a sense of moving solution across a membrane

Thanks!

Hydrostatic pressure = pushes things out.
Osmotic pressure = pulls things in.

In a capillary, capillary hydrostatic pressure pushes fluid out into the interstitium (edema) and capillary osmotic pressure pulls most of that fluid back in.
 
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