Osmotic Potential?

[sabdul]

Can someone please explain to me how this can be?

"Pure water is arbitrarily assigned an osmotic potential of zero. When a solute is added, the osmotic potential becomes negative."

Also. If anyone is using the EK General Chemistry books, on pg 89, can someone please explain how at the end of the last paragraph, it says "points A and B have the same water potential, but the osmotic potential of point B is less than that of point A."?

Thanks!

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[Bernoull]

Can someone please explain to me how this can be?

"Pure water is arbitrarily assigned an osmotic potential of zero. When a solute is added, the osmotic potential becomes negative."

Also. If anyone is using the EK General Chemistry books, on pg 89, can someone please explain how at the end of the last paragraph, it says "points A and B have the same water potential, but the osmotic potential of point B is less than that of point A."?

Thanks!

I think EK's discussion on water potential is beyond the MCAT but nonetheless here's the meaning.

Osmotic Potential:
Say u have pure water (distilled) in a beaker. Insert a semipermeable (fully permeable to water) membrane in it separating it into compartment A & B. Water molecules will freely move between both compartment and there will be no net movement of water molecules (i know this sounds super trivial) and since osmosis = net movement of water molecules, we have no osmosis taking place. The water is given an arbitrary osmotic potential of zero, this is the maximum osmotic potential (think of osmotic potential as the free energy or average kinetic energy they have to move about).

Now is we dissolve some solute into compartment A, the H20 molecule must solvate those ions and their ability to move freely and as fast is diminished and so is their average KE and osmotic potential decreases (from 0 to negative). Water molecules move from less negative (high) osmotic potential to more negative (low) osmotic. The value of a negative number decreases as its magnitude increases.

Water potential
Simply the osmotic potential of a cell which for equal to osmotic potential of the cytoplasm in animal cells. Cell walls of plant cell apply pressure potential therefore there water potential = osmotic + pressure potentials.

Back to our beaker with the 2 compartment. After adding solute to A, its osmotic potential (OP) decreases therefore water flows from higher OP to lower OP (B to A), this flow continues until an equilibrium is reached. The water level of A now increased and a pressure difference exists b/t A&B .. rho*g*Ha> rho*g*Hb. The difference of pressure between A and B is the pressure potential or osmotic pressure therefore OP (B) = OP (A) + PP at equilibrium. It's the PP that stops further net influx of water from B to A creating the equilibrium.

Therefore EK says they have the same water potential: WP = OP (B) = OP (A) + PP that is true.

EK also says OP (B) < OP (A) this is also true just note that EK added the solute to B whereas I added the solute to A (my apologies I just realized this) anyway the reverse is true for me OP (A) < OP (B) OR

OP (solution) < OP (pure water) well this was explained at the top but mathematically you can see that from the equation for WP u had to add PP to equal OP (B)..

I hope this helps, i gotta sleep now, its 2.30am here!!!!!!!!

 

[Corvette]

So basically, are osmotic POTENTIAL and osmotic PRESSURE inversely related?

As solute concentration increases, osmotic pressure increases and osmotic potential decreases?

This would make sense because water flows from low osmotic pressure to high osmotic pressure, so in an area of high solute (high osmotic pressure), there will be a low (negative) osmotic potential because water will not want to flow out.

 

[Bernoull]

So basically, are osmotic POTENTIAL and osmotic PRESSURE inversely related?

As solute concentration increases, osmotic pressure increases and osmotic potential decreases?

This would make sense because water flows from low osmotic pressure to high osmotic pressure, so in an area of high solute (high osmotic pressure), there will be a low (negative) osmotic potential because water will not want to flow out.

Essentially yes, water flows from relatively high OP (osmotic potential) to low OP and this flow of water results in osmotic pressure within the region with the low OP. SO lower OP corresponds to higher osmotic pressure.

You have a good understanding of the concept, the only thing I'll point out is that osmosis happens bcos of a difference in OP NOT osmotic pressure. Osmotic pressure is a consequence of osmosis as opposed to being a cause, actually osmotic pressure stops osmosis. Also within a system low osmotic pressure VS high osmotic pressure is a misnomer, there's only one osmotic pressure which is the difference of the hydrostatic pressures of the two compartments and it acts on the more concentrated solution to stop the net influx of water.