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Fun with Water Potential!
Botanists use the term water potential when predicting the movementof waterinto and out of plant cells. Remember,
the movementof water across a semi-permeable membraneasit followsits own concentration gradientis osmosis. But
plants are a unique case becausethey havea cell wall which prevents a plantcell from bursting if it takes on too much
water. Ifa plant cell is placed in a hypotonic environment,it will take on water UNTIL water pressure inside the cell
builds up to a point that it cannot take on any more. This increase in water pressure will actually push water molecules
back outof the cell against water's concentration gradient until the amountof water moving in due to solute difference
is equal to the water moving out due to water pressure—thisis a dynamic equilibrium!
Waterpotential is a measure of howlikely water is to move from one location (say outside the cell} to another(inside
the cell), For example, in the case above....a plant cell placed in a hypotonic solution.....water has a higherpotential
(tendency) to move INTOthe cell than it does to move OUTofit. Therefore, water potential is GREATER outside the cell
than inside the cell because the water outside the cell has the GREATER POTENTIAL TO MOVE. Justlike any solute will
diffuse DOWNits concentration gradient—from high to low concentration, waterwill always movefrom an area of
greater water potential to an area of lesser water potential:
Waterpotential is made up of 2 parts—solute potential and pressure potential. Add both togetherto get thetotal
water potential. See below.
Note: (we use the Greek letter “psi” or “W” to represent waterpotential.)
Solute potential + Pressure potential = Water potential
Wo+ Wo =W
*
Solute potential—(Y, }—the greater the concentration of a solute, the lower the water potential.—an inverse
relationship. Think ofit this way:if you place a plantcell in very salty water, waterwill leave the cell for the
environmentit’s in. Water had a greater tendency to move OUTthan in (the cell). Therefore, water potential
waslowerin the environment than in the cell. This means that the water in the environment had low tendency
to move(into the cell) than the other way around. Said another way, water that wasin the cell had a greater
potential to move (OUT) than the other way around. in summary,solute potential is the effect that solutes have
on a solution’s overall water potential.
¢
Pressure potential—( VY, )—physical pressure increases water potential—a direct relationship. If you have a
syringefilled with water and you press downon the plunger (increasing pressure), water will leave the syringe
through the needle (the only opening). Plant cells work the sameway,if water pressure increasesinside the
cell, that pressure will drive water back out. The cell wall will not break, butit is a little bit elastic and will bulge
outward. This is called TURGOR. The water pressure that causes turgor is called TURGOR PRESSURE. Lack of
turgor is called WILTING!
Nowlet’s add some numbersto this:
e Pure water has a water potential of ZERO.
e Solutes added to pure water decrease water potential—negative numbers.
e
e
Pressure increases water potential.
Water always moves from high water potential to low water potential.
WATER POTENTIAL PROBLEMS
Water potential = Solute potential + Pressure potential
Ys WY, = Bo
The solute potential (ws) = - iCRT, where | is the ionization constant, C is the molar concentration. R is the pressure
constant (R = 0.0831 liter bars/mole-K), and T Is the temperature in K (273 + °C).
Commonionization constants: sucrose (i=1), glucose (i=1), Nacl (i=2)
The pressure potential (w,) = 0 for pure water in an opencontainer. The fp = 1 fora plantcell (turgor pressure}.
1.
{facell’s Up = 3 bars and its W, = -4.5 bars, what is the resulting W/?
3+-4.5=-1.5 bars.
2. The cell from question #1 is placed in a beaker of sugar water with W, = -4.0 bars. In which direction will the net
flow of water be?
3.
Theoriginal cell from question # 1 is placed in a beaker of sugar water with W, = -0.15 MPa (megapascals). We
know that 1 MPa = 10 bars. In which direction will the net flow of water be?
4. The value for W in root tissue was found to be.-3.3bars. If you take the roottissue and placeit ina 0.1 M
solution of sucrose at 20°C in an open beaker, whatis the W of the solution, and in which direction would the
net flow of water be?
te
root solution
-(1)(0.831)(.1)(273+20)
= -2.44
5.
NaCl dissociates into 2 particles in water: Na* and CI’. If the solution in question 4 contained 0.1M NaCl instead
of 0.1M sucrose, whatis the
of the solution, and in which direction would the net flow of water be?
-(2)(0.0831)(.1)(273+20)
=-4.87
6. Aplantcell with a WY, of -7.5 bars keeps a constantvolume when immersed in an open-beaker solution that has a
W. of -4 bars. What is the cell’s Wp?
SHR
7.
At 20°C, a cell containing 0.6M glucoseis in equilibrium with its surrounding solution containing 0.5M glucose in
an open container. Whatis the cell’s Wp?
2
-(1)(0.0831)(.6)(293)
=-14.609
-(1)(0.0831)(.5)(293)
=-12.174
Pressure 2.44
8.
At 20°C, a cell with Y, of3 barsis in equilibrium with the surrounding 0.4M solution of sucrose in an open
beaker. Whatis the molar concentration of sucrose in the cell? (Bonus)
9. You are stranded on lifeboat with several other people in the middle of the ocean. You have run outof clean
drinking water and are very thirsty. The others have started to drink the seawater to quench their thirst. Since
you're waiting for rescue (and are a top notch AP Biology student), you decide to solve a couple of water
potential problems ta determine if drinking sea water is a good idea or not. ©
a.
The majority of dissolved ions in seawater is NaCl, roughly a 0.5M concentration. Theionization
constant of NaClis 2.0, Calculate the solute potential for seawater if you know that the wateris 2°C.
-(2)(0.0831)(0.5)(275)
=-22.8525
b.
Your own cells have a 0.15M concentration. Calculate the solute potential for your owncells, knowing
that body temperature is 37°C.
-(1)(0.0831)(.15)(310)
=-3.86415
c.
Using your knowledge about water potential, if you drink the seawater, wat will happen when the
seawater comesinto contact with your body’s cells? Note: Wpfor animalcells is zero.
Ve
If you were to drink the salt water, your cells would go from hypotonic to hypertonic making you more thirsty.
d.
ts drinking seawater a good idea for survival? What effect will drinking seawater have on you and your
chancesof surviving until you are rescued?
Drinking saltwater is a terrible idea. When you drink water you drink it to get less thirsty but when you
drink salt water it makes you more thirsty so it harms you more than it helps you.