1. Water Potential
Water potential quantifies the tendency of free (not bound to solutes) water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension.
Water potential has proved especially useful in understanding water movement within plants, animals, and soil.
Water potential is typically expressed in potential energy per unit volume and very often is represented by the Greek letter psi,  . (pronounced as “sigh” )
Water potential is the physical property that predicts the direction in which water will flow. The unit for water potential is the megapascal (MPa). By definition the  of pure water in a container open to the atmosphere under standard conditions (25C and 1 atm pressure) is 0 MPa.
One MPa is equal to about 10 times atmospheric pressure at sea level. The internal pressure of a living plant cell due to the osmotic uptake of water is about 0.5 MPa or about twice the air pressure inside an inflated car tire.

2. Water Potential
The addition of solutes to water lowers the water's potential (makes it more negative), just as the increase in pressure increases its potential (makes it more positive).
Pure water is usually defined as having an osmotic potential () of zero, and in this case, solute potential can never be positive.
Free water moves from regions of higher water potential to regions of lower water potential if there is no barrier to its flow.
Emphasize that the word “potential” refers to water’s potential energy which is water’s capacity to perform work when it moves from a region of higher water potential to a region of lower water potential.  = S + P where  is the water potential, S is the solute potential (directly proportional to its molarity and sometimes called the osmotic potential and the S of pure water is zero) and P is the pressure potential.
So, why do we have this annoying negative sign convention in the first place? Because adding a solute to water interferes with the intermolecular forces water molecules have for each other (hydrogen bonding being the most famous). If something dissolves in water, it is because water is more attracted to the solute particle than to neighboring water molecules, so the water molecules are less likely to “move”, thus their potential to do work is decreased hence the negative sign slapped upon the numbers!

3. Water Potential
The word “potential” refers to water’s potential energy which is water’s capacity to perform work when it moves from a region of higher water potential to a region of lower water potential.
The water potential equation is  = S + P where  is the water potential, S is the solute potential (directly proportional to its molarity and sometimes called the osmotic potential and the S of pure water is zero) and P is the pressure potential.
Emphasize that the cell contents press against the cell wall producing turgor pressure. Ask students what happens if a plant loses turgor pressure. (It wilts)

4. Water Potential P is the physical pressure exerted on a solution.
It can be either positive or negative relative to the atmospheric pressure.
Water in a nonliving hollow xylem cells is under a negative potential (tension) of less than −2 MPa.
BUT the water in a living cell is usually under positive pressure due to the osmotic uptake of water.
Emphasize that the word “potential” refers to water’s potential energy which is water’s capacity to perform work when it moves from a region of higher water potential to a region of lower water potential.  = S + P where  is the water potential, S is the solute potential (directly proportional to its molarity and sometimes called the osmotic potential and the S of pure water is zero) and P is the pressure potential.

5.  Solutes have a negative effect on  by binding water molecules.
Pure water at equilibrium
H2O
Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm:
Pure water
Membrane
Solutes
Positive pressure has a positive effect on  by pushing water.
Positive pressure
Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm:
Solutes and positive pressure have opposing effects on water movement.
In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:
Negative pressure (tension) has a negative effect on  by pulling water.
Negative pressure
Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm: 
Water moves from regions of high water pressure to regions of low water pressure! We’ll take these scenarios one at a time
If a solute dissolves in water it is because it is either ionic or polar in nature. Either way the solute particles find water very attractive and set up intermolecular attractive forces which result in less “free” water which reduces the capacity for water to “move, flow or do work”. This is a negative effect, hence the negative sign convention on the number. A 0.1 M sugar solution has a S of −0.23 MPa. So, as the solute concentration or molarity increases, so does the negative value of S. Why does the water move to the right side of the U-tube? Osmosis!
Campbell Figure 36.8 Effects of solutes and pressure on water potential () and water movement.

6.  Solutes have a negative effect on  by binding water molecules.
Pure water at equilibrium
H2O
Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm:
Pure water
Membrane
Solutes
Positive pressure has a positive effect on  by pushing water.
Positive pressure
Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm:
Solutes and positive pressure have opposing effects on water movement.
In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:
Negative pressure (tension) has a negative effect on  by pulling water.
Negative pressure
Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm: 
 If we apply a mechanical pressure to the right side of the U tube (in the absence of solutes), then the water is mechanically forced to the left side of the U-tube.
Campbell Figure 36.8 Effects of solutes and pressure on water potential () and water movement.

7.  Solutes have a negative effect on  by binding water molecules.
Pure water at equilibrium
H2O
Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm:
Pure water
Membrane
Solutes
Positive pressure has a positive effect on  by pushing water.
Positive pressure
Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm:
Solutes and positive pressure have opposing effects on water movement.
In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:
Negative pressure (tension) has a negative effect on  by pulling water.
Negative pressure
Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm: 
 Adding solute to the right side of the U-tube AND applying a positive mechanical pressure to the right side as well (like the one the cell wall applies to the plant cell) can balance the pressure since they have opposing effects on the pressure.
Campbell Figure 36.8 Effects of solutes and pressure on water potential () and water movement.

8.  Solutes have a negative effect on  by binding water molecules.
Pure water at equilibrium
H2O
Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm:
Pure water
Membrane
Solutes
Positive pressure has a positive effect on  by pushing water.
Positive pressure
Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm:
Solutes and positive pressure have opposing effects on water movement.
In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:
Negative pressure (tension) has a negative effect on  by pulling water.
Negative pressure
Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm: 
 Negative pressure (tension) as occurs during transpiration, causes a negative pressure. It is important to note that the water potential of air is EXTREMELY negative (about −65 MPa) and since water moves toward the more negative potential, transpiration is favored!
Campbell Figure 36.8 Effects of solutes and pressure on water potential () and water movement.

9. Water Potential vs. Tonicity
Connect this new concept of water potential to the more familiar concept of tonicity (which should be prior knowledge). Ask students leading questions to see what they know about the movement of water across the cell membrane AND how the resulting size and shape of animal vs. plant cells change.
Next, ask them to compare the water potential of the solutions depicted in the bottom 3 graphics.
Figure (c) has an intermediate amount of solute particles pictured in the solution surrounding the cell AND there appears to be an equivalent amount of solute particles within the cell so the water potentials for both have reached equilibrium (isotonic; water potential for both environments is negative but nearly equal in magnitude thus the net rate of water movement is about equal).
Figure (d) has more solute particles within the cell (thus lower potential) compared to fewer solute particles (thus higher potential) in the solution surrounding the cell. Therefore, water moves in through the semipermeable membrane and bursts the cell wall if it is damaged or weakened. (hypotonic; a lower water potential inside the cell since more solute particles are present than outside the cell. Water moves from higher potential to lower potential.)
Figure (e) has far more solute particles in the surroundings (thus much lower potential) than present within the cell. Water moves out of the cell, it shrivels (leaving the cell wall intact). Emphasize that figure (e) represents plasmolysis when this occurs in plant cells. (hypertonic; higher water potential outside the cell than inside the cell. Water moves from higher potential to lower potential.)

10. Once More With Feeling!
Initial conditions: cellular  greater than environmental 
0.4 M sucrose solution:
Initial flaccid cell:
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
P = 0
 S = −0.7
 P = 0
 S = − 0.9
 = − 0.9 MPa
 = − 0.7 MPa
 = − 0.9 MPa
Ask students to explain this diagram. Don’t settle for just the “what” answers, draw the “why” answers out of them!
Start easy…examine the initial conditions. Which water potential is higher? The INITIAL cell at -0.7 MPa or the 0.4 M sucrose solution at -0.9 MPa? The cellular potential is higher. Uh, oh! This is where the trouble starts! Negative numbers, ugh! Ask students to think about the number line with zero in the middle. The “higher or greater” potential is the one closest to zero on the number line.
Typical student response: The cell loses water and plasmolyzes. (not good enough)
Ask the student “WHY?” Because, water moves from high potential to low potential. (still not good enough)
Ask the student “What data are you basing that answer upon?” The fact that the water potential for the cell is MPa vs. the surrounding solution with a cell potential of -0.9 MPa, water moves out of cell. (almost good enough, answer is quantified)
Ask the student “So…what does that mean?” Water moves from an area of high water potential (−0.7 MPa) to an area of low water potential (−0.9 MPa), therefore water moves out of the cell into the surroundings and plasmolyzes. (much better when put all together!)
After plasmolysis is complete, the water potentials of the cell and its surroundings are the same.

11. Last Time, I Promise!
Initial conditions: cellular  less than environmental 
Distilled water:
Initial flaccid cell:
Turgid cell
at osmotic equilibrium
with its surroundings
 P = 0
 S = − 0.7
 S = 0
 P =
 = − 0.7 MPa
 = 0 MPa
 = − 0 MPa
Round 2, ask students to explain this diagram as well. Again, don’t settle for just the “what” answers, draw the “why” answers out of them!
Typical student response: The cell gains water and becomes turgid. (not good enough)
Why? The cell gains water and becomes turgid because the surrounding water potential is higher (0 MPa) than the cellular water potential (−0.07 MPa) and water moves from an area of high potential (surroundings) to low potential (cell), thus water moves into the cell making it turgid. (Better answer!)