1. Chapter 29
Water Potential

2. You Must Know
The role of water potential in predicting movement of water in plants.

3. Outside air  = -100.0 MPa Leaf  (air spaces) = -7.0 MPa
Figure 29.18
Xylem sap
Outside air 
Mesophyll cells =
MPa
Stoma
Leaf  (air spaces)
Water molecule =
-7.0 MPa
Atmosphere
Transpiration
Leaf  (cell walls)
Adhesion by hydrogen
bonding =
-1.0 MPa
Xylem
cells
Cell wall
Water potential gradient
Trunk xylem 
Cohesion
by hydrogen
bonding
−0.8 MPa 
Cohesion
and adhesion
in the xylem
Water molecule
Trunk xylem 
Root hair
−0.6 MPa 
Soil particle
Water
Soil 
Water uptake from soil
−0.3 MPa  3

4. Water potential is a measurement that combines the effects of solute concentration and pressure. Water flows from regions of higher water potential to regions of lower water potential.
To survive, plants must balance water uptake and loss. Water potential determines the direction of movement of water. Potential refers to water’s capacity to perform work. Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure.
Why can salty soil cause a water deficit in plants even though the soil has plenty of water? 4

5.   S  P  water potential
S = solute potential. It is directly proportion molarity
P = Pressure potential. It is the physical pressure on a solution.
Water potential is abbreviated as  and measured in a unit of pressure called the megapascal (MPa). Solute potential is also called osmotic potential.
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6.   0 MPa for pure water at sea level and at room temperature.
  S  P
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7. P =
Solution in an open container

8. s =

9. s
Ys = –iCRT
i = ionization constant (For sucrose this is 1.0 because sucrose does not ionize in water. For NaCl this is 2.0 because NaCl splits into Na+ and Cl- in water)
C = Molar concentration
R = Pressure constant (R = literbar/moleK
T = Temperature Kelvin (273 + °C)

10.  of the cell =  of the solution in the beaker
Given that the plant cell in the picture is maintaining a constant volume, what can you deduce about the water potential of the plant cell and the solution in the beaker?
 of the cell =  of the solution in the beaker
Video:

11. The tissue will ________ weight.
The net flow of water will be from the ________________ to the ________________
The tissue will ________ weight.
surrounding solution
tissue
gain

12. The tissue will ________ weight.
The net flow of water will be from the ________________ to the ________________
The tissue will ________ weight.
tissue
surrounding solution
lose

13. The tissue will ________ weight.
The net flow of water will be from the ________________ to the ________________
The tissue will ________ weight.
surrounding solution
tissue
gain

14. The tissue will ________ weight.
The net flow of water will be from the ________________ to the ________________
The tissue will ________ weight.
there is no net flow
stay the same

15. Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast.
Turgid
Flaccid
The protoplast is the living part of the cell, which also includes the plasma membrane. Water potential affects uptake and loss of water by plant cells. If a flaccid (limp) cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis. Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall. If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid (firm).
Turgor loss in plants causes wilting, which can be reversed when the plant is watered.
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16. Review
Water flows from regions of higher water potential to regions of lower water potential.
The net flow of water will stop when the water potential is equal.