# Water Potential student

```Water Potential
What you need to know!
• The role of diffusion (osmosis),
active transport, and bulk flow in the
movement of water and nutrients in
plants.
• How water potential explains
transpiration.
Water Potential Ψ
• Potential Energy of Water
• Determines direction of water flow
• Determined by two factors: pressure
potential (ΨP) and solute potential
(ΨS).
• The most common unit for water
potential problems is bars.
• The mathematical equation is:
Ψ = ΨP + ΨS
Pressure Potential ΨP
• Pressure potential will always be
provided for you
– Open systems are systems w/out
pressure potential
– Often cell walls will provide pressure to
a cellular example
• When not given you may assume it
is an open system
– ΨP = 0 bars
Solute Potential ΨS
• ΨS = -iCRT
• i = ionization constant
– Measure of how easily the solute breaks into ions
in water
– The value is always given and is normally either 1
or 2
• C = molar concentration of solute
– This is a measurement of the density of molecules
of the solute
• R = Pressure constant
– This will always be the same no matter what:
0.0831 L bars/M K
• T = temperature in Kelvin
– The temperature will always be given
– Sometimes it will be given in Celsius and you will
need to convert it into Kelvin: K = 273 + &deg;C
Water Potential Ψ
• Once you have both pressure
potential and solute potential simply
Ψ = ΨP + ΨS
• Make sure to include the units with
• Water will always flow from higher
water potential to lower water
potential
Practice: What is the water potential of a 0.5 M
sucrose solution at 20&deg;C in an open
system? The ionization constant of
the tenths place.
Practice: What is the water potential of a plant
cell with 0.3 M sucrose solution at
20&deg;C? The cell wall applies 2 bars
of pressure on the cellular solution.