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 + °C
Water Potential Ψ
• Once you have both pressure
potential and solute potential simply
add them together:
Ψ = ΨP + ΨS
• Make sure to include the units with
your answer
• 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°C in an open
system? The ionization constant of
sucrose is 1. Round your answer to
the tenths place.
Practice: What is the water potential of a plant
cell with 0.3 M sucrose solution at
20°C? The cell wall applies 2 bars
of pressure on the cellular solution.
Round your answer to the tenths
place.
Practice: Determine the net flow of water if the
plant cell from the second practice
problem is placed into the sucrose
solution from the first practice
problem what will be the net flow of
water? Justify your answer.