Water PotentialWater Potential

Osmosis & Plant cellsOsmosis & Plant cells

Plants & water potentialPlants & water potential

Plants can use the Plants can use the potential energy in potential energy in water to perform water to perform work.work.

Tomato plant Tomato plant regains turgor regains turgor pressure – cell pressure – cell pushes against wall pushes against wall due to uptake of due to uptake of waterwater

Plants & water potentialPlants & water potential

The combined effects of The combined effects of 1.) solute concentration1.) solute concentration 2.) physical pressure (cell wall)2.) physical pressure (cell wall)

can be measured as Water Potential can be measured as Water Potential == psi psi

is measured in megapascals (MPa)is measured in megapascals (MPa) 1 Mpa = 10 atmospheres of pressure1 Mpa = 10 atmospheres of pressure

Calculating Water PotentialCalculating Water Potential

= = PP + + SS

OrOr

Water = pressure Water = pressure + solute+ solute

Potential potentialPotential potential potential potential

Solute Potential Solute Potential S S

Solute potential is also called the osmotic Solute potential is also called the osmotic potential because solutes affect the direction of potential because solutes affect the direction of osmosis.osmosis.

S S of any solution at atmospheric pressure is always negative – why?

Answer = less free water molecules to do work

Solute Potential Solute Potential SS

Solutes bind water Solutes bind water molecules reducing molecules reducing the number of free the number of free water molecules water molecules lowers waters lowers waters ability to do work.ability to do work.

Pressure Potential Pressure Potential PP

P P is the physical pressure on a solution.

P P can be negative transpiration in the xylem tissue of a plant (water tension)

PP can be positive water in living plant cells is under positive pressure (turgid)

Standard for measuring Standard for measuring Pure water is the standard.Pure water is the standard.

Pure water in an open container has a Pure water in an open container has a water potential of zero at one water potential of zero at one atmosphere of pressure.atmosphere of pressure.

Water Potential: an artificial modelWater Potential: an artificial model

(a) addition of (a) addition of solutes on right side solutes on right side reduces water reduces water potential. potential. S S = -0.23= -0.23

Water flows from Water flows from “hypo” to “hyper”“hypo” to “hyper”

Or from hiOr from hi on left on left

to lo to lo on right on right

Water Potential: an artificial modelWater Potential: an artificial model

(b) adding +0.23 pressure with plunger (b) adding +0.23 pressure with plunger no net no net flow of waterflow of water

(c) applying +0.30 pressure increases water (c) applying +0.30 pressure increases water potential solution now has potential solution now has of +0.07 of +0.07

Water moves right to leftWater moves right to left

(d) negative (d) negative pressure or tension pressure or tension using plunger using plunger decreases water decreases water potential on the left.potential on the left.

Water moves from Water moves from right to leftright to left

Water Potential: an artificial modelWater Potential: an artificial model

Water relations in plant cellsWater relations in plant cells

(b) Flaccid cell in (b) Flaccid cell in pure waterpure water Water Water potential is into cellpotential is into cell cell becomes turgid cell becomes turgid

Water relations in plant cellsWater relations in plant cells

(a) Flaccid cell placed in (a) Flaccid cell placed in hypertonichypertonic solution solution Water potential is out of cell Water potential is out of cell plasmolysis plasmolysis

Calculating Solute potentialCalculating Solute potential

Need solute concentrationNeed solute concentration Use the equation Use the equation

S S = - iCRT= - iCRT

i = # particles molecule makes in wateri = # particles molecule makes in waterC = Molar concentrationC = Molar concentrationR = R = pressure constantpressure constant 0.0831 0.0831 liter barliter bar

mole mole ooKKT = temperature in degrees KelvinT = temperature in degrees Kelvin

= 273 + = 273 + ooCC

Solve for water potentialSolve for water potential(literal equation)(literal equation)

Knowing solute potential, water Knowing solute potential, water potential can be calculated by inserting potential can be calculated by inserting values into the water potential values into the water potential equation.equation.

= = PP + + SS

In an open container, In an open container, PP = 0 = 0

Hints & remindersHints & reminders

1. Remember water always moves from [hi] 1. Remember water always moves from [hi] to [lo].to [lo].

2. Water moves from hypo 2. Water moves from hypo hypertonic. hypertonic.3. [Solute] is related to osmotic pressure. 3. [Solute] is related to osmotic pressure.

Pressure is related to pressure potential.Pressure is related to pressure potential.4. Pressure raises water potential.4. Pressure raises water potential.5. When working problems, use zero for 5. When working problems, use zero for

pressure potential in animal cells & open pressure potential in animal cells & open beakers.beakers.

6. 1 bar of pressure = 1 atmosphere6. 1 bar of pressure = 1 atmosphere