WATER POTENTIAL

Osmosis• Osmosis = the movement of water molecules from a

region of higher water potential to a region of lower water potential through a partially permeable membrane. Osmosis is considered in terms of water potential and solute potential.

Water Potential• Water potential = a measure of the kinetic

energy of water molecules. Here, water molecules are constantly moving in a random fashion. Some of them collide with cell membrane, cell wall, creating a pressure on its known as water potential.

• The higher their kinetic energy the more they move and hit the membrane, therefore higher the water potential

Water potential

Why does water move?How does water move?

1.

2.

3.

Downhill

Fresh – salty

Hose, straw

4. Sponge

Pressure potential

Osmotic/solute potential

Matric potential

Components of Water Potential

1. Pressure potential: pushing (positive pressure, like the hose)

sucking (negative pressure, like a straw) Major factor moving water through plants

2. Osmotic, or Solute potential: reduction in water potential due to the presence of dissolved solutes

Dissolved substances dilute pure water, so

salty water has lower water potential (lower concentration) than pure water

Water Potential• Unit of measurement: megapascals (Mpa)

1 MegaPascal = 10 atm = 145.1 psi

• Water potential for pure water: 0

• Anything that lowers the “free energy”of water lowers it potential.

-dissolved solutes

Water potential = pressure potential + solute potential

Clarifying Water Potential Values

• (2) Factors to consider:

p = pressure potential (outside & inside)

s = solute potential

system = p + s

SO……

p results in “+” value

p results in “-” value

Water Potential Values• High water potential (+Value):

- less solute

- more water

- (hypotonic)

• Zero (0) Value:

- Pure water

• Low water potential (-Value):

- More solute

- less water

- (hypertonic)

****Water will move across a membrane in the direction of the lower water potential****

Calculating Solute Potential

• Variables involved: i, C, R, T

i = ionization constant: NaCl = 2.0 (Na+ & Cl-)

**for sucrose it will be 1.0 (it doesn’t ionize)

C = Molar concentration of your potato (graph)

R= rate constant: 0.0821 L · atm (bar)

mol · K

T = Temperature: K

Calculating the Solute Potential

(s) s = - iCRT• Sample Calc.

A 1.0 M sugar solution @ 22° C under standard atmospheric conditions:

s = -(1)(1.0mol)(0.0821 L · bar )(295K)

L mol · K

s = -24.22 bars

Typical Water Potential Values

• Outside air (50% humidity): -100 MPa• Outside air (90% humidity): -13 MPa• Leaf Tissue: -1.5 MPa• Stem: -0.7 MPa• Root: -0.4 MPa• Soil water: -0.1 MPa• Hydrated soil (Saturated) +2 - +5 MPa

** When the soil is extremely dry what happens to the water potential and water movement into the plant?

**Does the value become more negative or more positive?

Water Potential in Plants

Water Balance (pg. 117-118)• Osmoregulation~ control

of water balance

• Hypertonic~ higher concentration of solutes

• Hypotonic~ lower concentration of solutes

• Isotonic~ equal concentrations of solutes

• Cells with Walls:• Turgid (very firm)

• Flaccid (limp)

• Plasmolysis~ plasma membrane pulls away from cell wall

Dialysis Tubing Experiment

An Artificial CellPermeable to: monosaccharides & water

Impermeable to: Disaccharides