Unit 2 Topic 8 : Osmosis & Water Potential

Water Potential Equation

• Allows for quantification of water potential on BOTH sides of the membrane

• Dependent on 2 values:

  • Pressure potential

  • Solute potential

    **Water potential is their sum**

• Gives us a quantified way to determine where the water will move

• The water potential of pure (distilled) water is always 0

• WP = PP + SP

• Unit: bars

Pressure Potential

• How much pressure is being exerted on the cell or solution

• Exerted by either:

  • Atmosphere (for solutions in an open beaker)

  • Cell wall/membrane (for solutions inside of cells)

**For AP Bio purposes…they always set this equal to 0 UNLESS they tell you otherwise**

Solute Potential

• AKA Osmotic Potential/Osmotic Pressures

• How much solute is dissolved in the solvent (EX: water in the potato lab)

• ALWAYS a negative

  • Adding solute to a solution decreasing the water potential…always

• -iCRT

  • Ionization constant (# of ions the solute breaks into) x molar concentration x pressure constant (0.0831) x temp in Kelvin

    • Kelvin = celsius + 273

    • Covalently bonded molecules (like sugar) i = 1

    • Ionically bonded molecules (like NaCl) i = # of ions the compound breaks into in water (like 2 for NaCl)

      • Sucrose is always 1, salt is always 2