Overview
This lecture explains water potential, including how it predicts water movement in and out of cells, the factors affecting it, and related formulas.
Water Potential Basics
- Water potential measures the potential energy of water per unit area compared to pure water.
- Represented by the Greek letter Ψ (psi), water potential indicates where water will flow.
- Water moves from areas of high water potential to areas of low water potential.
- Pure water has a water potential of zero bars; anything added lowers its potential.
Factors Affecting Water Potential
- Water potential is the sum of solute potential (Ψs) and pressure potential (Ψp): Ψ = Ψs + Ψp.
- Solute potential drops (becomes more negative) as solute concentration increases.
- Pressure potential is the physical pressure on water, measured in bars; positive pressure pushes water out.
- In osmotic situations (e.g., salt on a slug), water moves toward areas with lower water potential.
Water Potential in Biological Systems
- Water moves up plants due to a gradient of water potential: soil > roots > stems > leaves > atmosphere.
- Evaporation at the leaf surface creates a low water potential, pulling water upward from the soil.
Solute Potential Equation
- Solute potential (Ψs) is calculated using: Ψs = -iCRT
- i = ionization constant (1 for sugar, 2 for NaCl)
- C = molar concentration (moles per liter)
- R = pressure constant (0.0831 liter bar/mol K)
- T = temperature in Kelvin (C° + 273)
- An increase in solutes or temperature lowers the solute potential.
Example Problem Process
- Plug values into the Ψs equation; remember to convert temperature to Kelvin.
- Cancel units to end with bars as the measurement.
- For open containers, pressure potential is zero; overall water potential equals the solute potential.
Key Terms & Definitions
- Water Potential (Ψ) — Measure of water’s potential energy compared to pure water, predicts direction of water movement.
- Solute Potential (Ψs) — Effect of dissolved solutes on water’s potential; always negative or zero.
- Pressure Potential (Ψp) — Physical pressure on water; can be positive or zero.
- Ionization Constant (i) — Number of particles a solute splits into in water.
- Molarity (C) — Concentration of solute, measured in moles per liter.
- Pressure Constant (R) — A fixed value, 0.0831 liter bar/mol K.
- Kelvin (K) — Temperature scale used in calculations; K = C° + 273.
Action Items / Next Steps
- Review the equations for water and solute potential.
- Practice solving sample water potential problems using Ψ = Ψs + Ψp and Ψs = -iCRT.
- Watch the osmosis video if unsure about osmosis concepts.