Lecture Notes: Water Potential

Introduction to Water Potential

• Water potential measures the potential energy of water per unit area compared to pure water.
• It helps determine water flow due to osmosis, gravity, pressure, and surface tension.
• Symbol for water potential is Ψ (Psi), resembles Poseidon's trident.

Components of Water Potential

• Water potential (Ψ) is equal to solute potential (Ψs) and pressure potential (Ψp).

Osmosis

• Osmosis is crucial for understanding water movement.
• Example: Salt on a slug causes water to move out due to changes in water potential.
• Water moves from an area of high water potential (inside the slug) to low water potential (outside).

Water Potential and Plant Physiology

• Pure water has a potential of 0 bars.
• Roots have a lower water potential due to solutes, causing water to flow into them.
• Water moves up a tree following a water potential gradient driven by evaporation at leaves.

Calculating Water Potential

• Water Potential = Solute Potential (Ψs) + Pressure Potential (Ψp).

Solute Potential (Ψs)

• Solute potential decreases with more solute (e.g., sodium chloride).
• Measured in bars, negative values indicate lower potential.

Pressure Potential (Ψp)

• Pressure potential arises from physical pressure exerted by cell wall.
• Positive values in bars reflect this inward pressure.

Solute Potential Calculation: Formula

• Formula: Ψs = -iCRT
  • i: Ionization constant (1 to 2), reflects ion formation in solution.
  • C: Molar concentration (moles per liter).
  • R: Pressure constant (0.0831 bar L/mol K).
  • T: Temperature in Kelvin (Celsius + 273).

Example Problem

• Calculate solute potential for a sugar solution:
  • Molarity (C): 0.2 M
  • Temperature (T): 22°C → 295 K
  • Ionization constant (i): 1 (sucrose doesn't ionize)
  • Solute potential example calculation yields ≈ -5 bars.

Overall Water Potential Example

• In an open beaker, pressure potential is zero.
• Overall water potential is equal to solute potential (-5 bars in example).

Conclusion

• Water potential helps in understanding water movement and plant physiology.
• Remember the trident symbol and the principles of osmosis and pressure to solve problems related to water potential.
• Poseidon analogy can aid in recalling these concepts.