Water Potential

Water Potential Overview

  • Definition: Water potential is the potential energy of water per unit area compared to pure water.

  • Measurement Unit: Measured in psi (Ψ) or bars.

    • Mnemonic: Think of Poseidon, the Greek god of the ocean, who carries a trident resembling the symbol for water potential.

Key Components of Water Potential

  • Formula: (Ψ = Ψ_s + Ψ_p)

    • Ψ_s: Solute potential (osmotic potential).

    • Ψ_p: Pressure potential.

Osmosis

  • Concept: Movement of water across a semipermeable membrane from an area of higher water potential (or concentration) to lower.

  • Example: Pouring salt on a slug causes it to shrivel as water moves out of the slug's cells due to osmosis.

  • Explanation of Movement:

    • When salt (sodium chloride) dissolves, it dissociates into ions (Na⁺ and Cl⁻).

    • These ions create a lower water potential outside the slug's cells (-40 bars) compared to inside (-5 bars).

    • Water flows from inside (higher potential) to outside (lower potential).

Water Movement in Plants

  • Example of Trees:

    • Distilled water poured at the base of a tree has a water potential of 0 bars.

    • Roots have a lower potential (-2 bars due to solutes), causing water to flow into the roots.

    • The water potential gradient continues to move water upward through stems and leaves due to evaporation (lower potential at leaves).

Understanding Solute and Pressure Potential

  • Solute Potential (Ψ_s):

    • Decreases with an increase in solute concentration.

    • Example: Adding sodium chloride reduces the solute potential, making it more negative.

  • Pressure Potential (Ψ_p):

    • Is the physical pressure inside the cell that affects water potential.

    • Positive value due to pressure exerted by the cell wall.

  • Combined Water Potential Calculation:

    • Example: If Ψ_s = -5 bars and Ψ_p = 2 bars, then (Ψ = -5 + 2 = -3 bars).

Solute Potential Equation

  • Equation: (Ψ_s = -iCRT)

    • i: Ionization constant (1 for sucrose as it doesn't ionize, 2 for sodium chloride).

    • C: Concentration in moles per liter.

    • R: Pressure constant (0.0831).

    • T: Temperature in Kelvin (Celsius + 273).

Example Problem

  • Situation: Calculate the solute potential for a 0.2 molar sucrose solution at 22 degrees Celsius.

    • Given Values:

      • (i = 1) (sucrose).

      • (C = 0.2) moles/liter.

      • (R = 0.0831).

      • Temperature: 22°C ➜ 295 K (22 + 273).

  • Calculation:

    • Begin substituting values into the equation (Ψ_s = -iCRT):

      • (Ψ_s = -1 imes 0.2 imes 0.0831 imes 295)

      • Calculate to find solute potential.

Summary of Water Potential

  • Water potential governs movement of water within biological systems.

  • Understanding osmosis, solute and pressure potential is crucial for applications like agriculture, plant biology, and ecology.