2.8 - Tonicity and Osmoregulation

Osmosis and Tonicity in Cellular Environments

• Osmosis Basics:

  • Definition: Osmosis is the passive transport of water through a selectively permeable membrane.

  • Water Movement: Water diffuses from an area of higher "free" water concentration (lower solute concentration) to an area of lower "free" water concentration (higher solute concentration). Essentially, water tends to "follow" the solutes.

• Tonicity and Its Effects:

  • Tonicity: Refers to the ability of a surrounding solution to cause a cell to gain or lose water, primarily based on the concentration of "nonpenetrating solutes" outside the cell compared to inside.

  • Isotonic Environment:

    • An animal cell in an isotonic solution experiences no net movement of water into or out of the cell. Water exchange is balanced; hence, the cell remains stable.

  • Hypertonic Environment:

    • In a hypertonic solution, the solute concentration outside the cell is higher than inside, causing water to move out of the cell, potentially leading to cell shrinkage or crenation.

  • Hypotonic Environment:

    • In a hypotonic solution, the solute concentration outside the cell is lower than inside, resulting in water moving into the cell, which can cause the cell to swell and possibly burst in the absence of structural protections.

• Cellular Responses in Different Environments:

  • Animal Cells:

    • Prefer isotonic conditions where the solute and water concentrations are balanced to maintain cellular integrity.

  • Plant Cells:

    • Thrive in hypotonic conditions where the uptake of water contributes to turgidity, an essential factor for structural support.

    • States:

      • Turgid: The ideal state for plant cells, where the cell is firm and healthy.

      • Flaccid: Occurs when cells lose water, becoming limp.

      • Plasmolysed: When the membrane pulls away from the cell wall due to severe water loss, causing the plant to wilt.

Understanding Water Potential in Cellular Processes

• Water Potential:

  • Definition: Water potential is a property that predicts the direction in which water will flow, incorporating factors like solute concentration and physical pressure.

  • Role in Plants: In plant cells, the central vacuole plays a vital role by exerting turgor pressure, helping to maintain the plant's structural stability.

• Water Potential Concepts:

  • General Principle: Water moves from regions of higher water potential to regions of lower water potential.

  • Solute Potential:

    • Represented as always zero or negative, influenced by the solute concentration.

    • Calculation Formula: Ψs​=−iCRT

      • i: Ionization constant (number of particles the solute dissociates into in solution)

      • C: Molar concentration

      • R: Pressure constant, 0.0831 liter MPa per mole K

      • T: Temperature in Kelvin (Celsius+273)

• Key Constants and Values:

  • Solute potential of pure water is 0.

  • Pressure potential of a solution in open air (like a beaker) is 0.

  • Ionization constant for salt (NaCl) is 2 (dissociates into Na+ and Cl-).

  • Ionization constant for sugar (e.g., glucose) is 1 (does not dissociate).