Tonicity and Osmoregulation Study Notes

Tonicity and Osmoregulation

Osmosis Review

  • Definition of osmosis: Movement of water across a semi-permeable membrane.

  • Visual representation of osmosis in cells.

Tonicity

  • Definition: Tonicity refers to the ability of an extracellular solution to cause a cell to either gain or lose water.

  • Key Factors:

    • Relies on the concentration of solutes that cannot pass through the cell membrane.

    • Types of solutions:

    • Isotonic: Equal solute concentration inside and outside the cell.

    • Hypertonic: Higher solute concentration outside the cell.

    • Hypotonic: Lower solute concentration outside the cell.

  • Osmoregulation: The process by which cells regulate solute concentrations and maintain water balance.

    • Different responses in animal cells vs. plant cells, fungi, and protists (which have cell walls).

Isotonic Solutions

  • Characteristics:

    • No net movement of water.

    • Solute concentration is equal inside and outside of the cell.

    • Example: Animal and plant cells exhibit stability and maintain shape.

Hypertonic Solutions

  • Characteristics:

    • Cells lose water to extracellular surroundings due to higher solute concentration outside.

    • Consequences:

    • Animal cells shrivel and may die.

    • Plant cells experience plasmolysis (vacuole shrinks, plasma membrane pulls away from the cell wall).

Hypotonic Solutions

  • Characteristics:

    • Cells gain water as solute concentration is lower outside.

    • Effects:

    • Animal cells may swell and lyse (burst).

    • Plant cells maintain turgor pressure, which is optimal for their function.

Water Potential

  • Definition: A physical property that predicts the direction of water flow, influenced by solute concentration and physical pressure.

  • Water flows from:

    • High water potential to low water potential.

    • Low solute concentration to high solute concentration.

    • High pressure to low pressure.

Water Potential Formula

  • Water potential () is given by the equation:  = s + p

    •  = Water potential (measured in megapascals (MPa) or bars).

    • ss = Solute potential (osmotic potential, always a negative number).

    • pp = Pressure potential (can be positive or negative).

    • Note: Pure water has s=0MPas = 0 MPa.

Solute Potential Calculation

  • Formula: s=−iCRTs = -iCRT

    • Variables:

    • ii = Ionization constant (number of particles formed in solution).

    • CC = Molar concentration (mol/L).

    • RR = Pressure constant (0.0831 liter bars/mol-K).

    • TT = Temperature in Kelvin (K = 273 + °C).

Practice Problems

  1. If NaCl concentration inside the plant cell is 0.15M and placed in 0.3M NaCl solution:

    • Answer: Water will diffuse out of the cell (into hypertonic solution).

  2. Solute potential of 0.3M sugar solution at 27°C:

    • s=−(1)(0.3)(0.0831)(300)s = - (1)(0.3)(0.0831)(300)

    • Calculation result: s=−7.48extbarss = -7.48 ext{ bars}.

  3. Water potential of the sugar solution from question 2:

    •  = -7.48 + 0 = -7.48 ext{ bars}.

  4. Solute potential of 0.2M NaCl solution at 30°C:

    • s=−(2)(0.2)(0.0831)(303)s = - (2)(0.2)(0.0831)(303)

    • Calculation result: s=−10.07extbarss = -10.07 ext{ bars}.

  5. Water potential for root tissue placed in 0.2M sucrose solution at 20°C:

    • Calculate ss:

    • s=−(1)(0.2)(0.0831)(293)s = - (1)(0.2)(0.0831)(293)

    • s=−4.87extbarss = -4.87 ext{ bars}.

    • Calculate water potential:

    •  = s + p = -4.87 + 0 = -4.87 ext{ bars}.

    • Conclusion: Water moves into the solution from root tissue.