AP Biology: Tonicity and Osmoregulation

Key Concepts

• Tonicity: Refers to the ability of a solution to cause a cell to gain or lose water.

  • Isotonic Solution: No net movement of water; cell remains the same.
  • Hypotonic Solution: Water enters the cell; cell may burst (lysis in animal cells, turgid in plant cells).
  • Hypertonic Solution: Water exits the cell; cell may shrivel (crenation in animal cells, plasmolysis in plant cells).

• Osmoregulation: The control of water balance within a cell or organism.

Factors Influencing Osmosis

1. Solute Concentration: Determines the direction of water movement.
2. Physical Pressure: Also affects osmosis.

Water Potential

• A property predicting the direction of water flow.
• Formula: ( \Psi = \Psi_s + \Psi_p )
  • ( \Psi ) (Psi): Represents water potential.
  • ( \Psi_s ): Solute potential (concentration).
  • ( \Psi_p ): Pressure potential.
• Direction of Water Movement: Moves from regions of high water potential to low water potential.

Example Calculations

• Plant Cell Example:
  • If water potential outside is 0.5 megapascals and inside is 0, water moves into the cell.
  • If outside is -0.5 megapascals and inside is 0, water moves out, causing plasmolysis.

Solute Potential Formula: ( \Psi_s = -iCRT )

• Variables:
  • i: Ionization constant (number of ions formed).
    • Example: NaCl dissolves into 2 ions; i = 2.
  • C: Molar concentration (molarity, M).
  • R: Pressure constant (0.0831 L⋅bar/mol⋅K).
  • T: Temperature in Kelvin (Celsius + 273).

Important Notes

• Solute potential of pure water is 0.
• Pressure potential of a solution in open air is 0.
• Ionization constant of salt is 2; sugar is 1.

Example Problem Solutions

1. Water Potential Calculation:

   • Given: ( \Psi_p = 2 ) megapascals, ( \Psi_s = -3 ) megapascals.
   • Calculation: ( \Psi = \Psi_s + \Psi_p = -3 + 2 = -1 ) megapascals.

2. Direction of Water Flow:

   • Plant cell in a solution with ( \Psi_s = -5 ) megapascals.
   • Water moves from higher water potential (cell) to lower (solution), flowing out.

3. Complex Example:

   • Plant cell water potential = -3.5 megapascals.
   • Solution: 0.2 M NaCl at 20°C.
   • Calculate ( \Psi_s = -iCRT = -2 \times 0.2 \times 0.0831 \times (20 + 273) ).
   • Result: ( \Psi_{solution} = -9.7 ) megapascals.
   • Water flows out from the cell as ( \Psi_{cell} = -3.5 ) is higher.

Practice

• Continue practicing water potential calculations.
• Refer to provided formulas for AP exam.

Final Note

• Regular practice with these concepts and calculations will ensure readiness for exams. Consult instructors for any questions or additional assistance.