Osmosis and Water Potential

  1. Osmosis is the diffusion of water down its concentration gradient across a selectively permeable membrane

  2. Water is moving from an area of low solute (left hand side) to and area of high solute (right hand side) across the semi permeable membrane. This increases the level of water on the right hand side of the U-Shaped tube.

  3. Tonicity: the ability of an extracellular solution to cause a cell to gain or lose water

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

    • Cells can be in three types of solutions:

      • Isotonic

      • Hypertonic

      • Hypotonic

    • Osmoregulation: cells must be able to regulate their solute concentrations and maintain water balance

      • Animal cells will react differently than cells with cell walls, like plants, fungi, and some protistsCells immersed in an isotonic solution have no net movement of water

        • The concentration of nonpenetrating solutes inside the cell is equal to that outside the cell.

          • Water diffuses into the cell at the same rate water moves out of the cell, resulting in a stable internal environment that supports cellular functions.

          • Cells immersed in a hypertonic solution lose water to their extracellular surroundings

            • The concentration of nonpenetrating solutes is higher outside of the cell

              • Water will move to the extracellular fluid

                • Cells shrivel and die

Cells immersed in a hypotonic solution gain water

  • The concentration of nonpenetrating solutes is lower outside of the cell

    • The cell will gain water 

      • Animal cells swell and lyse

      • Plant cells work optimally

Water potential: a physical property that predicts the direction water will flow

  • Includes the effects of solute concentration and  physical pressure

    • Water will flow from areas of:

      • high water potential to areas of low water potential

      • Low solute to areas of high solute concentration

      • High pressure to areas of low pressure

Solute potential (osmotic potential)

  • An increase in solutes causes binding to more free water

    • This reduces water potential

  • Expressed as a negative number

  • Pure water is 0 MPa

  • 1MPa = 10 bars

Pressure potential

  • The physical pressure on a solution

  • Can be + or - relative to atmospheric pressure

  • “Open air” means 𝚿p= 0 MPa

  • Pressure constant

    • 0.0831 liter bars/mol-K

    • 0.00831 liter MPA/mol K