Osmosis

• Water potential is the pressure created by water molecules in a solution
• Water potential of pure water is 0kPa
• When solute concentration increases, the solution's water potential decreases

  When solute concentration decreases, the solution's water potential increases
• Osmosis


  Osmosis is the net movement of water from an area of higher water potential to an area of lower water potential, through a selectively permeable membrane
• Osmosis is a passive transport mechanism because it doesn't require ATP
• Omosis in animal cells

  • When a cell has a water potential lower than the surrounding solution, osmosis causes the cell the swell
  • This causes the cell to burst and undergo lysis
• Osmosis in plant cells

  • The cell wall prevents the protoplast inside the cell from bursting
  • So the cell becomes turgid
• When a cell's water potential is higher than the surrounding solution, osmosis causes the cell to shrivel up

  In plant cells, the cell becomes plasmolysed
• When a cell has exactly the same water potential as the surrounding solution, water moves in and out of the cell at the same rate
• Isotonic solution

  • No net movement of water in or out of cells by osmosis
  • So cells don't gain water and undergo lysis
  • So cells don't lose water and shrivel
• Method: water potential of plant tissue

  1. Place plant tissue in solutions with different concentrations of solutes
  2. Record the change in mass
  3. Plot calibration curve
  4. Record the solute concentration where there is no change in mass of the plant tissue
  5. This solute concentration has the same water potential as the plant tissue
• Advantage: plants in hot dry climates to have a lower water potential in leaves

  • A lower water potential in the leaves means the leaves have a smaller water potential gradient with the surrounding air
  • This means that less water is lost in transpiration