osmosis

• osmosis is the net movement of water from a region of higher water potential to a region of lower water potential, across a partially permeable membrane
• water moves down the water potential gradient
• water potential is measured in units of pressure (kPa)
• pure water has a water potential of zero
• more solute decreases the water potential, making a more concentrated solution
• water potential of a solution will always be less than zero
• osmosis in animal cells: hypotonic solution

  • less negative ψ outside the cell
  • more negative ψ inside the cell
  • so water moves into the cell by osmosis
  • across the partially permeable membrane
  • cell swells -> lysis
• osmosis in animal cells: isotonic solution

  • ψ is equal inside and outside the cell
  • so there is no net movement of water
  • so cell stays the same
• osmosis in animal cells: hypertonic solution

  • less negative ψ inside the cell
  • more negative ψ outside the cell
  • so water moves out the cell by osmosis
  • across the partially permeable membrane
  • cell crenation occurs
• osmosis in plant cells: hypotonic solution

  • less negative ψ outside the cell
  • more negative ψ inside the cell
  • so water moves into the cell by osmosis
  • across the partially permeable membrane
  • cell become turgid
• osmosis in plant cells: isotonic solution

  • ψ is equal inside and outside the cell
  • so there is no net movement of water
  • so cell stays the same
• osmosis in plant cells: hypertonic solution

  • less negative ψ inside the cell
  • more negative ψ outside the cell
  • so water moves out the cell by osmosis
  • across the partially permeable membrane
  • cell plasmolysis occurs
• plant cells in a hypotonic solution do not burst as the cell wall is fully permeable therefore can withstand the pressure inside the cell
• when a plant cell is in a hypertonic solution the protoplast becomes incipient