osmosis

• osmosis - the net movement of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• water potential -

  • The ability of water to move from one solution to another (from a high to a lower water potential).
  • Pure water has the highest water potential! 
  • As we add solutes (substances dissolved in the water like sugar), the ability of water to move away is reduced.  Its water potential has been lowered!  It effectively has a lower concentration of water because it has a higher solute concentration.
• • If there is a soluble solute present e.g. salt or sucrose then water molecules stick to it.
  • This effectively stops them from moving freely
  • This means the more solute there is the lower the concentration of water (or the lower the potential of water to move)

  
• • Selectively permeable means it can control the passage of different substances in and out of the cell  e.g. through differences in size (sugar). Sugar for example is too big to enter the cell directly and so needs a carrier to carry the substance through.
• • Partially and semipermeable means it has gaps that are small enough to keep out most solutes but allows water through
• Isotonic - The osmotic concentration of solutes is equal in and out of the cell.
• Hypertonic - The osmotic concentration of solutes outside the cell is higher than inside the cell
• Hypotonic -The osmotic concentration of solutes outside the cell is lower than inside the cell
• • There is now a pressure pushing downwards on the water in the right tube.  This stops more water particles moving from the left and into the right side.
  • The 2 sides now have the same water potential – if you include the pressure of the container! 

  
• pressure potential is essential for plants -

  • So increasing the pressure on the water increases the water potential (even when solutes are present).
  • This idea is very important in plants that use water in their cells to maintain the plants structure.
• water potential in plant cells -

  • Water moves into plant cells via osmosis
  • This creates hydrostatic pressure
  • The rigid cell wall pushes back against this pressure; this is called pressure potential
  • The pressure potential increases if you add more water until they cancel each other out and no more water moves in
  • The cell is now said to be turgid (or in a state of tugor)

  
• plasmolysis -

  • In a hypertonic solution the plant cell will lose water
  • The cytoplasm and vacuole start to shrink away from the rigid cell wall – this is called incipient plasmolysis
  • Total plasmolysis is where the protoplasm (nucleus and cytoplasm) shrinks away from the cell wall due to severe water loss
• • Water potential – a measure of the potential for water to move out of a solution. Pure water = 0 therefore most solutions have a negative water potential
  • Turgor pressure – The inward opposing pressure caused by the cell wall resisting an expanding protoplasm (cytoplasm and nucleus) – usually positive
  • Osmotic potential – the potential of water to move across a partially permeable membrane – always negative

  
• The combination of the osmotic potential (water moving from the protoplasm) and the pressure potential (from the cell wall pushing back) = the water potential of the plant cell