Osmosis

• Osmosis is the diffusion of water molecules across a partially permeable membrane from an area of higher water potential (higher concentration of water molecules e.g. dilute) to an area of lower water potential (lower concentration of water molecules e.g. concentrated)
• Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution
• Water molecules are small and can easily diffuse through the cell membrane, but large solute molecules cannot. Pure water has a water potential of zero. Adding solutes to pure water lowers its water potential - the the water potential of any solution is always negative.
• The more negative the water potential, the stronger the concentration of solutes in the solution
• If two solutions have the same water potential they are said to be isotonic. Cells in an isotonic solution will not gain or lose any water - there is no net movement of water molecules because there is no difference in water potential between the cell and surrounding solution.
• If a cell is placed in a solution that has a higher water potential, it will swell as water moves in by osmosis. Solutions with a higher water potential compared with the inside of the cell are called hypotonic.
• If a cell is placed in a solution that has a lower water potential it may shrink as water moves out of it by osmosis. Solutions with lower water potential than the cell are called hypertonic
• Factors that affect osmosis?

  1. water potential gradient - the higher itis, the faster the rate f osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases - rate of osmosis levels off
  2. thickness of exchange surface - thinner the exchange surface the faster the rate
  3. the surface area of the exchange surface - the larger the surface area, the faster the rate of osmosis