Osmosis

• • Osmosis is the diffusion of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential.
  • Pure water has a water potential of zero. Adding solutes to pure water lowers its water potential — so 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're said to be isotonic. Cells in an isotonic solution won't lose or gain any water - there is no net movement of water particles because there's no difference in water potential between the cell and the surrounding solution
• • If a cell is placed in a solution that has higher water potential it will swell as water moves into it by osmosis. Soultions with 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 a lower water potential than the cell are called hypertonic
• Factors affecting the rate of osmosis

  • The water potential gradient — the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time.
  • ƒThe thickness of the exchange surface — the thinner the exchange surface, the faster the rate of osmosis.
  • ƒThe surface area of the exchange surface — the larger the surface area, the faster the rate of osmosis.