Osmotic, or Water Potential is simply a measure of the tendency for water to cross a selectively permeable membrane from low concentration to high concentration of solute. (or from high to low concentration of water) Distilled water has the highest potential (zero). When water has substances dissolved in it, the water molecules have less potential to move, and the osmotic potential is NEGATIVE.

Water molecules always move from less negative to more negative water potential. (or higher potential, to lower potential) Less NegativeMore Negative -200 MPa-400 MPa Remember…the “less” negative the number is, the greater the osmotic/water potential is.

What is the highest possible osmotic potential? What is the only fluid to exhibit this potential? Which of these two fluids has the greatest osmotic potential? A B -14 MPa -12 MPa

The osmotic potential of a cell is known as its WATER POTENTIAL. For animal cells, the water potential is the osmotic potential of the cytoplasm. An animal blood cell with water potential of –50 MPa is placed in a solution… Osmotic potential of the solution is -20 MPa. If the osmotic potential of the solution is less negative than the water potential of the cytoplasm (the solution is hypotonic), osmosis will occur, and water will move into the cell from the solution. This is known as endosmosis. The result will be haemolysis (the blood cell will burst, or “lyse”) -50 MPa -20 MPa

If the osmotic potential of the solution is more negative than the water potential of the cytoplasm in the animal blood cell, (ie…the solution is hypertonic), osmosis will occur and water will move OUT of the cell. This is called exosmosis. The result will be crenation (the cell will shrivel up) Water potential of cytoplasm= -50 Osmotic potential of solution = KPa -50 KPa

If the osmotic potential of the solution is the same as the water potential of the cytoplasm (the solution is isotonic), there will be no net osmosis Water potential of cytoplasm= -50 Osmotic potential of solution= -50 The cell is in dynamic equilibrium with its environment, and osmosis WILL occur, but no measureable difference on either side of the membrane will be noticed.

What is haemolysis, and when is it likely to occur? Describe the conditions under which exosmosis would occur. When water from a hypotonic environment moves into a cell, what type of osmosis has occurred? What happens when the cell’s cytoplasm has an osmotic potential of -25, and the environment does as well? What “state” is that cell and isotonic environment in?

In animal cells, the water potential is equal to the osmotic potential of the cytoplasm, but this is different in plant cells… Plant cells have a cell wall, which exerts an inward pressure when the cell is turgid. This is known as the pressure potential. This must be considered when calculating osmotic potential. A plant cell with water potential –50 is placed in a solution which has an osmotic potential of -20… The solution is hypotonic, so net endosmosis occurs and the cell becomes fully turgid. Animal cells, without a cell wall, would lyse, without that protection. -50 KPa -20 KPa

If the solution is hypertonic, net exosmosis occurs and causes plasmolysis (the cell membrane pulls away from the cell wall. The cell wall stays intact). Remember, animal cells will “crenate”, or shrivel, when placed in a hypertonic solution. This cell wall is very protective of plant cells, and they remain safe in most environments as a result! -80 KPa -50 KPa

If the solution in which the plant cell is placed is isotonic, no net osmosis occurs. The cell is not plasmolysed, but it is not fully turgid either. -50 KPa

Why is osmotic potential different in plant cells than it is in animal cells? Which solution has higher osmotic potential? When plant cells are placed in a solution with higher osmotic potential, what would you expect to occur? What is crenation, and what happens to plant cells instead of this? -50 KPa -40 KPa