1. Osmosis

2. Osmosis
Is the net movement of water from a high concentration to an area of low concentration through a semi-permeable (selectively / partially) membrane.
(It is passive – down a concentration gradient)

3. Something to think about!
Water molecules move in both directions across the membrane at the same time!
At any given moment more water molecules are moving in one direction rather than the other!
Osmosis is thus the net movement of water molecules!

4. Partically / semi / selectively permeable membrane!
Controls the passage of different substances in and out of the cell eg. through differences in size (sugar)! Too big to enter the cell directly and so needs a carrier to carry the substance through.

5. 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 has a lower concentration of water! It has a higher solute concentration!

7. Direction of
Water movement
Direction of
Water movement
Middle water potential
Highest water potential
Lowest water potential

9. Explain what is happening and why the water is moving as it does?
Both sides have reached equilibrium! Water potential is the same on both sides of the membrane

10. What would happen if I put a stopper in the side that is gaining water before both sides get to the same water potential? L R L R

11. How pressure effects water potential
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!

12. What would happen to the pressure if I pushed the stopper downwards into the water?
The water would move through the membrane and into the left side of the tube!
The pressure of the stopper increased the water potential on the right side and so the water moved down the gradient until the water potential balanced out on both sides!

13. 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.

14. Extension
Water potential. Water moves across a selectively permeable membrane from higher water potential to lower by osmosis.
Water potential (MegaPascal, or Ψ) is the sum of pressure potential (ΨP) and solute potential (ΨS):
Ψ = ΨP + ΨS.
The addition of solutes reduces water potential (to a negative value).
Application of physical pressure increases water potential.
The increased physical pressure can reverse the effects of the solutes .
A negative pressure reduces water potential (on the other side).