Osmosis is defined as the net movement of free water molecules through a partially permeable membrane down a water potential gradient. It occurs when a cell is placed in a solution with a different osmotic concentration - the concentration of solutes that affect water movement. If the solution has a lower osmotic concentration than the cell (hypotonic), water will move into the cell. If it has a higher concentration (hypertonic), water will move out. Net water movement is always from the hypotonic to the hypertonic solution.

1. Osmosis: a special case of
diffusion
Chapter 2A

2. Learning objectives
 Understand what is meant by osmosis in
terms of the movement of free water
molecules through a partially permeable
membrane, down a water potential
gradient.

3. What is osmosis?
 Can be defined as the net movement of
free water molecules through a partially
permeable membrane, down a water
potential gradient.
 Solvent = water, partially permeable
membrane = cell membrane or organelle
membranes

4. Water potential
 A measure of the
potential for
water to move
out of a solution
by osmosis.
 Measures the
concentration of
free water
molecules.

5. Osmotic concentration
 Osmotic concentration- a measure of
the concentration of the solutes in a
solution that have an osmotic
effect.(soluble substances in water)

6. Modelling osmosis in cells

7. Osmotic concentrations
 Isotonic solution= the osmotic
concentrations are the same, no net
movement of water
 Hypotonic solution(dilute solution)= the
osmotic centration is lower
 Hypertonic solution(concentrated
solution)=the osmotic concentration is higher

8.  Net movement of water is always from
HYPOTONIC to HYPERTONIC solutions.

9. Osmosis in animal cells

10. Osmosis in plant cells