Osmosis

All cells are surrounded by a cell membrane which is partially permeable

• Water can move in and out of cells by osmosis

• Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane

• In doing this, water is moving down its concentration gradient

• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Osmosis Experiments

Immersing plant cells in solutions of different concentrations

• The most common osmosis practical involves cutting cylinders of root vegetables such as potato or radish and placing them into distilled water and sucrose solutions of increasing concentration

• The cylinders are weighed before placing into the solutions

• They are left in the solutions for 20 - 30 minutes and then removed, dried to remove excess liquid and reweighed

If the plant tissue gains mass:

• Water must have moved into the plant tissue from the solution surrounding it by osmosis

• The solution surrounding the tissue is more dilute than the plant tissue (which is more concentrated)

• If plant tissue loses mass:

  • Water must have moved out of the plant tissue into the solution surrounding it by osmosis

  • The solution surrounding the tissue is more concentrated than the plant tissue (which is more dilute)

• If there is no overall change in mass:

  • There has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal

  • Remember that water will still be moving into and out of the plant tissue, but there wouldn’t be any net movement in this case