Osmosis Experiments (Cambridge O Level Biology)

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

Osmosis Experiment Diagram

Potatoes are usually used in osmosis experiments to show how the concentration of a solution affects the movement of water, but radishes and carrots can be used too

• 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 and has a higher water potential than the plant tissue (which is more concentrated)
  • The plant tissue will become turgid
    • This is because water molecules, inside the cell, push the cell membrane against the cell wall, increasing the turgor pressure in the cells which makes them turgid 
• 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 and has a lower water potential than the plant tissue (which is more dilute)
  • The plant tissue will become flacid
    • This is because the cell membrane is pulled away from the cell wall and the cell can no longer support itself; the cell is said to be plasmolysed
• 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

Investigating osmosis using dialysis tubing

• Dialysis tubing (sometimes referred to as visking tubing) is a non-living partially permeable membrane made from cellulose
• The tubing can be used to model and investigate the process of osmosis outside of a cellular environment
• Pores in this membrane are small enough to prevent the passage of large molecules (such as sucrose) but allow smaller molecules (such as glucose and water) to pass through by diffusion and osmosis
• This can be demonstrated by:
  • Filling a section of dialysis tubing with concentrated sucrose solution
  • Suspending the tubing in a boiling tube of distilled water for a set period of time
  • Noting whether the water level outside the tubing decreases as water moves into the tubing via osmosis
    • Water moves from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

Osmosis Experiment Diagram

An example setup of a dialysis tubing experiment