Thursday, 17 October 2013



  • Osmosis is the net movement of water molecules from a region of high water potential (water concentration) to a region of lower water potential, across a partially permeable membrane as a result of random movement.
  • e.g Osmosis in the large intestine and in root hair cells.

Osmosis can be show in an experiment using Visking tubing:
  • A Visking tubing bag full of sucrose solution is placed into a beaker of water and a capillary tube is added to the Visking tubing.
  • Water molecules go into the bag as there is higher water potential outside the bag than in and they are smaller molecules. The sucrose molecules cannot get out of the bag as they are too big. The dilute sucrose solution then climbs up the capillary tube as the water enters.
  • The more concentrated sucrose solution added at the beginning the steeper the gradient and so water enters the Visking tubing faster.

Osmosis in potato chips:
  • Five potato chips are measured and put into test tubes of water and sucrose. Each test tube has a different amount of sucrose in it.
  • The potato chips are weighed before and after the experiment. This shows how much water has entered or left the chip. A graph is then drawn to show where the isotonic point is.

  • Water potential is a measure of the number of free moving water particles.
  • Non-free water molecules are attracted to solute molecules forming a shell.