Talk:School Science/Apparatus for demonstrating osmosis

Wikipedia edit history

 * 14:19, 2005 Feb 27 SimonP m (Category:Science experiments)
 * 23:18, 2004 Mar 3 Naddy m (typo)
 * 17:56, 2003 Nov 14 Reddi m (intro)
 * 17:53, 2003 Nov 14 Reddi m (img; See Also)
 * 13:14, 2003 Jul 6 MyRedDice m (osmosis demonstration)
 * 16:28, 2003 Jun 28 User:62.254.128.6
 * 16:25, 2003 Jun 28 User:62.254.128.6
 * 16:22, 2003 Jun 28 User:62.254.128.6
 * 23:06, 2003 Apr 16 Theresa knott m (Adding links to osomis with eggs and potatos)
 * 22:59, 2003 Apr 16 Theresa knott (Diagram plus instructions on how this simple apparatus is assembled and used )

The opening paragraphs are simply wrong. Although Visking is a partially permeable membrane the molecular weight cut-off of Visking tubing is generally rated at 12-14kD. Sucrose or most other sugars would easily pass through this membrane which is why it is used as a model of the gut. That is starch is broken down by enzymes in the tube and sugars are allowed to pass through the membrane wall.

The osmosis experiment relates to something slightly different. It is the movement of water from a high water concentration to a low one, aiming ultimately to equilibrium. Should there be a higher concentration of sugar solution inside of the membrane then the tendency would be for the water molecules to come into the membrane and a rise in the level of liquid in the capillary tube, in plant cells this would result in turgidity. Although the osmotic potential (into the membrane rather than out) may stop the sugars escaping they are small enough to come out and the dyes may well be seen in the outside bath. Similarly if the tubing contains a smaller concentration of sugars on the inside than the outside one would expect water to come out of the membrane leading to flaccidity but the concentration on the outside needs to be substantially higher and the sugars may still go through.