To demonstrate Osmosis.
Attach a semi-permeable membrane such as parchment paper or cellophane carefully over the mouth of a thistle-funnel. Invert, and fill the cup of the funnel with sugar solution. Then immerse the funnel in distilled water in a beaker. Clamp the arm of the thistle-funnel in a vertical position. Mark the level, and leave for some time.
The level of the solution in the funnel rises and the membrane bulges out.
This is because water has entered by osmosis. Permeable and semi-permeable membrances and the phenomenon of osmosis are found in cells. The wall of the plant cell is permeable, but the layers which form the protoplast and the lining of the vacuole are semi-permeable. Hence when a cell containing a concentrated solution is surrounded by pure water or a very weak solution, the water will penetrate the cell. The greater the difference between the concentrations of the inner and outer solutions, the more is the rate of entry. The concentration of a solution is a measure of its osmotic pressure, the force with which it can absorb water molecules when in contact with water.