Question

Tie a piece of semipermeable membrane around the mouth of the thistle tube using a tight rubber band or string. Dissolve a teaspoonful of sucrose in about 20 milliliters of water. Invert the sealed thistle tube into a beaker containing water and hold it securely using a ring stand and clamp. You may have to use some tissue around the clamp to secure the tube tightly without breaking the glass. Pour the sugar-water solution into the stem of the thistle tube so the level of the sugar water within is even with the level of the water in the beaker. Set this experiment aside until the next day. Answer the following questions: What do you observe after twenty-four hours? What two molecules (molecular species) are involved in this experiment? In which region was the concentration of water greater—inside or outside the thistle tube—when you first set up the experiment? Explain. Based on what you have learned about membranes and osmosis, how could you explain the results of this experiment?

Answer 1

1. After twenty-four hours, the water level is higher on the side with sugar than on the side without sugar.

2. Sucrose (C12H22O11 ) and water (H2O)

3. The concentration of water is higher outside the thistle tube at the start of the experiment. That side of the membrane is 100% water, while the inside of the membrane is a solution of water and sugar.

4. Water moves through the semipermeable membrane from high concentration to low concentration to reach equilibrium in a process called osmosis. The sucrose cannot pass through the membrane because the molecules are too large, but water molecules are small enough to permeate the membrane. By the end of the experiment, the water has moved through the membrane into the thistle tube so that the sugar has been diluted with more water and the concentrations are equal.