(Illustration taken from Google Images)

Our new topics include the principles of diffusion, osmosis, and active transport. Read “The Cell in Its Environment” in chapter 1 section 4 (pages 40 to 44) of your textbook Cells and Heredity).

Let’s review a few facts before you try some activities.

1. Molecules are always in motion, except theoretically, at a temperature of absolute zero. Thermal energy and the rate of molecular motion are directly proportional. As one increases, the other does too.

2. If molecules move more rapidly, their collisions will be more forceful and frequent. This will cause them to spread further apart. This is why molecules tend to spread from areas of greater concentration to areas of lesser concentration. This is called diffusion.

3. Selectively permeable cell membranes allow some molecules to pass through while restricting others.

4. Osmosis is the diffusion of water molecules through a selectively permeable membrane.

5. During active transport most of the molecules tend to move across a membrane from an area of lesser concentration to an area of greater concentration. Because this is the opposite of diffusion, energy is necessary for this movement.

Try these two demonstrations to see the effects of diffusion and osmosis.

Demonstration #1 -- Diffusion

1. Fill a clear drinking glass with fresh water.

2. Allow it to set for at least one minute.

3. Gently add one drop of blue food coloring.

4. Watch the food coloring spread rapidly due to movement of the falling drop.

5. Soon the energy from the fall will be diminished. Then, the slow, even spreading of the food coloring molecules will be caused by the diffusion of molecules. This will continue until the liquid becomes one shade of light blue. The molecules will continue to move but no further change in color will result.

Demonstration #2 -- Osmosis

1. Get two raisins. They should be about the same size and as similar as possible.

2. Placed one raisin on a paper towel.

3. Put the other raisin in a cup of fresh water for at least thirty minutes.

4. Remove the soaking raisin. Gently, pat it dry with another paper towel.

5. Then, place both raisins on the same dry paper towel.

6. Using a metal spoon, press the moisture out of both raisins.

7. As you probably expected, the raisin that was previously soaked released more water. The process of osmosis caused the cells in this raisin to absorb more water.

8. If you use two prunes, the results will be even more dramatic.