Diffusion in a Liquid
1. Obtain a Petri dish and add enough water to cover the bottom.
2. Place the dish on a ruler so that the metric scale crosses the center of the dish.
3. Allow the water to remain still for one minute, then add a crystal of potassium permanganate to the center of the dish.
4. Measure how far the molecules diffused after 10 minutes. Distance should be measured from the crystal to the edge of the purple area (or diameter of the purple area/2).
5. Calculate the rate of diffusion per hour.
6. Observe the dish at the end of the laboratory period. Did the potassium permanganate diffuse throughout the entire dish by the end of the laboratory period?
Diffusion in a Gelatin
Several drops of dye have been added to tubes containing a clear gelatin.
1. Obtain one of these previously-prepared tubes and measure how far the dye diffused.
2. Record the number of hours that the dye has been diffusing.
3. Calculate the rate of diffusion per hour.
4. Obtain a tube that had been prepared last semester. What happened?
The center of cell membranes contains the nonpolar fatty acid tails of phospholipid molecules. Because of this large nonpolar area, charged particles and large polar molecules cannot diffuse across the membrane. Small polar molecules such as water can diffuse across the membrane.

Osmosis is the diffusion of water across a differentially permeable membrane (see "Diffusion" above).
It occurs when a solute (example: salt, sugar, protein, etc.) cannot pass through a membrane but the solvent (water) can. Water always moves from where it is most concentrated (has less solute) to where it is less concentrated.
In general, water moves toward the area with a higher solute concentration because it has a lower water concentration.

In the container on the left side of the diagram, water will enter the cell because it is more concentrated on the outside. In the center drawing, water is more concentrated inside the cell, so it will move out. If the solute concentration is the same inside as it is out, the amount of water that moves out will be approximately to the amount that moves in.
Osmotic pressure is the force of osmosis. In the diagram above, the cell on the left will swell. The pressure within the cell is osmotic pressure.
1. Cut a piece of dialysis tubing approximately 15 cm in length.
2. Moisten the tube with water and then clamp one end. Plastic or foam clamps may be used. Instructions for using the foam clamps are below.Click an image to view an enlargement.
3. Rinse the tubing under water so that it can be opened.
4. Fill the tube 1/2 full with 50% molasses solution.
5. Clamp the other end of the tube. Be sure that you leave plenty of room in the tube for water to enter.
6. Rinse the tube under water and let it drip for about 10 seconds to remove excess moisture.
7. Place a plastic weighing tray on the scale and zero the scale. Place the bag in the plastic and record its weight to the nearest 0.1 g. Do not place the bag directly on the metal weighing pan of the scale and do not drip liquids on the scale because this could damage the scale.
8. Place the bag in a beaker containing distilled water.
9. Weigh the bag again every 10 minutes (after 10, 20, 30 and 40 minutes). Be sure to use a plastic weighing tray and to zero the scale before placing the bag in the tray.
10. Record your data in the table in the answer sheet.
11. Plot your results using a computer graphing program such as Create A Graph.
12. Did the rate of gain appear to be constant? You can answer this question by seeing if the graph is a straight line.
13. What do you predict would happen to the bag after one day?
1. Cut two strips of potato about the size of a French fry. They should be no thicker than 0.5 cm.
2. Put one of the strips in a test tube that contains enough 10% NaCl to cover the potato.
3. Put the other strip in a test tube that contains enough distilled water to cover the potato.
4. Remove the strips from the test tubes after about 60 minutes and examine the potatoes. Is one of them limp? Is one firm? Record your observations in the table on the answer sheet.
5. Explain your observations on the answer sheet.
Below: The potato strip on the left was in fresh water and the strip on the right was in NaCl for 60 minutes.

Plasmolysis in Elodea
1. Prepare a wet mount of an Elodea leaf using 10% NaCl instead of water.
2. Observe the cells under scanning and low power immediately after you prepare the slide.
3. Keep the slide under the microscope and observe it periodically for 10 minutes. It may be helpful to use a brighter light to view the cells. As the cells contents shrink, the chloroplasts will appear to clump together.
4. Describe what happened to the cells.
5. Why did this happen?
Below: Elodea cells in 10% NaCl. 400X, 200X. Click on the photographs to view enlargements. Notice that the contents of the cells have shriveled into a ball. The cell wall is rigid and cannot shrink. Click here for further explanation.

Blood
1. Use a toothpick to place a small amount (approx. 1/2 drop) of sheep blood or rabbit blood on each of three slides. You may need to transfer the toothpick from the blood to the slide several times in order to get approximately 1/2 drop of blood but be careful not to use too much blood because there will be too many cells to see anything under the microscope.
2. Add a drop of 0.9% NaCl to one of the slides, a drop of 10% NaCl to a second slide and a drop of distilled water to the third.
3. Put a cover slip on each and observe the cells under high power beginning with the 0.9% NaCl slide..
4. Record your observations and an explanation why they occurred in the table in the answer sheet.
 | Rabbit blood mixed with 0.9% NaCl X 400. |
 | Rabbit blood mixed with 10% NaCl X 400. Notice that the cells appear to be caved in or otherwise distorted due to a loss of water. |