Diffusion and Osmosis

Dialysis Tubing Investigation

Exploration:

Students are given time to think, plan, investigate, and organize collected information.

Objective:

To investigate the movement of various molecules across a selectively permeable membrane in order to describe how an organism maintains life and interacts with the environment.

Background Information:

A selectively permeable membrane allows only certain particles to pass through it. Like a cell membrane, dialysis tubing is made of a substance that is selectively permeable. Dialysis tubing is used in renal dialysis, a process that filters soluble waste materials from the blood in patients whose kidneys have failed. Removal of wastes is critical to maintaining a state of homeostasis within the body. Kidney patients will die without the ability to cleanse their blood artificially through renal dialysis.

Materials Needed:

5-inch length of dialysis tubing goggles

2 pieces of string Lugol’s iodine

1 clear plastic 10-16 oz. cup (or 250 mL beaker) tap water

1% glucose solution in dropper bottle glucose test strip or tape

1% starch solution in dropper bottle 10 mL graduated cylinder

Procedure:

1.  Fill your plastic cup or beaker about ¾ full with tap water.

2.  Observe the color of the water and test the water with the glucose test strip to determine if glucose is present. Compare the color of the test strip with the chart on the test strip package. A complete color change results in a positive test for glucose. Record your result in the Data Table.

3.  Add 20 drops of Lugol’s iodine solution to the water in the cup.

4.  Tie one end of the dialysis tubing with the string. Be sure the knot is very tight, so that no leakage occurs.

5.  Open the other end of the dialysis tubing by holding under tap water and rubbing it between your thumb and index finger.

6.  Use 1 graduated cylinder to measure 5 mL of starch solution. Pour this into the dialysis tubing. RINSE the graduated cylinder thoroughly in water.

7.  Use the second graduated cylinder to measure 5 mL of glucose solution and pour this into the dialysis tubing. RINSE the graduated cylinder thoroughly in water.

8.  Use a second piece of string to securely tie the open end of the tubing. You have now created a sealed sac that models a cell. The dialysis tubing represents the cell membrane of your model cell

9.  Rinse the dialysis tubing in water to remove any liquid that it clinging to the outside.

10.  Observe the color of the starch and glucose in the dialysis tubing and record your observation in the Data Table.

11.  Place the sac in the plastic cup/beaker.

12.  For several minutes, observe any changes in the color of the liquid inside the sac. Record the final color in the Data Table.

13.  Observe the color of the liquid in the cup. Record the final color in the Data Table.

Data Table:

Glucose Test / (+/-) / Solutions / Color
Water in cup (Step #2) / Initial color of
water + iodine in cup
Water in cup (Step #12) / Initial color of glucose + starch in dialysis tubing
Final color of water + iodine in cup
Final color of glucose + starch in dialysis tubing

DIFFUSION AND OSMOSIS LAB QUESTIONS:

  1. Where was the glucose at the beginning of the experiment?
  1. Where did you expect the glucose to be at the end of the experiment? Explain.
  1. Where was the iodine at the beginning of the experiment?
  1. Where did you expect the iodine to be at the end of the experiment? Explain
  1. Where was the starch at the beginning of the experiment?
  1. Where did you expect the starch to be at the end of the experiment? Explain
  1. Explain how you know the iodine molecules diffused in our lab while starch did not.
  1. Why did the starch molecules not diffuse?
  1. Explain how water is a special form of diffusion.
  1. What type of membrane is the dialysis tubing used in kidney machines? Remember that it selects what passes through it. Explain how this set-up works since it rids the body of wastes.
  1. Someone who drown in Utah’s Great Salt Lake. What would the body look like after 2 weeks? Explain.
  1. Pickles are made from cucumbers. They are soaked in various flavors in crocks or containers for long periods of time. Explain how the whole cucumber can eventually taste totally different from its taste after pickling.
  1. Compare and contrast diffusion and osmosis. This means to tell how they are alike and how they are different.