Osmosis+&+Diffusion


 * OSMOSIS AND DIFFUSION LAB **
 * Problem:** Will glucose, starch, water or iodine pass through a semi-permeable membrane?


 * Theory:** In the cells of plants and animals, protoplasm is limited or restrained from the environment by the existence of a membrane. These membranes, by necessity, must be relatively permeable to assist in such life processes as respiration and excretion. Membranes must permit some substances to pass through, yet prohibit other substances from doing so. They may allow rapid diffusion of some substance or very slow diffusion of others. Diffusion is the movement of particles from where they are more concentrated to where they are less concentrated. Osmosis specifically refers to the diffusion of water through a membrane. Membranes can be referred to as differentially permeable, selectively permeable, or semi-permeable.

In this lab, you will perform an experiment that will illustrate diffusion and osmosis. You will be able to determine through your observations that in a mixture of substances some substances will diffuse through semi-permeable membranes and some will not.


 * NOTE:** Before doing this experiment note that the following solutions or items will be the tools used to indicate the presence of certain substances.

__Lugol’s Solution__ contains iodine, which is an indicator for the presence of starch. Its golden-brown color turns blue-black as a positive reaction to starch.

The __Glucose Testing Strips__ react quickly to small amounts of glucose. The reacting area is the small bright yellow rectangle on the very end of the plastic strip. A green color indicates a positive test for glucose. DO NOT TOUCH THE TESTING AREA OF THE STRIP WITH YOUR FIGERS BEFORE USING.

1 semi-permeable membrane 1 plastic cup 1 Glucose Test Strip (sectioned for the # of tests you need) 1 Stir Rod Tap Water Glucose and Starch solution
 * Procedure:** You will need:

1. Fill the plastic cup with water to within 5 cm of the top. Test the water for the presence of glucose by dipping the yellow reacting area of one of the test strips in the water. Remove the strip and observe any color change Record the following in your notebook: The color of the strip is _______________________. Is glucose present? ______________

2. Add 30 drops of Lugol’s Solution to the water in the cup and stir. Set aside for later use.

3. Thoroughly wet one end of the membrane with water from the **beaker or sink** and rub the end between the thumb and forefinger. This should open the tube. When opened, the tube should be thoroughly moistened by allowing water to run through it.

4. Tie a know VERY TIGHTLY close to one end of the tube.

5. Put a solution (3 ml of LIQUID STARCH into the tube through the open end, then add about the same amount (3 ml) of GLUCOSE SOLUTION) into the dialysis tubing.

6. Hold the top of the tube closed by pinching it together and RINSE THE TUBE (VERY IMPORTANT) under running water to remove any glucose or starch on the outside. Gently squeeze the tube to mix the contents.

7. Tie another knot VERY TIGHTLY just above the level of the solutions inside the membrane.

8. Carefully place the filled membrane into the cup of water containing the Lugol’s Solution. Use a fine-point permanent marker to mark the outside of the cup showing the level of the contents (water and tube) inside the cup. Also mark the level of contents in the tube on the outside of the membrane. After 4-5 minutes, look at the contents inside the membrane tube and at the liquid inside the plastic cup.

9. What changes (if any) do you notice:

a. Inside the membrane? b. To the water in the cup? c. Test the water in the cup for glucose. Results?

On the basis of the above tests and observations, you should be able to explain and prove the following:

1. What happened to the iodine that was in the cup? 2. How do you know? 3. What happened to the starch? 4. How do you know? 5. What happened to the glucose? 6. How do you know? 7. Did any water diffuse through the membrane? What is this called? 8. How do you know? 9. Name the material(s) that diffused through the membrane. 10. Name the material(s) that did not diffuse through the membrane. 11. Suggest a reason for the fact that some materials were able to mass through the membrane, while others were not.