| Diffusion | Osmotic Pressure Protocol | Transpiration |

We have seen how DIFFUSION is behind the movement of all molecules and small particles in the universe. We will now proceed on a pathway to cells. In particular we are interested in learning about membranes. So let us consider the effects of placing a simplified membrane between two different solutions and see what happens. Imagine the membrane as a sort of grill or fence with small holes in it. In the first case, the components of the solution are able to fit through the holes. Of course, this will slow down the diffusion of the components through the membrane. Additionally, if water is on both sides of the membrane, the Laws of Thermodynamics dictate that the total system - the solutions on both sides of the membrane - will strive to mix and reach equilibrium (i.e.: to become the same). The process of the diffusion of molecules through such a membrane is called OSMOSIS or DIALYSIS.

A very interesting and ultimately useful case arises in a multicomponent dialysis system when only certain components (including water) can pass through the holes of the membrane, while other, very large components cannot pass through. The system seemingly cannot obey the laws of thermodynamics, but, upon further inspection, it does. How? Well, wait for that answer until we get there further on. We will look at osmotic pressure inside of membranes shaped like sausages, which are closed systems - AND inside that sausage are molecules too large to squeeze through the membrane's holes.

So let's get started! Your instructor will demonstrate to you the mechanics of using dialysis tubing. This is a semi-permeable "cellophane" (cellulose acetate) membrane through which small molecules can pass but larger ones (approx. 10,000 dal and above) are unable to penetrate. This makes for a simplistic model of a cell membrane. (Real cell membranes differ in that their membrane surfaces are covered with charged molecules - usually negatively charged.) "Reverse osmosis" is also used for desalinization of sea water, and regular osmosis is used even with gases as "fuel cells" produce electrical energy. See stocks such as BLDP, FCEL and PLUG.)

DIALYSIS

1. OUTSIDE of the tubing (see figure, below):
   1. To 675 ml 22% sucrose solution add 75 ml of 1% starch solution (resulting in a solution that is 20% sucrose and 0.1% starch); test a very small amount of this for a positive iodine reaction. It should give a dark color.
   2. Place this solution in a liter graduated cylinder, and attach a bubbler to continuously mix the fluids in the cylinder. If excessive foaming results, add a small amount of ethanol.

2. Put these INSIDE the dialysis tubing (see figure below):
   1. 250 ml of 0.1% starch solution; at this point, take a small sample and test with iodine. It should give dark reaction complex.
   2. YELLOW food coloring
   3. Approx. 3 to 5 ml of Alpha-Amylase (ginger root aqueous extract): add this at the very last moment before sealing the tubing; thoroughly rinse bag under running water to wash away any exterior dribbled amylase
   4. Tie one end of a 2 ft long string to the knot at one end of the dialysis sausage, and tie a pencil to the other end of the string.

   

3. Using the string, lower the filled and tied dialysis bag into the cylinder, and make sure that the rising bubbles are massaging the side of the dialysis bag. (Note that the pencil prevents the string from slipping into the cylinder.)

4. Either add to or subtract some of the exterior liquid so that the total volume (sausage plus liquid in cylinder) is 1,000 ml.

5. At timed intervals,* fish out the bag to read and record the volume of the cylinder.

6. At this point while you are not too busy and only taking occasional volume readings, begin reading the next part of today's lab assignment.

7. At the end of osmosis lab, test the fluids both inside and outside the bag for the iodine stain.

8. Data collection:
   • Plot the bag's volume with respect to time.
   • Follow the yellow's diffusion from the bag
   • At the end, how did the two starch analyses go (inside vs outside)?

RAMIFICATIONS

Write a short summary of the experiment and discuss what happened and the ramifications.

| Supplies | Osmotic Pressure Protocol | VAST-2001 |