Diffusion is a process by which molecules move from a region of high concentration of those molecules to a region of lower concentration of those same molecules. Diffusion can take place from one region of an open space to another region in the same space, or across a partial barrier (for instance, the selectively permeable cell membrane) separating to spaces, or volumes.

A selectively permeable (SP) membrane is one that permits certain, typically small, molecules to pass through, but holds back other, typically larger, molecules. The plasma or cell membrane fits this category, as do certain artificial and natural membranes. An example of an constructed artificial SP membrane is cellulose dialysis tubing, which is essentially for sausage casings. High quality dialysis tubing is, of course, also used to filter waste products from the blood of patients with kidney failure. Cell membranes are more selective than this artificial membrane, however, as the cells have the ability to truly select what passes through, not just passively allow some materials to pass and others not.

Osmosis is a special case of diffusion involving the passage of solvent molecules (usually water) through a SP membrane. Osmosis takes place if the dissolved molecule concentration (solutes; for example, salts) is higher on one side of the membrane than the other. Osmosis is the net movement of water from the side of the membrane where solute concentration is lower (and therefore water concentration is higher) to the side of the membrane where solute concentration is higher (and therefore water concentration is lower) B in other words, diffusion of water. Another way of saying this is that water moves from the hypotonic (less solute) side to the hypertonic (more solute) side. This will cause an increase in fluid volume on one side of the membrane and a decrease in volume on the other. For example, if a cell (which contains numerous solutes too large to pass through the membrane) is placed in distilled (pure) water, then the cell=s volume will increase (the cell will expand).

An artificial Acell@ can be made from a piece of dialysis tubing by filling it with a solution and tying off both ends. Such a Acell@ should respond similarly to a normal cell when placed in various solutions. Osmosis (and diffusion of other particles small enough to pass through the membrane) will continue in such a system until a (dynamic) equilibrium is reached. Movement of water from one side of the tubing membrane to the other will result in a change in pressure (tonicity) exerted by water on the membrane, which will cause the cell to swell or shrink.

You will be asked to build three model Acells@ using dialysis tubing, which will be used to demonstrate osmosis. Each Acell@ will be constructed to contain a different solution, and all three will each be placed in a separate beaker of water. Weight of each Acell@ will then be recorded at regular intervals over the period of an hour, to see if any changes have occurred indicating a change in the volume of the cell. Two of the solutions that will be used inside the cells are a 15% sugar and a 5% sugar solution B these should indicate that osmotic rate (and pressure) is dependent on the strength of the concentration gradient across the membrane. Water will be the primary substance diffusing through the membrane, though is should be noted that, given enough time (longer than this lab period) the sugar (sucrose) will also diffuse through. The third Acell@ will contain water only (not a solution), and so should be isotonic with the water in the beaker.

Materials& Methods (Procedure):

1. Obtain three pieces of dialysis tubing.

2. Twist one end of each and tie a knot (as will be demonstrated).

3. Squirt one of the unknown solutions (A, B, or C) into this bag so that the bag is partially filled. Make sure you keep track of which bag contains which solution!!

4. Tie off the other end of each bag; try to avoid trapping air bubbles in the bag.

5. Cut off the excess string and tubing and rinse each bag with water.

6. Follow steps 1 - 5 with the other two bags/solutions.

7. Sponge excess water off of the outside of each bag. Make sure you keep track of which bag contains which solution (A, B, or C)!!!

8. Weigh each bag on the balance and record the weight (mass) in the table below to the nearest 0.1 gram. (This will be the reading for A0 min.@)

9. Place each bag in a beaker containing enough water to cover the bag. Label the beakers at this point (A, B, or C), making sure you keep track of which bag contains which solution!!

10. At ten minute intervals (for 60 minutes), remove each bag from its beaker, sponge, weigh, and record the weight in the table. Then immediately return the bags to their beakers.

11. Figure the cumulative percent weight change at each time interval (using the formula provided below) and record this on the table.

12. Plot the cumulative % weight change vs. time on a graph (as will be demonstrated).

13. Determine which solution is which, and indicate this.

You will be required to turn in the graph, plus include your determination of solution composition (for instance, ASolution A is water@). With each determination, you will need an explanation as to why you indicate that solution A, B, or C is the water, 5% sugar, or 15% sugar. This assignment is worth 25 points.

                                                                Cumulative %

Cumulative %     =         (Weight at a given interval - initial weight)       X     100

Weight Change                                     Initial weight

Constructing Graphs B Figures

Graphs, pictures, maps, etc. are all called figures in scientific literature. When constructing your graph, there are several items that must appropriately completed or you can have points taken off. All figures are labelled as a AFigure@, and also given an appropriate caption. An example for the graph you will generate in this assignment could be:

    AFigure. Percent change in weight observed in sixty minutes for three dialysis bags containing      different solutions.@

Captions must be descriptive; that is, they must describe something about what is contained within the graph (figure).

When constructing the graph itself, the independent variable goes on the horizontal axis and the dependent variable goes on the vertical axis. Each axis must be labelled, and include appropriate units for whatever the variable is that is represented on the axis. You will then plot appropriate points on the graph, and draw lines approximating the trends represented by the points. Do not connect all of the points, unless they all fall on a nice smooth curve; just eyefit the line for each bag in this experiment. Then label each line on the graph, or use a key to indicate which line represents which bag. It would be a good idea to plot the points for the different bags using either different symbols or different colored points, so that anyone can clearly differentiate between the points. I highly recommend you use graph paper, as you will also need to get the scaling correct as well (I will explain this in class). Computer generated graphs are okay, but be aware that a lot of programs explicitly connect the points, which could cost you grade points. If you so desire, you can put your results for each bag on different graphs, but this would reduce the comparative aspects of a single graph, and you would then have three separate figures (Figure 1, Figure 2, Figure 3) that you would have to label, plot, etc.