Lab: Diffusion and osmosis

This lab offers an opportunity to investigate the processes of diffusion and osmosis in a model membrane system, and in particular the effect of solute concentration on water potential as it relates to living plant tissues.

Prep

 * Review the concepts related to diffusion and osmosis using the website Lab 1: Diffusion and Osmosis (see Concepts 1-8). Note that we will be performing the experiments described in "Experiment 1" and "Experiment 3" on the website, but in reverse order.
 * Printout the instructions below for the two activities included in this lab.

Water Potential of Potato Cores
In this experiment you will use potato cores placed in different molar concentrations of sucrose to determine the water potential of potato cells.

Materials needed:

 * Clear plastic drinking cups
 * Sharpie Markers for labeling cups
 * Funnels
 * Distilled water
 * 1.0-molar sucrose solution (to be used as a stock solution. Use 342g sucrose/liter of water)
 * To make 250 mL of each molarity:
 * 1.0 M: use full strength.
 * 0.8 M: 200mL stock + 50mL water
 * 0.6 M: 150mL stock + 100mL water
 * 0.4 M: 100mL stock + 150mL water
 * 0.2 M: 50mL stock + 200mL water
 * Plastic wrap
 * Rubber bands
 * Cork borer (varying diameters for optional exercises)
 * Potatoes
 * Optional (for comparison): sweet potatoes, turnips, jicama, carrots, winter squash, apples, radishes
 * Electronic balance (triple beam will work)

Procedure:

 * 1) Pour about 100 mL of each of the following sucrose solutions, distilled water, 0.2 molar, 0.4 molar, 0.6 molar, 0.8 molar, and 1.0 molar, into separate plastic cups. Label cups.
 * 2) With the cork borer, cut 6 cylinders of potato from the center. Make sure there is no skin on the cylinders. Each cylinder should be about an inch long.
 * 3) Tare the balance and weigh the 6 cylinders QUICKLY. Record the mass and immediately place the potato into the appropriate cup. Cover the cup with plastic wrap, put a rubber band around the top of the cup, and let stand for at least 1 hour.
 * 4) After standing time complete, remove, gently blot, and determine the final mass of the cylinders. Record this mass and calculate the percent change:
 * $$percent \ change \ in \ mass = \frac{final \ mass - initial \ mass}{initial \ mass} \times 100$$
 * 1) Aggregate results across experimental trials, if there are others. (A trial is one repetition of the experiment.)
 * 2) Graph the results (molarity of sucrose solution versus percent change in mass).

Data table
Before beginning the experiment, design a data table to record the initial and final observations/measurements for each element in the experiment.

Analysis extension
Calculate the water potential of the potato cells. Note that the water potential of the solution at equilibrium will be equal to the water potential of the potato cells.

Diffusion and osmosis through a selectively permeable membrane
In this activity you will investigate osmosis and diffusion of small molecules through dialysis tubing, an example of a selectively permeable membrane.

Materials needed:

 * Distilled water
 * 15% glucose solution
 * 1% starch solution
 * Iodine, potassium iodide solution* (IKI)
 * Glucose testing strips
 * 250mL beaker or 9oz cup
 * Graduated pipette
 * Dialysis tubing
 * Goggles
 * Gloves


 * *Iodine, potassium iodide solution is harmful if inhaled or swallowed. It causes burns to skin and eyes. Goggles should be worn when using this solution. See links to Hands-on Science Project: Chemical Safety Database for further safety information:
 * iodine
 * potassium iodide

Procedure:

 * 1) Fill a 250 mL beaker or 9 oz cup two-thirds full with distilled water.
 * 2) Add 20 drops of IKI solution to the beaker/cup of water.
 * 3) Test the water for glucose by dipping a glucose test strip into the water. If glucose is present, the strip will turn purple in approximately 1 min. The glucose test strip will not change color when glucose is absent. Record your answer in your data table.
 * 4) Obtain a 30 cm piece of 2.5-cm dialysis tubing.
 * 5) Hold the section of dialysis tubing under running water, until it is pliable.
 * 6) Once the tubing is piable, tie off one end of the tubing to form a bag. (Be gentle, as the tubing tears easily.)
 * 7) Open the other end of the bag. (Rub the end between your fingers until the edges separate.)
 * 8) Place 10 mL of the 15% glucose solution and 10 mL of the 1% starch solution into the dialysis tubing.
 * 9) Test the 15% glucose/1% starch solution, in the dialysis tubing, for the presence of glucose using a glucose test strip. Record the results in your data table.
 * 10) Tie off the other end of the bag, leaving sufficient space for the expansion of the contents in the bag. (Again, be gentle as the tubing tears easily.)
 * 11) Record the color of the solution in your data table.
 * 12) Rinse the tube thoroughly to wash off any glucose or starch which may have spilled onto the outside of the tubing.
 * 13) Immerse the bag in the beaker/cup of water-IKI solution.
 * 14) Allow your set-up to stand for approximately 30 minutes or until you see a distinct color change in the bag or in the beaker.
 * 15) Record the final color of the solution in the bag, and of the solution in the beaker, in your data table.
 * 16) Test the liquid in the beaker and in the bag for the presence of glucose. Record the results in your data table.

Data table
Before beginning the activity, design a data table to record the initial and final observations/measurements for each element in the demonstration.

Some ideas for conclusion/discussion

 * 1) Which substance(s) migrated into or out of the dialysis tubing? Why?
 * 2) Which, if any, substance(s) did not diffuse through the membrane? Why?
 * 3) Did osmosis occur? How can you tell?
 * 4) How is it that dialysis tubing is similar to a plasma membrane, if at all?
 * 5) Molecules of similar substance are about the same size, whereas molecules of different substances are known to have different sizes. By looking at your results and determining which molecules did and did not diffuse across the dialysis membrane, can you determine the relative sizes of molecules, that is, which are bigger and which are smaller? Can all of the molecules be ordered by size?