13 Lab Protocol: Diffusion and Osmosis
Exercise 1: Observing semipermeable properties of dialysis tubing
Answer the following questions by discussing with your group members in your Student Data Sheet before beginning the experiment below.
- Calculate the molecular weight of a starch molecule with 50 glucose monomers: (C6H10O5)50.
- What is the molecular weight of the starch indicator potasium iodide (iodine) KI3?
- Which substance is larger? Which substance will move more easily through a semipermeable membrane?
- Form a hypothesis.
Dialysis tubing allows passage of small but not large solute molecules
- Prepare the beaker you will run this experiment in by adding 3 drops of iodine to 50 mL of water.
- Obtain a piece of dialysis tubing from a beaker of water and tie a knot on one end of a strip of dialysis tubing.
- Add 3 mL of 1% starch solution into the dialysis tubing and tie a knot on the other end.
- Rinse the bag in water to wash off any starch on the outside of the bag before immersing the bag into the beaker you prepared in step 1.
- Incubate for 30 or more minutes and observe the outcome. Compare the outcome to your previous predictions.
Exercise 2: Observing the effect of tonicity on osmosis
- Add 200 mL water to one beaker and 200 mL 5% sucrose to another beaker. Label each beaker with tape.
- Prepare 6 dialysis bags (pre-soaked in water) each containing 5 mL of the following solutions (2 each): 1% sucrose, 5% sucrose, 20% sucrose. After tying the second end, immerse each dialysis tube in water for a few seconds one at a time, blot dry on a paper towel, and weigh to the nearest 0.1 g. Record the weight of each tube on a data table.
- Place one bag of each type in each beaker and start a timer. After 15 minutes remove the bags from each beaker, blot dry and weigh.
- Return bags to the proper beakers. After an additional 15 minutes, remove bags from beakers, blot dry and weigh.
- Record your results in your Student Data Sheet.
Exercise 3: Observing the effect of tonicity on animal cells
- Each group will label 3 test tubes: 0% NaCl, 0.9% NaCl, 5% NaCl.
- Add 1 mL of the appropriate NaCl solution to each tube
- Add 50 µL sheep blood to each tube and mix gently.
- Describe the appearance of each tube.
- Transfer 1 drop of each tube onto a microscope slide and place a cover slip.
- View under a light microscope using the 63X (or 40X) objective lens to visualize cells. View the 0.9% NaCl tube first.
- Take a photograph and upload it to the Photo Drop. Describe what you see.
Exercise 4: Determine the concentration of solutes in a plant tissue
This week, you will have the opportunity to plan and carry out your own experiment. Your goal is to identify the osmolarity of a vegetable of choice (identify an isotonic solution). You will first plan the steps of the experiment and explain them to instructor. Starting materials are: water, 10% NaCl stock solution, vegetable material and an electronic balance. Please use at least 2 -5 grams of vegetable slices per beaker. They should be uniformly cut – use a coring tool or a razor blade. Be sure to blot dry the slices before you weigh them.
Each group is to plan and carry out an independent experiment.
A variety of different plant specimens (each group will choose one). Potato, carrot, apple, celery,
Solutions with different NaCl concentrations
Balance (scale), weigh boat, razor blades, pipets, paper towels, forceps
Data Table to determine the osmolarity of _________________.
| Concentration of NaCl in beaker | Initial weight of vegetable | Weight after 15 minutes | Weight after 30 minutes | Weight after 45 minutes |
| 0% NaCl | ||||
| 0.9 % NaCl | ||||
| 5% NaCl | ||||
| 30% NaCl |
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