Exercise 8: Diffusion and Osmosis
Procedure 8.1: Brownian movement
This will be set up as a demonstration. Indian ink is used instead of carmine red. Observe the demonstration and answer the following questions:
1. Describe the movement of the pigment particles.
2. How does the movement of the particles change when temperature is increased?
3. What is absolute zero and why is it significant relative to diffusion?
Procedure 8.2: Diffusion as affected by molecular weight
This will be set up as a demonstration. The two compounds used in the demonstation are Potassium Permanganate (KmnO4) and Methylene Blue (C16H18CIN3S.3H20). Your lab book contains an incorrect molecular weight for one of these. Calculate the molecular weight of each one. Methylene Blue = ______Potassium Permanganate = ______
Observe the agar plates and answer the following questions:
1. Did the substance with the higher or lower molecular weight diffuse faster?
2. Do molecules stop moving when diffusion stops? Why? Hint: think about dynamic equilibrium. At what theoretical point do molecules stop moving (we discussed this in class).
Procedure 8.3: Diffusion across a selectively permeable membrane.
Important Note: Skip to procedure 8.4. After setting up procedure 8.4, return to procedure 8.3. Watch your time carefully to assure you do not miss any measurement points for 8.4. Throughout this experiment you will be tying knots in the dialysis tubing to form the bags instead of using string as directed in the lab book’s procedure. BE ESPECIALLY CAREFUL TO THOROUGHLY RINSE THE OUTSIDE OF THE DIALYSIS TUBING AFTER TYING THE KNOTS. Perform experiment as written. Note that dialysis tubing, unlike the plasma membrane, is selectively permeable based on size only. Record results in the following table
Beaker / Contents / ObservedColor / Cause of Observed Color Change
1 / 10 drops of 1 M NaOH
in 200mL of water.
Dialysis tubing with 3 drops of
Phenylphthalein in 10 mL of H2O
2 / 20-40 drops of iodine in 200 mL of water
Dialysis tubing with 10 mL of starch solution
Which of the following substances passed through the bag?
Phenolphthalein ______, NaOH ______, starch ______, Iodine ______
What determined whether a substance passed through the tubing?
You did not test for the movement of water through the membrane. Do you think water passed through the membrane? ______Why or why not?
Procedure 8.4 Osmosis along a free-energy gradient (from high to low concentration)
Follow procedure as written. Assume that the dialysis tubing is not permeable to sucrose.
Fill in the following table.
Changes in Weight of Dialysis Bags as a Result of OsmosisElapsed time / 0 min / 15 min / 30 min / 45 min / 60 min
Initial
weight / Weight / Weight*
change / Weight / Weight
change / Weight / Weight
change / Weight / Weight
change
Beaker 1
(50% sucrose) / Bag A
(1% sucrose)
Beaker 2
(1% sucrose) / Bag B
(1% sucrose)
Bag C
(25% sucrose)
Bag D
(50% sucrose)
*Weight change values should reflect the change in weight from the previous weight—not the total change in weight from the beginning of the experiment.
Answer the following questions:
1. Predict the net movement of water for each of the following:
Beaker 1 (50% sucrose)
Bag with 1% sucrose—into bag out of bag equilibrium
Bag with 70% sucrose-- into bag out of bag equilibrium
Bag with 50% sucrose-- into bag out of bag equilibrium
Beaker 2 (1% sucrose)
Bag with 1% sucrose—into bag out of bag equilibrium
Bag with 25% sucrose-- into bag out of bag equilibrium
Bag with 50% sucrose-- into bag out of bag equilibrium
2. Which of the experimental bags have the steepest concentration gradients relative to the surrounding environment?
3. Which of the bags should gain the most weight?
4. Which of the bags should lose the most weight?
5. Should the steepness of the concentration gradient affect the rate of diffusion? How? Does your data support this?
6. Will the bags eventually stop gaining or losing weight? When
Use the terms hypertonic, hypotonic, or isotonic to answer the following questions.
7. The solution in Bag A is ______to the solution in Beaker 1. Bag B is ______to Beaker 2. Bag C is ______to Beaker 2. Bag D is ______to Beaker 2.
8. You would expect water to move from a ______solution to a ______solution.
9. In solutions that are ______to one another there should be no net movement.
Procedure 8.5: Graphing you results
You can do this procedure on your own. The graph should be included in your formal lab write-up. While it is not a requirement, I recommend that you prepare the graph on your computer. This is an important skill and one I am sure you will use often in the future.
Procedure 8.6: Please omit this section
Procedure 8.7: Please omit this section
Procedure 8.8: Plasmolysis
This will be set up as a demonstration. Answer the following questions.
1. How will water entering the cell affect the size of the central vacuole?
2. What is plasmolysis?
3. When a cell is placed in a ______solution the central vacuole will shrink causing the cytoplasm to pull away from the cell wall.
4. What causes the crispness of celery and carrots? What causes them to wilt?
Draw sample cells from the elodea showing the effects you observed when the cell was placed in hypotonic, hypertonic, and isotonic solutions.
Author: Dr. Bette Jackson, FGCU, Ft. Myers, FL 33965