Aimee Papa
10/14/21
Laboratory 5: Diffusion and Osmosis - Transport Across Cell
Membranes
Objectives:
 To understand the mechanisms used to move molecules across membranes including simple
diffusion facilitated diffusion, osmosis, and active transport.
 To apply the principles of molecular movement across membranes to living plant and animal cells.
Introduction
Through a series of exercises, the principles of moving molecules across selectively permeable
membranes will be studied. Each exercise will reveal a different, yet related, aspect of transportation
into and out of cells. Bear in mind that these principles are put into practice continually by cells trying to
function and maintain homeostasis in an ever- changing environment. Please review the concepts of
diffusion, osmosis, and active transport in Chapter 5 of your text.
In part A of the lab, we will demonstrate osmosis using potato cylinders placed into different
concentrations of salt. Depending on whether the solutions are hypotonic, isotonic, or hypertonic, the
osmosis will occur into or out of the potato. Measure the weight of the potato before and after
submersion in the solution will allow us to determine the direction of water movement.
In part B of the lab, we will determine the effect of time on diffusion using colored agar blocks.
The colored agar blocks will incubate in vinegar for different amounts of time. At each time point, an
agar block will be removed, sliced in half, and the distance of diffusion of the vinegar determined.
Part C will allow us to microscopically observe the changes due to a hypotonic, isotonic, and
hypertonic solutions on the structures of both plant and animal cells.
In part D will model the difference between facilitated diffusion and active transport using a kit
with pipe cleaners and beads representing molecules of interest.
Procedure
Part A: Osmosis
1.
Obtain five test tubes and five cylinders of potatoes.
2.
Label the test tubes A through E.
3.
Describe the appearance and texture of the potato cylinders on the In-Lab Report Sheet.
4.
Weigh a potato cylinder and place it in one of the test tubes. Record the weight of the potato in
Table 1 of your In-Lab Report Sheet.
5.
Repeat step 4 for all remaining potato cylinders.
6.
In test tube A, add enough distilled water to completely cover the potato.
7.
In test tube B, add enough 0.5% NaCl solution to completely cover the potato.
8.
In test tube C, add enough 1% NaCl solution to completely cover the potato.
Aimee Papa
10/14/21
9.
In test tube D, add enough 2% NaCl solution to completely cover the potato.
10. In test tube E, add enough 5% NaCl solution to completely cover the potato.
11. Let the potatoes sit for one hour.
12. Remove the potatoes from the test tubes, pat dry with a paper towel, and weigh each potato
cylinder, recording the results in table 2 of the In-Lab Report Sheet.
13. Calculate the percentage change in weight % 𝐶ℎ𝑎𝑛𝑔𝑒 = 100 ×
(𝐹𝑖𝑛𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡−𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡)
𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡
14. Record these values on the In-Lab Report Sheet.
15. Describe the appearance and texture of each potato cylinder on the In-Lab Report Sheet
16. Discard potatoes in labeled trash container on counter and pour salt solutions down the sink.
Part B: Diffusion
1. Obtain one tub and 4 agar cubes. Unwrap the plastic wrap from the cubes and discard.
2. Disperse the agar cubes in the tub and pour enough vinegar in to completely cover the cubes.
3. Please wear gloves when handling the agar cubes. After the cubes have been in the solution for 15
minutes, remove one agar cube, slice it in half, and using a ruler, measure how far from the edge
of the cube the vinegar has travelled. (The cubes start out blue because they contain bromophenol
blue, a pH indicator. They turn yellow as the vinegar comes in contact with the dye.) Record your
data. Use mm, not inches. Record the data on Table 2 on the In-Lab Report Sheet.
4. Repeat step 3 at 30 minutes, 45 minutes, and 60 minutes. (While you are waiting proceed to other
parts of the laboratory).
5. Calculate the rate of diffusion at each time point by dividing the distance traveled by the vinegar in
mm by the total time in minutes. Record on Table 1 on the In-Lab Report Sheet.
6. Discard the agar cubes in the trash and pour the vinegar down the sink.
Part C: Comparing animal and plant cells exposed to hypotonic and hypertonic solutions
1. You will view animal blood cells in three different kinds of solutions: pure blood, blood with 10%
NaCl added, and blood with 0.9% NaCl added. These will be set up for you at demo scopes.
 Sketch several red blood cells from each slide on your In-Lab Report Sheet. Notice the size and
shape of the cells – not the amount of cells on the screen!
2. Obtain three clean slides and label them A, B, and C. Divide and conquer: have each person in your
lab group prepare one of the three slides and view with the microscope.
3. Prepare the plant slides as follows:

Slide A: place an Elodea leaf on a slide and add a drop of solution A (100% DI water). Add a
cover slip. Observe the cells under the highest magnification. Sketch several cells on your In-Lab
Report Sheet. Notice the size of the vacuoles and the distribution of chloroplasts. Can you see
any of the chloroplasts moving?
Aimee Papa
10/14/21

Slides B: Follow the same procedure as Slide A above, using solution B (10% NaCl) instead. Let
the slide set for 5 minutes before viewing under the microscope.

Slide C: Follow the same procedure as Slide A above, using solution C (0.9% NaCl) instead.
4. Clean up: place all slides in the “used slides” container. Wash hands thoroughly and put
microscopes away. Wipe down the surface of your lab counter with detergent to clean up any
solutions that might have leaked or spilled.
Part D: Modeling membrane transport
1. Your model kit contains small beads, pipe cleaners, and some small square “solutes.”
2. The small white ball represents oxygen. The beads with bent pipe cleaners running through them
are phospholipids – you should have 8 of them. Arrange them into a bilayer.
3. Model the movement of oxygen across the membrane by diffusion– does it need a protein “gate”?
Why or why not? If so, construct one using a different colored pipe cleaner.
4. Answer the questions in Part D on your in-lab report sheet
5. Use your kit to model facilitated diffusion, using your solutes and bilayer and by making a protein
gate using other pipe cleaners in the kit. What kinds of molecules pass through a membrane by
facilitated diffusion?
6. Now use your kit to model active transport. You will need to construct a protein transporter using
some of the extra pipe cleaners in your kit. Since active transport requires energy, represent the
cellular energy requirement (ATP) by requiring that you must physically change the shape of
(work=energy input) the transporter before the molecules can pass through. Use your book to help
you.
7. Call your instructor over when each member of your group is ready to demonstrate how each of
the processes – diffusion, facilitated diffusion, and active transport.
8. Have your instructor initial your In-Lab Report Sheet
Aimee Papa
10/14/21
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Aimee Papa
10/14/21
Lab 5: Diffusion & Osmosis In-Lab Report Sheet
Your name: Aimee Papa
Section____________
Your group members’ names:
______________________________________
______________________________________
______________________________________
______________________________________
Part A: Osmosis in potatoes
1. Describe the initial appearance and texture of the potato cylinders.
2. Make a hypothesis for this experiment:
3. Describe the final appearance and texture of each potato.
A.
B.
C.
D.
E.
Aimee Papa
10/14/21
Concentration of
NaCl (%)
Initial weight
(gm)
Final weight (gm)
% change in
weight
0
4.39g
4.72g
7.5%
0.5%
4.64g
4.78
3.0%
1%
4.45g
4.46g
0.22%
2%
4.50g
4.18g
-7.1%
5%
3.94g
3.18g
-19.3%
Table 1. Osmosis in potatoes due to incubation in different concentrations of table salt. Potato
cylinders were incubated in different concentrations of NaCl for 1 h. Record the weights of the
potato cylinder before and after the incubation here.
4. What does the sign of the % change indicate (what is the difference between positive and negative
values)? Positive values means that there is a percentage increase in mass of the potato. A negative
values mean that there is a the percentage decrease in the mass of the potato.
5. Why is it important to use “% change” rather than just the difference in weight?
6. Which concentration(s) of NaCl were hypotonic to the potato?
7. Which concentration(s) of NaCl were hypertonic to the potato?
8. Graph the data. Using your graph, find the concentration of solution that would be isotonic to the
potato. Mark the point on the graph that corresponds to an isotonic concentration.
9. Based on your graph, what concentration of NaCl is isotonic to the potato cells?
10. Thoroughly explain why we see a change in the appearance and textures of the potatoes after
incubation in the different solutions (Describe the movement of water).
Aimee Papa
10/14/21
Part B : Diffusion
Time (min)
Distance (mm)
Rate (mm/min)
15
4mm
0.267 mm/min
30
6mm
0.2 mm/min
45
7mm
0.156 mm/min
60
8mm
.133 mm/min
Table 2. Diffusion of vinegar into an agar cube. Agar cubes were incubated at room temperature with vinegar for
the indicated amount of time. Record the results here.
11. How do you know that diffusion occurred (visually, what happened)?
12. Did the rate of diffusion change with time? Describe what happened to the rate over time..
Part C: Comparing animal and plant cells exposed to hypotonic and hypertonic solutions
13. Sketch a few red blood cells under the highest magnification in the circles below. Be sure to depict
their overall size and shape (of their membranes), as compared to one another. Ignore the
differences in the NUMBER of cells on the slide & the swirling movement of the water (this has
nothing to do with osmosis)
Blood Only
Blood in 10% NaCl
Aimee Papa
10/14/21
Blood in 0.9% NaCl
14. Which of the three solutions was hypertonic to the red blood cells? Thoroughly explain your
answer, describing the movement of water in or out of the cells, and the overall appearance of the
cells.
15. What would happen to a patient’s red blood cells if they were given intravenous medications that
were dissolved in pure water rather than physiological saline (0.9% NaCl)?
16. Sketch a few Elodea cells under the highest magnification in the circles below. Be sure to show
the overall size and shape of the cells. Also notice the distribution of chloroplasts and the size of
the vacuole.
Slide A: 100% DI H20
Aimee Papa
10/14/21
__________________X
Slide B: 10% NaCl
_________________X
Slide C: 0.9%NaCl
____________________X
17. Explain what happened to the cells that were placed in 10% NaCl, and why.
18. Animal cells and plant cells appear different from each other when exposed to hypotonic or
hypertonic conditions. What difference between plant and animal cells accounts for this?
19. Elodea is a plant that has evolved in pond water. Thus we should expect it to be fairly well-adapted
to its habitat. Would you expect pond water to be isotonic, hypo-tonic, or hyper-tonic to Elodea
cells and why?
Part D: Modeling Membrane transport
20. Can oxygen can diffuse freely through the plasma membrane? Why or why not? What chemical
property allows this to occur?
Aimee Papa
10/14/21
21. Instructor’s Initials: