Lab #: ______ The Movement of Water Through Cell Membranes
Date:_______
Purpose: To observe both macroscopically and microscopically how living cells are affected by the
movement of water through the cell membrane (osmosis)
All living organisms are made up of cells. The cell membrane provides a barrier between the cell and it’s
environment. In part 1 of this lab you will observe how movement of water through the cell membrane can
change the firmness, size, and mass of living tissue.
Living organisms remain alive only if they are able to maintain a proper chemical balance within their cells.
If this balance is upset the organism is disturbed, begins to function poorly, and may even die. In part 2 of
this lab you will observe what changes result inside elodea cells (the cells of a common water plant) when
they are placed in a salt solution.
Procedure – Part 1
1. Use a cork borer to obtain several cylinders of potato, each with a diameter of 0.5 cm.
2. Remove all peel from each of the cylinders. Then carefully use a razor blade to cut 9 identical cylinders
(#1 – 9) each 2 cm long.
3. Feel the potato pieces and record your observations on their initial crispness in your data chart.
4. Place pieces #1 – 3 in a weigh boat and find the combined mass of the pieces. (You do not need to
mass the pieces individually. Place all 3 in the weigh boat together.) Record the initial total mass.
5. Place pieces #4 – 6 in a weigh boat and find the combined mass of the pieces. (You do not need to
mass the pieces individually. Place all 3 in the weigh boat together.) Record the initial total mass.
6. Place pieces #7 – 9 in a weigh boat and find the combined mass of the pieces. (You do not need to
mass the pieces individually. Place all 3 in the weigh boat together.) Record the initial total mass.
7. Obtain 3 petri dishes.
8. Use masking tape to label 1 dish tap water, 1 dish 5% salt, and 1 dish distilled water.
9. Fill each dish with the appropriate solution.
10. Place potato cylinders #1 – 3 in the tap water dish.
11. Place potato cylinders #4 – 6 in the 5% salt dish.
12. Place potato cylinders #7 – 9 in the distilled water dish.
13. Make sure all potato pieces are covered by their solution. If they are not, add more of the
appropriate solution.
14. Cover the dishes and let them sit for at least 30 minutes (or overnight). (Go on to Part 2 during this
waiting period)
15. After the 30 minutes (or the next day), remove potato cylinders #1 - 3 and place them on a piece of paper
towel to blot the excess water.
16. Find the final total mass of cylinders #1 - 3 and record it in your data chart.
17. Feel the potato cylinders and write down your observations on their final crispness.
18. Repeat steps 15 – 17 for cylinders #4 – 6.
19. Repeat steps 15 – 17 for cylinders #7 – 9.
Procedure – Part 2
1. Make a wet mount of a leaf from the growing tip of an elodea plant. Study it briefly under low power of
your microscope, then change to high power. Carefully, study the cells near the midrib of the leaf. Note
the position of any organelles that are visible. Make a careful, detailed sketch of a few typical cells on
high power. Be sure to label your sketch with the name of the sample and magnification.
2. Carefully remove the coverslip, use a paper towel to absorb the water and replace it with two drops of a
5% salt solution (95% water). Replace the coverslip and let the slide stand for a minute or two. Observe
the cells again under high power. Be careful to note any differences from the way the cells looked in
your original wet mount. Make a careful, detailed sketch of a few typical cells on high power. Be sure to
label your sketch with the name of the sample and magnification.
3. Now remove the coverslip again and absorb the salt water with a fresh piece of paper towel. Gently use a
pipette to rinse the salt solution away with tap water. Also rinse the coverslip well. Add a few drops of
distilled water and blot again. Add 2 more drops of distilled water and replace the coverslip. Wait a few
minutes, then make a careful, detailed sketch of a few typical cells on high power. Be sure to label your
sketch with the name of the sample and magnification.
Questions – Part 1
1. For each group of potato cylinders, use your knowledge of osmosis to predict the motion of water
through the cell membranes (into or out of the cells) that you would predict would occur. Explain each
prediction.
2. Did the mass of any of the potato cylinders change during the experiment? If so, describe the change for
each group (i.e. did it increase or decrease?).
3. Did the crispness of any of the potato cylinders change during the experiment? If so, describe the change
for each group.
4. Tell if your results support each prediction you made in question 1 or refute it. Explain by citing specific
data about changes in mass, size, and texture to support your answer.
Questions – Part 2
1. What changes did you noticed in the appearance of the elodea cells throughout the lab? Be specific,
referring particularly to the position of the chloroplasts as seen in each solution.
2. Based on your knowledge of osmosis, draw diagrams to illustrate how you believe water moved (into or
out of) the elodea cells in each step.
3. What do you think would happen to the elodea cells if they were left in the salt solution for several
hours? Explain.
4. What do you think would happen to a living bacteria or mold cell if it were placed in a 5% salt solution
for several hours? Explain.
5. Salt solutions are sometimes sprayed on paths to kill weeds. Based on your hypothesis from question #3,
why do you suppose this practice is successful?
6. Strawberry jam contains a very high concentration of sugar (and therefore a low concentration of water).
How might this sugar help to prevent the jam from spoiling? (Hint: Food spoils when bacteria or mold
cells begin to grow in the food. Think about your answer to #4.)
7. Based on your observations in part 2 of the lab, predict what you would have seen if you had looked at
potato cells from the piece that had been soaked in salt water in part 1 under the microscope. (Answer in
terms of how spread out or clumped together you would expect any visible organelles to be.)