Name_______________________________________________
Osmosis in Plants Biology
LAB
Problem
What effect does the concentration of solutes outside of plant cells have on the cells?
Introduction
The main ingredient in most eye drops is saline solution, which is a solution of salts dissolved in
water. You might think that salt water would irritate the eyes. But saline solution and cells at the
surface of your eyes are isotonic, which means that they have the same concentration of solutes.
Because saline solution and the cells in your eyes are at “equal strength,” using eye drops does not
change the balance of water in the eyes. What happens when solutions are not isotonic with the cells
that they bathe?
In this lab, you will make a wet mount of red onion cells and observe the cells. Then, you will
compare the effect of a 10 percent salt solution and of distilled water on the onion cells.
Skills Focus
Observe, Compare and Contrast, Draw Conclusions
Materials
• red onion
• forceps
• metric ruler
• scissors
• paper towel
• iodine solution
• microscope slide
• coverslip
• dissecting probe
• microscope
• 10% salt solution
• distilled water
Safety
Iodine solution can irritate the eyes and skin and can stain clothing. Wear safety
goggles, gloves, and a laboratory apron while handling any solution that contains
iodine. Rinse off any solution that spills on your skin or clothing. Do not direct the
points of the scissors toward yourself or others. Use the scissors only as instructed.
Handle slides and coverslips carefully to avoid breaking them and cutting yourself.
Review the rules for handling a microscope.
COMPLETE THE PRE-Lab questions on page 3 before you continue!
Name_______________________________________________
Osmosis in Plants Biology
LAB
Procedure
1.
Use your fingers to separate one of the curved layers from the onion. Bend the layer until the shiny surface
cracks. Look for a thin, clear layer of tissue at the place where the surface cracked. Use forceps to peel away some of
the tissue. Use a scissors to cut away the tissue, if necessary.
2. While your partner uses the forceps to hold the tissue sample, put on your goggles, apron, and gloves.
3. Place a drop of iodine stain in the center of a clean glass slide. Use the forceps to place the onion sample in the
drop of iodine on the slide and to gently flatten the tissue so that no air bubbles are trapped underneath the tissue.
4. Place a coverslip at an angle so that one edge touches the drop of iodine solution. Use a probe to slowly lower the
coverslip over the onion tissue.
5. Remove your goggles. Use the low-power objective to locate some onion cells. Then, switch to high power and
focus with the fine adjustment. CAUTION: Always view the microscope from the side when you move an
objective.
6. Use the space in the table to draw the onion cells you see in the field of view. Choose one cell and
label the cell wall, nucleus, and cytoplasm. The cytoplasm will be yellow. Note whether the
cytoplasm completely fills the cell.
Make your drawing in the space labeled “CONTROL”.
7. Put a few drops of salt solution at one edge of the coverslip. Hold a small piece of paper towel at
the other end of the coverslip to draw the salt solution underneath the coverslip.
8. After 3 minutes, observe the onion cells. Use the space below to draw the cells you see in the field of view.
Choose one cell and label the cell wall, nucleus, and cytoplasm. Note whether there is any empty space between
theell wall and the cell membrane. Include a drawing in the table in the space labeled “After
Adding Salt Solution”
9. Put a few drops of distilled water at one edge of the coverslip. Hold a new piece of paper towel at the other end of
the coverslip to draw the distilled water underneath the coverslip.
10. After 3 minutes, observe the onion cells again. Use the space below to draw the cells you see the field of view.
Choose one cell and label the cell wall, nucleus, and cytoplasm. Note whether the cytoplasm completely fills the cell.
Include a drawing in the space labeled “After Adding Distilled Water”
Name_______________________________________________
Osmosis in Plants Biology
LAB
PRE-LAB Questions
1. Review What is osmosis?
2. Apply Concepts When water is distilled, any solutes dissolved in the water are left behind. Is distilled water
hypertonic or hypotonic compared to tap water? Explain.
3. Infer What is the purpose of the iodine solution?
Analyze and Conclude
1. Compare and Contrast How were the cells different after you added the salt solution? What happened after you
added the distilled water?
2. Draw Conclusions What caused the changes you observed when the onion cells were surrounded by the salt
solution? Was the salt solution hypertonic or hypotonic compared to the cells?
Draw Conclusions What caused the changes you observed when the onion cells were surrounded by
distilled water? Was the distilled water hypertonic or hypotonic compared to the cells?
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4. Infer A red blood cell can swell until it bursts. What prevented the onion cells from bursting when
they swelled?
_____________
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5. Relate Cause and Effect Some communities no longer use salt to prevent a buildup of ice on
roads. Road salt can pollute water supplies and harm plants that grow along the side of roads. Explain
how road salt could harm these plants.
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Name_______________________________________________
Osmosis in Plants Biology
LAB
Control
After Adding
Salt Solution
After
Adding
Distilled
Water
Extend Your Inquiry
Design an experiment to determine what percentage of salt in a solution produces a solution that is
isotonic.