Guided Inquiry • Skills Lab
Osmosis
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
• scissors
• paper towel
• distilled water
• microscope slide
• coverslip
• microscope
• 10% salt solution
• iodine
Pre-Lab Questions
Please answer in complete sentences in your notebook.
1. Review What is osmosis?
2. Apply Concepts When water is distilled, any solutes (like salt or sugar) dissolved in the water are
left behind. Is distilled water hypertonic or hypotonic compared to tap water? Explain.
3. Hypothesize Read through the procedure. Then answer the following questions using the “if…
then… because…” format (use the terms hypertonic and hypotonic in your explanations):
a. What do you think will happen when salt water is applied to the onion? Why?
b. What do you think will happen when distilled water is applied to the onion? Why?
1
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.
3. Place a drop of iodine 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. Slowly lower the coverslip
over the onion tissue.
5. Use the low-power objective to locate some onion cells. Then, switch to medium power and focus
with the fine adjustment, and finally, switch to high power.
6. Focus so you see a few cells clearly. Carefully draw a picture of 1-2 cells. Title your
drawing with the type of solution used, and the power the cells are being viewed
under power (ex. Onion in iodine solution under high power [400x]). Label the cell
wall, nucleus, and cytoplasm.
Effects of Salt
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 low power to view the onion cells again.
Switch to medium power and then high power.
9. Focus so you see a few cells clearly. Carefully draw a picture of 1-2 cells. Title your
drawing with the type of solution used, and the power the cells are being viewed
under. Label the cell wall, nucleus, plasma membrane, and cytoplasm.
After Adding Salt Solution
10. 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.
11. After 3 minutes, observe the onion cells again. Use low power to view the onion cells again. Switch
to medium power and then high power.
12. Focus so you see a few cells clearly. Carefully draw a picture of 1-2 cells. Title your drawing with
type of solution used, and the power the cells are being viewed under. Label the cell wall, nucleus,
plasma membrane (if visible), and cytoplasm.
Write-up
1. Create a word document. Head the document with your name, the date, and your period.
2. Title the lab, “What effect does the concentration of solutes outside of plant cells have on the
cells?”
3. Title the first section, “Introduction.” Write a paragraph summarizing what you know about
osmosis and how cells respond in hypertonic, hypotonic, and isotonic solutions. End this
paragraph with your hypotheses (If… then… because…) that you wrote in your pre-lab.
4. Title the second section “Analysis” and answer the following questions in complete sentences:
a. Compare and Contrast How were the cells different after you added the salt solution?
What happened after you added the distilled water?
b. 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?
c. 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?
d. Infer A red blood cell can swell until it bursts. What prevented the onion cells from
bursting when they swelled?
5. Attach the pictures you drew in class. Make sure each drawing has a title and the appropriate
parts are labeled.
Write-up
1. Create a word document. Head the document with your name, the date, and your period.
2. Title the lab, “What effect does the concentration of solutes outside of plant cells have on the
cells?”
3. Title the first section, “Introduction.” Write a paragraph summarizing what you know about
osmosis and how cells respond in hypertonic, hypotonic, and isotonic solutions. End this
paragraph with your hypotheses (If… then… because…) that you wrote in your pre-lab.
4. Title the second section “Analysis” and answer the following questions in complete sentences:
a. Compare and Contrast How were the cells different after you added the salt solution?
What happened after you added the distilled water?
b. 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?
c. 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?
d. Infer A red blood cell can swell until it bursts. What prevented the onion cells from
bursting when they swelled?
5. Attach the pictures you drew in class. Make sure each drawing has a title and the appropriate
parts are labeled.