Lab #7 Osmosis in Onion Cells

Purpose

• To study osmosis using onion cells.

Background Information

• Osmosis is the movement of water through a selectively permeable membrane.

• Water moves from a low concentration of solutes to a high concentration.

• In the lab you will use a 15% NaCl (salt) solution. This solution has more solutes than the inside of the cell and is hypertonic.

• You will also use distilled water which is hypotonic when compared to the cell.

Hypothesis

• What will occur to the onion cell if placed in a hypertonic solution (salt water)?

• What will occur to the onion cell if placed in a hypotonic solution (distilled water)?

Materials

• Microscope

• Slide & cover slip

• Onion

• 15% NaCl water

• Distilled water

• Pipette

Procedure

1. Remove a small section of tissue from the epidermis (skin) of the onion on the purple side. Once removed the epidermis is thin and almost transparent.

2. Prepare a wet mount using the epidermis.

3. Make drawings and observations for the onion in low, med, and high power.

Procedure

4. Add 2-3 drops of 15% NaCl solution under the cover slip. Observe and draw the cells in low, medium and high power.

5. Add several drops of distilled water and wait about 2 minutes. Has the cell returned to normal?

6. Clean your slides and return the materials.

Data

Treatment Observations

Starting

With 15% NaCl

With distilled water

• Start

• 15% NaCl

• Distilled water

Conclusion

1. Summarize the lab and explain if the result support or reject your hypothesis.

2. How did the 15% NaCl solution affect the structure of the onion cell? Give a reason for why this occurred.

3. In the winter, grass often dies near the roads that have been covered in salt to remove the ice. What do you think is the reason the grass dies?

Conclusion

4. Define the following: isotonic, hypertonic, and hypotonic solution.

5. How do animal cells and plant cells react in a hypotonic solution?