Name: _____________________________________________________ Date: ________________________________ Block: ____________
Those Poor Onion Cells!
A Study in Osmosis
Objective: Students will observe and analyze the effects of osmosis on living cells.
Problem: How does an onion skin cell respond to external solutions of different salt concentrations?
Procedure:
1. Prepare a wet mount of red onion skin cells, using water. The preparation may be no more than 2
cells thick.
2. Focus onto the cells at 100x. Hold the slide in place using the stage clips.
3. Do not move the slide for the remainder of the experiment.
4. Prepare a drawing of 5 cells showing the cell wall, cytoplasm, cell membrane, and nucleus. Notice
especially the shape and color of the cytoplasm.
5. Change the surrounding solution to 5% salt using the
replacement technique.
a. Do not change the position of the slide on the
stage.
b. Place a small square of paper towel on the
slide next to the coverslip. The towel should
start getting damp as it absorbs water from
under the cover slip.
c. Place a drop of the new solution (5% salt) on
the opposite side of the cover slip. Some of
the new solution should “leak in” under the
cover slip.
d. If the paper towel becomes soaked, replace it.
6. Make a second drawing of the same 5 cells, showing the same structures.
7. Use the replacement technique to surround he cells with tap water. Make a third drawing of the
same 5 cells.
8. When you are finished, make sure that any liquid on the stage or other microscope parts are
thoroughly wiped dry. Only then should you put away your microscope.
Analysis Questions:
1. How did the cells change? Use complete sentences to describe the changes for each of the cell parts
under the given conditions. If the part didn’t change, say so.
a.) Cell wall in 5% salt solution.
b.) Cell membrane in 5% salt solutions.
c.) Cytoplasm in 5% salt solution.
d.) Cell wall in tap water.
e.) Cell membrane in tap water.
f.) Cytoplasm in tap water.
2. Explain how osmosis is caused the changes you observed in the salt solution and tap water.
3. Use the terms hypertonic, hypotonic, and isotonic to correctly describe the two solutions used in the
replacement technique. Defend each answer by describing how the concentrations of salt differed inside
the cell and outside the cell.
4. How would the response of an animal cell differ from the of a plant cell if we exposed an animal cell to
these solutions?
Microscope Drawing Sheet