Download Observing Osmosis in Plant Cells

Name __________________________________________
Date ____________
Activity: Observing Osmosis in Plant Cells (Red Onion)
Goals:
1. To demonstrate what happens when water is drawn out of plant cells
due to a change in the environment.
2. To describe plasmolysis.
Background Information: Have you ever noticed that after a particularly snowy and/or icy
winter there is quite a lot of dead vegetation on the sides of the road on near sidewalks where salt
has spread to prevent cars and pedestrians from slipping? All of this is a result of plasmolysis,
the contraction (shrinkage) of cytoplasm in plant cells due to the loss of water through osmosis.
What causes this damaging osmosis in which the cytoplasm shrinks in the cell? Salt is the
answer, but how does it happen?
Materials: Red onions, concentrated salt solution, eyedroppers, blank slides, cover slips, paper
toweling
Procedure:
What I Know: Reflect on what you now know about osmosis and write a sentence or two about
how it might relate to the question of plants dying after a snowy or icy winter when lots of salt is
used on roadways and sidewalks.
1. Peel away the membrane from the outer side of a layer of onion.
2. Cut the piece of membrane so that it will fit in the center of a blank slide.
3. Place a drop of water on the slide, place the onion membrane on the water, and carefully
put a cover slip on top.
Part 1.
1. Observe the onion cells first under low power (40x) and then get your best focus – either
under medium (100x) or high (400x) power. (Water from the wet mount procedure will
move into the onion cells by means of osmosis because the concentration of pure water is
higher on the outside of the cells than it is on the inside of the cells. The cells are said to
be very full or “turgid.”
2. Using your best power for your observations draw the onion cells as they appear mounted
with fresh water (Before Plasmolysis).
Caption:
Mounted in fresh water -before
plasmolysis
______X
Part 2.
1. Remove your onion cell slide from the microscope stage.
2. Place a small drop of salt solution on the side, just to the left of the cover slip
3. Place a piece of paper towel on the slide, just to the right of the cover slip. Hold the paper
towel in place as the salt solution will be drawn across the cover slip from left to right.
This technique is known as “irrigation.”
4. Return the slide to the microscope.
5. Observe the onion cells first under low power (40x) and then get your best focus – either
under medium (100x) or high (400x) power. (Salt water from the irrigation procedure
will have pulled water out of the onion cells because the concentration of pure water is
now higher in the cells’ cytoplasm than it is in the salty water. The cells will shrink and
become “flaccid” or limp. You should actually see the vacuole, which is normally full of
water, shrink.
6. Using your best magnification, draw the onion cells as they appear mounted in salt water
(After Plasmolyisis).
Caption:
Mounted in salt water - after
plasmolysis
______X
What I Did: Summarize the procedure you used. Be very specific.
What I Learned: Describe what you learned about what happens to cells when water moves
out. Refer to the goals for the activity and be as specific as possible.
What I Wonder: Pose a why or how question that you still have about the process of
osmosis in cells.
Questions: Answer the following questions in complete statements.
A. Have you ever noticed that a plant will wilt when it needs water, and if it’s not too
late, will “come back to life” when it receives that water? What do you think causes a
dry plant to wilt? What causes it to “come back to life” after watering. Include a
diagram to illustrate your explanation.
B. Some plants thrive (grow well) at the shore – think of dune grass. What allows them
to tolerate a salty environment that would kill most plants? (You may have to do
some research for this answer.)