Download LAB: The Wizard of Oz(mosis)

The Wizard of Oz(mosis)
Background: Water molecules are so small, they move across and within cells at will. Maintaining
proper osmotic balance is important for cells. In the human body, the organs called the kidneys help
maintain the correct salinity (“saltiness”) of blood. This in turn, keeps our bodies’ cells properly
hydrated. Plant cells, on the other hand, are bathed by water from the environment. In this lab, you will
investigate how cells of a vegetale react to salt water. And, can they recover? Think about organelles
which must function in a plant cell’s ability to achieve osmotic balance.
Question: How will the cells of a vegetable (red onion skin cells specifically) react, at the microscopic
level, to saltwater?
Hypothesis: Write a proper hypothesis for this experiment.
Prelab Questions:
1. Define Osmosis.
2. What happens to a cell if you put it in a hypotonic solution? Which way is water moving?
3. What happens to a cell if it is put in in a hypertonic solution? Which way is water moving?
4. How is osmosis different from facilitated diffusion? How is it the same?
Materials:
TINY piece of red onion skin
fresh water (it has no salt)
compound microscope
15% salt solution (very salty)
paper towel
glass slides & cover slips
Procedure:
1. Take a scale of red onion, snap it backwards and peel off a little piece (far smaller than a cover slip!) of
its purple skin tissue. Make a wet mount of this tissue using tap water. Apply a cover slip.
2. Look at the cells under the microscope on 100x or 400x. Locate and zero in on an area where you can
clearly see the cells (they must be purple). Notice how the vacuole, containing purple pigments, appears
to fill each cell. Draw this view as Figure 1. Don’t forget a title, caption, and labels.
3. After making the Fig. 1 sketch, temporarily remove your slide from the stage of your microscope. Set it
on a piece of paper towel on your lab table. Gently take off the cover slip and add a big drop of
saltwater on top of the red onion skin.
4. Return the cover slip (gently!) to the red onion skin (soaking in saltwater), and find a good view of cells
on 100x or 400x again. Let it sit for about a minute. Draw this view as Fig. 2 and label properly.
Capture the change in appearance that is due to osmosis.
5.
Remove the saltwater by adding several drops of fresh water to the little piece of red onion skin. (As
with Step 3, do this with the slide on paper towel on your lab table).
6. Watch and wait – you can actually SEE the water moving back into the vacuole.
7. Once again, capture the effect of osmosis in your drawing. Your final drawing would of course be
Figure 3! Once again, do not forget to label properly.
8. Clean off your slides, cover slip, and work area. Please insure that no saltwater has been left on your
microscope.
Data Collection:
Your data section will only have three circles for your microscope observatios. Label them Figure 1, Figure 2
and Figure 3, but give them a proper title (i.e: Fig. 1: Red Onion Cell in Tap Water). Leave room for captions
and labeling around the circles. (captions and labels should include: what you are looking at, magnification,
osmotic treatment, and cellular structures visible, what is happening)
Conclusion:
Write a conclusion using your guidelines in the lab notebook.
Post-Lab Questions:
1. What is the red /purple area you saw in the cell?
2. What happened when you added the salt solution to the cell? What happened when you added tap water
for the second time? Explain these using the words “hypotonic” and “hypertonic”.
3. Red blood cells (and other animal cells) placed in a distilled water solution usually swell up and
burst. What prevented the red onion cells from swelling up and bursting when they were
placed in the distilled water?
4. Draw a diagram to indicate the movement of salt and water across the onion cells’ membrane
when they were placed in the salt solution. Use arrows to properly indicate the direction of
osmosis.
5. Using what you learned in class about osmosis, explain why do grocery store owners spray
fresh fruits and vegetables with water. (Think about osmosis and what characterists people
look for when buying fruit.)
6. When agricultural fields in lowland areas experience an influx of seawater, are crops in danger? If well
water becomes tainted with seawater, could this harm a person’s lawn or garden?