LAB: Osmosis in Plants
BACKGROUND:
Diffusion of water molecules across a cell's outer membrane from areas of high water
concentration to areas of low water concentration is called osmosis. This movement of
water may be harmful to cells. It can result in cell water loss (plasmolysis) when living
cells are placed into an environment where the water concentration inside the cell is
higher than outside the cell (which means that the solute concentration inside the cell is
lower than outside the cell). However, most cells live in an environment where
movement of water in and out of the cell is about equal. Therefore, there are no harmful
effects to the cell.
The cells of the aquarium plant Elodea normally contain 1% salt (NaCl) and therefore
99% water (H2O) on the inside of their cells. The NaCl water used in this experiment
contains 6% NaCl and therefore 94% H2O.
Because there is a higher water concentration inside the plant cells than in the salt water
mount, water will leave the cells moving from high concentration to low concentration.
OBJECTIVES:
� Prepare a wet mount of an Elodea leaf in distilled water and a wet mount of Elodea
leaf in salt water for microscopic observation
� Observe and diagram the cells in fresh water and in salt water
� Compare normal cells in aquarium water to plasmolyzed cells in salt water
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A: PROCEDURE FOR OSMOSIS:
1. Prepare a wet mount of an Elodea leaf by adding a drop of tap water to the leaf.
Cover with a cover slip.
2. Focus the slide under the scanning objective (4X).
3. Move to the 10X objective and focus the microscope further. Adjust the
diaphragm to the amount of light that allows you the clearest visualization. (If you
can move to the 40X objective and still see the cells clearly then do so as well).
You want to be able to see the cell wall, chloroplasts, and central vacuole if
possible.
4. Formulate a hypothesis predicting what will happen when elodea cells are placed
in salt water. Write your hypothesis in the analysis section (#1).
5. Leaving the slide on the microscope stage, place a drop of 6% salt on the left side
of the slide. The drop should touch the side of the cover slip. Place a small piece
of paper towel on the right side of the slide to draw the salt water across the slide.
This will expose the elodea to a hypertonic solution.
6. Observe the effects of the salt solution on the Elodea cells. You may notice the
movement of particles in the water as the water is wicked to the opposite side of
the slide. Do not let these particles distract your attention-focus on what occurs
inside the Elodea cells. Note any changes in color, size, or shape of the cell
structures. Continue observing two or three minutes. It is important to wait a few
minutes in order to allow osmosis to occur.
7. Draw a single elodea cell as it appears under the 40X objective after it has been
exposed to a hypertonic solution in the analysis section (#2).
8. Formulate a hypothesis predicting what will happen to these same elodea cells if
they are placed in a distilled water solution. Write your hypothesis in the analysis
section (#1).
9. Flood the Elodea cells with a hypotonic solution by placing a drop of distilled
water on the left side of the slide. The drop should touch the edge of the cover
slip. Place a small piece of paper towel on the right side of the slide to draw the
distilled water across the slide.
10. Observe the effects of the distilled water on the Elodea cells. Note any changes in
cell size or structure. Make a sketch of your observation in the analysis section
(#2).
11. Carefully observe the location of chloroplast in relation to the cell wall of both
leaves.
ANALYSIS OF RESULTS
1. Hypothesis (distilled water):
Hypothesis (salt water):
2. Draw a diagram of a single cell an Elodea leaf with distilled water, and an Elodea
leaf with salt water and afterwards with distilled water. Remember to label your
diagram including magnification. Also remember to label the cell wall, the cell
membrane, and chloroplasts.
Elodea with salt water
Elodea with distilled water
3. Describe the effects of placing an Elodea cell in a hypertonic solution. In your
description, include the difference in the concentration of water outside and inside
the cell.
4. Describe the effects of placing an Elodea cell in a hypotonic solution. In your
description, include the difference in the concentration of water outside and inside
the cell.
5. In which solution were the Elodea cells most turgid?
6. Describe the structure of a plant cell wall. Was there any difference in
appearance of the cell wall in both plant cells? Was there any difference in the
appearance of the cell membrane in both plant cells?
7. Compare the location of chloroplasts in a plasmolyzed cell in salt water vs. a cell
placed in distilled water.
APPLICATION QUESTIONS
1. Roads are sometimes salted to melt ice. What does this do to plants around the
roadside and why?
2. Before you receive an IV, they must know that the solution they are giving you is
isotonic to your cells. What would happen to you if you were given a hypertonic
solution? What about a hypotonic solution?
3. Red blood cells (and other animal cells) placed in a distilled water solution
usually swell up and burst. What prevented the elodea cells from swelling up and
bursting when they were placed in the distilled water?