The Effects Of Osmotic Balance And Imbalance In Living Cells
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Wayne Brown Collins High School
1313 So. Sacramento Blvd.
Chicago, Illinois 60626
1. Students will review the processes of diffusion and osmosis.
2. Students will prepare a wet mount of an elodea leaf in tap water and a wet
mount of an elodea leaf in salt water for microscopic observation.
3. Students will observe and diagram cells of both wet mounts.
4. Students will distinguish between osmotic balance and osmotic imbalance
in elodea leaf cells.
microscope cover slips medicine droppers
microscope slides elodea (water plant) water
6% salt solution
Begin the lesson by either reviewing the definition of diffusion and osmosis
and/or by discussing the previous lab on the same topic.
Diffusion - the gradual movement (spreading out) of molecules from an area of
greater (higher) concentration to an area of lesser (lower) concentration.
Osmosis - the movement of water and dissolved materials from an area of greater
(higher) concentration to an area of lesser (lower) concentration through a
Divide the activity into Part A, osmotic balance, and Part B, osmotic imbalance.
Students may work individually or in pairs.
Part A: Osmotic Balance
1. Prepare a wet mount of an elodea leaf in tap water for microscopic
2. Observe the cells of an elodea leaf under low and then high power of
your microscope. Locate a single cell along the leaf edge. Observe the
location of the chloroplasts in relation to the cell wall.
3. Save this slide for Part B.
Part B: Osmotic Imbalance
1. Prepare a second wet mount using another elodea leaf. Use 6% salt
solution instead of tap water.
2. Let the wet mount stand for two to three minutes and then observe cells
of this leaf under high (dry) power.
3. Place the tap water slide on the microscope stage next to the saltwater
4. Compare the saltwater leaf to the tap water leaf by simply switching
back and forth from one slide to the other while viewing under high
5. Check the saltwater slide carefully. Observe the location of the
chloroplasts in relation to the cell wall when viewed under high power.
6. Diagram, on a sheet of unlined paper, a single cell from each slide.
Label the cell wall, plasma membrane and chloroplast in both cells.
When living cells are placed into any environment where there is a higher water
concentration inside the cell compared to outside the cell, water loss called
plasmolysis, can result. This may be harmful to the cell. However, most cells
live in an environment where the movement of water in and out of the cell is
equal. Therefore the cell is in "osmotic balance" and there are no harmful
effects to the cell.
Use the following information to help you answer these questions:
(a) Elodea cells normally contain about 1% salt and 99% water.
(b) Tap water normally contains about 1% salt and 99% water.
(c) Salt solution contains 6% salt and 94% water.
1. What observable change is seen in the cells showing an osmotic
Explain your answer.
2. What observable change is seen in the cells showing an osmotic balance?
Explain your answer.
3. In the wet mount using salt solution, where was the higher concentration
of water located at the start of this experiment? Where was the lower
4. According to osmosis, water moves from areas of high concentration to
areas of low concentration. In which direction should water move in the
cell of Part B? What evidence do you observe to support this?
5. What term describes the condition of a cell when it loses water?