Lesson Plan on Osmotic Balance and Imbalance

This biology lesson plan begins here... Teacher Lesson Plan on Diffusion and Osmosis.

Strategy

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 semi-permeable membrane.
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 observation.
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 slide.
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 power.
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.

Conclusion

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.

Evaluation

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
imbalance?
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
concentration?
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? 3rd party contributor

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