The Effects Of Osmotic Balance And Imbalance In Living Cells

Wayne Brown                    Collins High School
                               1313 So. Sacramento Blvd.
                               Chicago, Illinois 60626
                               312-534-1500

Objectives:

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.

Materials needed:      
                                                          
microscope                    cover slips                 medicine droppers
microscope slides             elodea (water plant)        water
6% salt solution
 
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?