Practical 2 - Osmosis

NoS: Experimental design—accurate quantitative measurement in osmosis experiments are essential. (3.1)

S2: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. (Practical 2)

 6 microscope slides and cover slips, 

•   6 concentrations of sodium chloride solution (0.0M, 0.25M,0.5M, 1.0M, 1.5M and 2.0M), 

•   1 dropping pipette and paper tissue 

•   epidermis samples of two varieties of onion, 

•   microscope 

1. Label six slides with the six salt concentrations. 

2. Put two drops of each salt solution onto each slide. 

3. Add a small piece of epidermal tissue from an onion to each slide, and then a cover slip. 

4. Study the onion cells on the first slide under the microscope at x100 magnification. 

5. Wait 2 minutes to allow osmosis to take place. 

6. Count the total number of cells in the eyepiece view. 

7. Count the number of plasmolysed cells in the same view. 

8. Repeat steps 6 & 7 at two different places on the slide, so you have three repeats. 

9. Repeat steps 6 to 8 for each concentration of salt solution. 

10. Record all your results in a table. 

11. Write a short paragraph of qualitative observations, ie. things you observed about the cells during the experiment in addition to the numbers of cells. 

1. a)  Calculate the % of plasmolysed cells on each slide.

2. ( % plasmolysed = no. plasmolysed / total no. of cells x 100 ) 

3. b)  Make a table of these values. 

4. c)  Write a short explanation of how you did the percentage calculation. 

5. d)  Plot an x-y scatter graph of salt concentration against % of plasmolysed cells for each onion. 

6. e)  Draw a best fit line over the data on each scatter graph. 

7. f)  Work out using your best fit line the sodium chloride concentration which would result in 

8. 50% plasmolysis for each type of onion. This represents the concentration of the cytoplasm of the cell. 

•   Describe the two scatter graphs. 

•   Give a short explanation of how you calculated the solute concentrations using the graphs. 

•   Explain if the results show a difference in cytoplasm concentration between each onion? 

•   What limitations did you find in the method? 

•   How could you improve the experiment to reduce each limitation?