Sweet Onions and Strong Onions

Aim

To find out the concentration of the cytoplasm of two types of onion cell using plasmolysis

Hypothesis

There are more chemicals dissolved in the cytoplasm of strong onions than "sweet" onions;  it is this which gives the onion its strength.  Where there is a high concentration of onion chemicals the concentration of water in the cell will be lower.  Therefore the cells will start to plasmolyse at different salt concentration.

Prediction

Write a prediction for plasmolysis in the two types of onion based on your preliminary experiment results........__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __

Apparatus;  six slides, teat pipette, six concentrations of salt solution (0, 0.5, 1.0, 1.5 and 2M), samples of two onions, microscope

Method

1. Label five test tubes with the concentrations of salt solution you have available 
2. Put 5 cm³ of each salt solution into each test tube.
3. Cut six thin slices of onion, including its thin epidermis and put them into the test tubes
4. Leave the cells for 15 minutes to allow plasmolysis to occcur.
5. Prepare six slides of the onion cells from each test tube. (as we did last lesson)  Be sure to put the salt solution from the test tube on your slide.
6. Count the total number of cells, and the number of plasmolysed cells in the view at 400x magnification.
7. Repeat step 6 at a different place on the slide.
8. Repeat step 6 again at another place on the slide.  
9. Record your results in a table of your own design.
10. Calculate the  overall % of plasmolised cells on each slide.
11. Design a results table for these results for each onion.
12. Plot an x-y scatter graph of salt concentration against % of plasmolysed cells.
13. Work out using you graph where 50% plasmolysis would be for each onion.
     This is the concentration of the cytoplasm of the cell.

Conclusion

• Do your results agree with the hypothesis?
• Is there a difference in the concentrations of the cytoplasm of each cell?
• How accurate do you think the method is?
• How could you improve the experiment?

sample results