Cell Membranes
Activity #3
Investigating the Effect of Different Concentrations of Water Surrounding a Living Cell
Procedure for Activity #3
- Prepare a wet mount of a leaf of the aquatic plant, Elodea, or a single layer of the
white onion skin (not the outer, brown layer) peeled off the onion with a forceps.
- Cover the tissue with a coverslip, withdrawing any excess liquid with a piece of paper
towel placed at one end of the coverslip.
- Focus and center the specimen under low power, then switch to high power.
- A partner should place a drop of 20% sucrose solution at the edge of the coverslip as you
remain focused on the plant cells. To draw the sugar solution under the coverslip, a
piece of paper towel is placed on the side of the coverslip opposite to where the sugar
solution was placed. Observe changes in the plant cells as the sugar solution is drawn under the
coverslip. Note the changes with a simple sketch of the cells before and after sugar solution
is applied.
- Repeat the procedure in Step #4 with the SAME tissue specimen, but this time apply DISTILLED
WATER in the same location as the previous sugar solution. Observe any changes over the next
several minutes. It may be necessary to refocus the microscope as the distilled water is drawn
in with the paper towel.
- Relate your observations to the results of Activity #1.
- Repeat steps #1-4 but use a 5% salt solution (5 grams salt, 95 grams water) in place of the
sugar solution.
- Repeat steps #1-4 but use physiological saline (salt) solution.
Data Analysis for Activity #3
- Did the cells enlarge or shrink for each of the chemicals applied to the cells? How would
you explain those changes in terms of salt or sugar moving into the cell compared with water
moving out of the cell? Salt has the formula NaCl and the sucrose has a formula of
C12H22O11. Would this account for any differences in the effect of
salt versus sugar on the cells (remember that a cell is more than 90% water by weight).
- In irrigation projects, there is the danger that the application of water containing many
different dissolved chemicals (collectively called salts) will cause the soil to become
saturated with enough salts that in turn will kill some plant species. Based on this activity,
how would you explain the effect?
- Bacteria cause food to spoil since the bacteria are feeding on the food and excreting waste. Bacterial decay of food can sometimes be prevented by adding salt, sugar, and even acid in the form of vinegar to the food. Explain how these chemicals act as a “preservative” of food. What mechanism do you think is at work, based on the results of this activity?
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