Exercise 6.2 - Osmosis

LEVEL I

Materials

• Elodea, guard cells from plant leaves or animal blood
• Solutions ranging from 0.05 M to 0.50 M each of sucrose, sodium chloride, potassium chloride and calcium nitrate
• Razor blades
• Depression slides
• Microscope
• Osmometer (Optional)

Procedure

1. Place 0.5 ml of 0.50 M NaCl into the center of a depression slide and add either a small piece of an Elodea leaf, the stripped lower epidermal layer of a leaf containing guard cells, or a small drop of blood.

2. Place the slide on the microscope and observe the cells for swelling or shrinking.
   Swelling is difficult to determine with Elodea since the cell wall inhibits swelling. Cell shrinking can be observed as a pulling away of the cell membrane from the cell wall. In guard cells, the swelling will open the stomates, while shrinking will cause the stomates to close. Red blood cells react quickly to changes in the environmental salt concentrations and will shrink or swell. Shrinking is observed as a wrinkling or crenulation of the cell, and swelling may proceed to the point where the cells burst (plasmolysis).

3. Repeat steps 1 and 2 sequentially with 0.4, 0.3, 0.2, 0.1 and 0.05 M solutions of NaCl. Note which solution induces cellular shrinking or swelling.
   One solution will result in neither shrinking nor swelling. This is known as the isotonic solution. Those causing the cell to shrink are known as hypertonic, and those causing swelling are hypotonic.

4. Repeat steps 1-3 with each of the remaining salts and with sucrose. Note which concentration of each is the equivalent of the isotonic NaCl.
   Tonic refers to the NaCl equivalence of a solution. It is more correct for other salts and especially for organic non- electrolytes to use the terms isosmotic rather than isotonic.

5. Based on the molarity of the salt solutions, calculate the osmolarity of each solution and compare the osmolarity of each solution that is isosmotic to the cells under study. Multiplying the osmolarity by 22.4 will yield the osmotic pressure in atmospheres.

Optional

Use an osmometer to determine the precise osmolarity of each solution. Osmometers measure the freezing point depression, which in turn can be related to an equivalent solution of NaCl. If the osmometer is available, follow the manufactuer's instructions for its use.

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