Diffusion is the spontaneous movement of a substance from an area of high concentration to an area of lower concentration. The diffusion of water across a membrane is called osmosis. The process of osmosis is essential for maintaining homeostasis. When a cell is in an isotonic solution, the concentration of solutes is the same outside the cell as inside.
Therefore, there is no net movement of water. If a cell is placed in a hypotonic solution (one that has a lower concentration of solute than the cell’s cytoplasm), water will move into the cell to equalize the concentration gradient. If a cell is in a hypertonic environment (one in which the environmental concentration of solute is greater than the solute concentration within the cell), then water will move out of the cell. In this experiment, you will expose living cells to a hypertonic solution and observe the results.
To prepare the salt solution, measure 10 mL of water and pour it into the beaker.
Measure 1 gram of salt and add it to water. Mix the solution.
Cut the onion into rings, break one of these rings and peel off some of the thin outer membrane.
Prepare a wet mount and view the slide under low power - sketch a few cells for comparison later.
Add a few drops of the prepared salt solution to one edge of the coverslip and use a small piece of paper towel against the other edge to draw the solution across the sample.
Once the paper towel is saturated, it can be removed. This is also a common staining procedure. Spend a few minutes observing the cells. Compare the cells with your sketch and note any changes. If you added enough salt solution, you should see that the cytoplasm and cell membranes have pulled away from the cell walls. This process is known as plasmolysis and only occurs in plant cells.