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Osmosis

Introduction

In this experiment you will expose living cells to a hypertonic solution and observe the results.

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 and it is essential for maintaining homeostasis or balance in a living organism. When a cell is in a solution where the concentration of solutes is the same outside the cell as inside, it is in what is known as a isotonic solution. In an isotonic solution, there is no net movement of water since the solution is uniform throughout. However, in both hypotonic and hypertonic solutions, there is considerable movement of water to equalize the concentration gradient:

  • Hypotonic Solution: If a cell is placed in a hypotonic solution, then the concentration of solute is higher in the cell's cytoplasm than the surrounding solution. As a result, water will move into the cell in order to equalize the concentration gradient.

  • Hypertonic Solution: A hypertonic solution is the opposite. In this instance, the surrounding solution has a greater concentration of solute than the cell cytoplasm. Water will move out of the cell.

Materials

  • Microscope slide and cover slip
  • Red Onion
  • 1 gram Salt
  • 10ml Water
  • 50ml or larger beaker
  • Pipette or dropper
  • Paper towel

Instructions

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. View the slide using a low power objective lens (4x or 10x) and 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.

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. Feel free to add more salt to your solution if required.

To rehydrate the cells, simply add several drops of water to one edge of the coverslip and draw the water across, as before. As the salt is washed away, the cells will return to their original state although this may take several minutes.

Osmosis also occurs in animal cells with dire results. Animal cells exposed to hypertonic solutions begin to shrivel as the water leaves the cells in a process known as crenation. Generally, crenated cells are not recoverable and die! You can test this using blood cells.