The passage of water molecules through a selectively permeable membrane is a special case of diffusion known as osmosis. Osmosis results in the movement of water from an area of lower concentration of dissolved material to an area of higher concentration of dissolved material. For a demonstration of this principle, a purple onion cell is going to be bathed in different concentrations of salt (NaCl) water.
How do solutions of various salt concentrations influence osmosis in relation to an onion cell?
What will happen to the onion cell if it is placed in salt water? What will happen to the onion cell if it is placed in distilled water?
If ..the onion cell is placed in salt water then… the cell will shrivel up
The independent variable is the salt solution and distilled water
The dependent variable is whether the cell shriveled or not
thin onion strip
small plastic cover slip
iodine stain pipette
paper towel strip
STEPS 1. Carefully remove the outer layer of cells (epidermis) of a piece of red onion. 2. With the slide on your desktop, place a drop of water on the center of a clean dry slide. 3. Using the tweezers, place the onion in the middle of the drop. 4. While holding the cover slip upright, carefully place one edge of the cover slip next to the water. 5. Slowly lower the upper edge of the cover slip onto the water. The objective is to minimize or eliminate air bubbles under the cover slip.
6. Examine under low power. Sketch a drawing of the cells.
7. Take the slide of the stage of your microscope and place it on your desktop. Add several drops of salt solution to one side of your cover slip while placing a small piece of paper towel along the opposite edge of the cover slip. 8. Observe the slide to see the effects of the saline (salt) solution on the onion cells. Record in observations. 9. Replace the sodium chloride solution with distilled water in the same way that the salt solution was added
Observation and drawings
The drawings above depict what happened during the osmosis in an onion cell experiment. They are the illustrations of what I saw through the microscope in the three different trials. In all three of the drawings, the cell walls, cell membranes, and cytoplasm are labeled. The one on the left is the original, in which water was in the wet mound slide. As you can see, there is little space between the cell wall and membrane, and this is because the cell had a lot of water in it, making the inside large.
The middle picture is the view after I put the salt solution in the slide, removing the water. The result of this, as it can be seen, was that the cell membrane significantly shrunk, making the inside of the cell with the cytoplasm very small. This is because when the salt difised in, the water diffused out. The drawing on the right shows what happened when I then replaced the salt solution with distilled water. This produced basically the identical view as the original, since it was back to being mainly water again, with the cell wall and membrane very close to each other. This happened because once the water was put back in, the salt diffused out.