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.
Problem: How do solutions of various salt concentrations influence osmosis in relation to an onion cell? Hypothesis
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
Materials: microscope thin onion strip glass slide small plastic cover slip iodine stain pipette tweezer paper towel strip small beaker Salt solution
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.