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| Plasmolysis in a plant cell. |
Title: Plasmolysis and Deplasmolysis in onion cells.
Purpose: To be able to observe the process of osmosis and distinguish between hypertonic, hypotonic, and isotonic solutions. To show that water molecules move in and out of a cell by osmosis.
Equipment: red onion, distilled water, slides, salt, microscope.
Background: Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells. Some substances can cross the cell membrane without any input of energy by the cell. The movement of such substances across the membrane is known as passive transport. The simplest type of passive transport is diffusion.
Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. A second type of passive transport is osmosis. Osmosis is the process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration. Molecules must move about in a cell in order for a cell to survive.
Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out of the cell and into a hypertonic solution (high solute concentration) surrounding the cell as shown in the figure below. During plasmolysis the cellular membrane pulls away from the cell wall. In the next lab exercise you will examine the details of the effects of highly concentrated solutions on diffusion and cellular contents.
Procedure:
1. Prepare a slide of onion membrane. Add 2 drops of distilled water, cover with a coverslip and observe under the microscope (high power, AK 200x). Draw a picture of your observations below and label the cell wall, cell membrane, and cytoplasm.
2. Using the prepared slide from above and a dropper, place a drop of salt water at one edge of the coverslip. Place a small piece of paper towel on the opposite edge to get the salt water solution under the coverslip. Repeat with a second drop of salt water solution. Observe the onion cells under the microscope (high power) for 3-4 minutes. Draw a picture of your observations below and label the cell wall, cell membrane, and cytoplasm.
3. Using the prepared slide from above and a dropper, place a drop of distilled water at one edge of the coverslip. Place a small piece of paper towel on the opposite edge to get the distilled water under the coverslip. Repeat with a second drop of distilled water. Observe the onion cells under the microscope (high power) for 3-4 minutes. Draw a picture of your observations below and label the cell wall, cell membrane, and cytoplasm.
Result: write usual one!
Conclusion:
1. Explain (in detail) the changes you observed in the distilled water. Please be sure to discuss the positioning of the cell wall, cell membrane, and cytoplasm.
2. Explain (in detail) the changes you observed in the salt water. Please be sure to discuss the positioning of the cell wall, cell membrane, and cytoplasm.
3. In the winter, grass often dies near roads that have been salted to remove ice. What causes this to happen?
