Osmosis Lab: Potatoes and Elodea
Cells have a need to regulate their internal environment. They need to be able to this because cells often find themselves in environments where the concentration of dissolved solids outside the cell is different from the number of dissolved solids inside the cell. Since the solids can not move across the membrane, the cell responds by moving water either into or out of the cell in an attempt to balance the number dissolved particles. This process is called osmosis, the diffusion of water across a cell membrane. So if a cell finds itself in an environment where there are a lot of dissolved particles outside compared to inside (a hypertonic environment), water will begin to flow from an area of high concentration inside the cell to an area of low concentration outside the cell. The cell will lose water (and mass), start to shrivel up and eventually die. This process is called plamolysis. It is the result of osmosis. On the other hand, if the cell is in an environment where there are more dissolved particles inside the cell than outside (a hypotonic environment), water will diffuse into the cell across the membrane. The cell will swell up and eventually burst. This is also the result of osmosis. An ideal environment is one where there is roughly the same number of dissolved particles inside the cell as outside (an isotonic environment). This is the environment that cells are normally found in and have adapted themselves to. In an isotonic environment the amount of water diffusing into a cell equals the amount of water diffusing out. The cell remains the same size and is a happy cell. Hypertonic environments are more common than hypotonic ones for members of the bacteria, protists, fungi, and plants. Think about jelly. It sits in your refrigerator week after week without going bad. No mold, no bacterial colonies. Just jelly. You can’t do that with practically anything else. The reason is that to microorganisms, the jelly is a very toxic place. Even though it is loaded with sugar that should make it an inviting place to live, the concentration of the sugar molecules is so high that when cells land on it they undergo plasmolysis and die. Can you explain why? And how about drinking salt water next time you are marooned at sea? Is this a good idea? Why not? While you do this lab and interpret the results, you will be asked to answer those types of questions. Think about what happened to the potato and the elodea. Think about bacteria cells and your cells. Are they any different in terms of osmosis and their response to hypertonic and hypotonic environments? Background Questions:
Osmosis- Diffusion of fluid through a semi-permeable membrane to move from high concentration to low concentration. Hypertonic environment- A highly concentrated environment.
Plasmolysis- Causes cells to shrivel up and die because water diffuses out of the cell to an area of high concentrated solids. Hypotonic environment- a low concentrated environment.
Isotonic environment- having the right balance of concentration between the cell and it’s environment.
Lab 1: The Potato Lab
1. Using a cork borer (caution: cork borers are sharp), make 7 potato cores each 3-cm long. Using a razor blade (caution: razor blades are sharp), cut off the skin. The core should be 3 cm long after this cutting. 2. Find the mass of each core. Record it on the data table. 3. Fill 8 cups with water of different molarities of salt. Use just enough water to cover the potato core. 4. Place one core into each cup. Wait about an hour.
5. While waiting, go on to the Plasmolysis of Elodea Lab
6. After an hour, remove the core from each tube and pat them dry. 7. Reweigh each core and record the data on your data table
Data Table: Potato Lab
Mass of Core Before
Mass of Core After
Change In Mass
0.0M (pure water)
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