Plasmolysis in epidermal cells of a red onion (duration in real time about 10min)
The epidermis cell under turgescent conditions
The first frame following the title shows an epidermal cell of a red onion in rest. It is possible to distinguish the cell wall and the vacuole that appears red because of the presence of anthocyan. The vacuole is surrounded by the tonoplast, while the cytoplasm is bordered by the plasma membrane. Both membranes are so thin that they cannot be visualized with a light microscope. They are permeable for water but not for salts and large molecules: these membranes are semi-permeable.
At the beginning of the movie the vacuole has the highest concentration salts and molecules, and, opposite, the lowest concentration of water. Water tends to adjust an equal concentration everywhere. Therefore it is as if the vacuole would suck water to even out the concentration of water molecules. By taking up water the vacuole gains pression, like a balloon. This pressure is transduced to the surrounding cytoplasm that, in its turn, presses on the rigid cell wall. The cell shows turgor pressure.
Immersion in KNO3 and plasmolysis
In the next sequences the strip of epidermis cells is plunged into a 10% solution of KNO3. Now the highest concentration of dissolved particles is located outside the cell. The concentration of water is thus higher inside the vacuole and in the cytoplasm than outside the cell. Again, water wants to be "spread" with an equal concentration everywhere. Therefore, more and more water flows out of the cell. As a consequence, the vacuole looses her "over pressure" and the cell becomes flacid. After a while so much water has left the cell that even the plasma membrane retracts and the vacuole shrinks. This phenomenon is called plasmolysis. The number of anthocyan molecules within the vacuole remains constant, as these particles cannot pass the tonoplast. However, the total volume of the vacuole has decreased impressively because of water lost. The concentration (quantity divided by volume) of anthocyan in the vacuole is now higher than first; at severe plasmolysis, like in the final sequences, the vacuole appears stronger red than in resting condition, as at the beginning.
Imaging/web text: Department of General instrumentation (KUN) (E.S. Pierson, B. van der Linden)
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