|MadSci Network: Botany|
Some of the confusion might be the similar terms hypotonic and hypertonic. It can get confusing because when a cell is hypertonic relative to the surrounding solution, the surrounding solution is hypotonic relative to the cell. Let's apply the -tonic terms just to the solution for clarity. Hypertonic solution - cell less salty than surrounding solution, net water movement out of cell Isotonic solution - cell and surrounding solution equally salty, no net water movement Hypotonic solution - cell saltier than surrounding solution, net water movement into cell It depends on whether it is a plant or animal cell. Here's what to expect: Plant cell Hypertonic solution - Plasmolysed cell Isotonic solution - Nonturgid or wilted cell Hypotonic solution - Turgid cell (Usual environment) Animal cell Hypertonic solution - Cell shrinks Isotonic solution - Normal (Usual environment) Hypotonic solution - Cell swells and may burst The differences are due to the fact that plant cells have a cell wall and animal cells do not. Remember that the usual condition is different for plant and animal cells. When considering effects of changing the external solution, the normal assumption is you are transferring a cell from its usual environment to a different environment. A plant cell is normally "saltier" than its external environment so the cell swells slightly due to osmosis as water enters the cell. The cell wall limits swelling and allows a positive turgor pressure or tugor potential to develop. Osmotic potential plus turgor potential equals the water potential. If the osmotic potential was -8 and the turgor potential was +2 then the water potential would be -6. A plant cell transferred from a hypotonic solution to an isotonic solution will lose a little bit of water, and the turgor potential will go to zero. With zero turgor, the cell is wilted. The water potential will be the same inside the cell and in the external solution. A plant cell transferred from a hypotonic solution to a very hypertonic solution will immediately lose a large volume of water by osmosis. The cell volume is greatly reduced, and the cell membrane shrinks away from the cell wall. This is often demonstrated under a microscope with an elodea leaf or onion bulb epidermis. An animal cell is normally as salty as its external solution. Animal cells do not have cell walls so have no turgor pressure. Plants use turgor pressure for support. An animal cell transferred from an isotonic to a hypertonic solution will shrink as water moves out of the cell via osmosis. An animal cell transferred from isotonic to hypotonic will gain water via osmosis and swell. If the solution is dilute enough, the cell may burst because it has no cell wall to prevent the cell membrane from stretching as the cell expands. Turgid and flaccid are terms that really only apply to cells with cell walls, such as plant cells. Animal cells may shrink or swell when placed in hypertonic and hypotonic solutions, respectively, but they never have turgor because the cell membrane is too weak. A cell wall is required for a cell to have a turgor potential. Plasmolysis is a term that applies only to cells with cell walls. Plasmolysis of plant cells is a very extreme and rather artifical situation found mainly in biology classes. In a typical plasmolysis demonstration, an elodea leaf on a microscope slide is flooded with a concentrated (10% by weight) salt solution. Water rapidly exits the cell, and the cell membrane shrinks away from the cell wall. However, the cell membrane is still surrounded by solution. Under natural conditions, plants are rarely exposed to concentrated salt solutions. When plant cells are subjected to extreme drought, their cells may shrink but the outside of the cell membrane would be surrounded by air, not solution. Plant cells in isotonic and hypertonic solutions both lack turgor. References Animal versus Plant Cells in Isotonic, Hypertonic and Hypotonic Solutions Hypotonic Plasmolysis definition Photos of Red Blood Cells in Hypotonic Solution Photos of Red Blood Cells in Isotonic Solution Photos of Red Blood Cells in Hypertonic Solution Photos of Plasmolysis in Elodea Leaf Cells and Onion Bulb Epidermal Cells Plasmolysis
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