Water Concentration and Change in Turgor Pressure in Plant Cells (page 2)
Design Your Own Experiment
- What is the effect of turgor pressure on plant cells? Demonstrate the change in turgor pressure in plant cells by cutting across the bottom of different plant stems, such as flowers and celery. Stand the stalks in an empty glass at room temperature. Prepare a second stalk of each plant in like manner, but stand these stalks in a glass half filled with distilled water. Allow the stems to stand in the glasses for 24 hours. Observe the change in the appearance and firmness of the stalks. Display before and after photographs of the stalks to represent the changes of turgor pressure as a result of the gain and loss of water from cells.
- How does turgor pressure affect the movement of plants? Water movement through the petals of morning glories creates enough pressure to cause the petals to spread open. Cut a 6-×-2-inch (15-×-5-cm) strip from a newspaper. Cut a petal shape at both ends of the strip. Fold the petals toward the center of the strip and crease the paper so that the petals lay flat. Drop the folded paper, petal side up, on top of water in a bowl (see Figure 7.3). Notice what happens. More information about the opening of flowers due to turgor pressure can be found in the experiment titled "Morning Glory," (p. 30) in Janice VanCleave's 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments (New York: Wiley, 1992).
- How does heat affect the rate and quantity of water absorption by cells? Into each of three Styrofoam® cups, pour cold, room-temperature, and warm water, respectively. Place raisins and a thermometer in each cup. Note: Do not make the water so hot that it ruptures the cell membrane of the fruit. Use aluminum foil to seal off the top of the cups. Observe the size of the raisins in each cup as often as possible until the temperature of the water in the cups is the same. Use your observations to determine whether the temperature affected the rate and quantity of water absorbed by the raisins.
- Membranes of cells continue to be semipermeable as long as they are alive, but liquids are easily extracted from plants after their tissues have been killed. Demonstrate this by using two fresh beets. Grate one of the beets and boil the second beet in a saucepan of water. Even when grated, the fresh beet cells retain their fluids, but much of the liquid coloring matter inside the beet is extracted by killing the cells in the hot water.
Get the Facts
- Osmotic pressure can move small quantities of water into stems, but the tons of water raised to the tops of giant sequoias and even the small quantities of water lifted to the petals of morning glories require more than osmotic pressure. Plants do not pull or pump the water through their systems, but the water does flow up the stems of plants. The movement of this water to the summit of giant trees or the top of small plants is a result of transpiration, capillary action, cohesion, and adhesion. Find out more about this upward movement of water against the downward pull of gravity.
- Effective responses in animals require internal communication. The nervous systems in higher organisms send messages that result in movement due to muscle relaxation and contraction. Plants have no nervous systems or muscle cells, and quick responses are almost nonexistent in the plant kingdom. However, a few plants, such as the mimosa, respond rapidly to being touched. It appears that a stimulus passes through the plant's stem as the leaves fold in a wave of motion from the tip to the base. Find out more about rapid plant motion. What causes this rapid movement of mimosa leaves? Nerves are not involved, but are there any electrical impulses in plants? Is this motion due to tropism or nastic movements?
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