Turgor Pressure Definition
Turgor pressure is the force exerted by stored water against a cell wall. Fungi, protists, bacteria, and plants all secrete various extracellular molecules form together to create a solid wall on the outside of their cells. As water fills the cells, it pushes against the cell membrane and cell wall, producing turgor pressure. While the cell walls of bacteria and protist cells simply keep the cells from exploding in a hypotonic environment, multi-celled organisms like fungi and plants use their turgor pressure to create various forms.
Plants and fungi regulate the turgor pressure in their cells by directing water into specialized vacuoles. The vacuoles are hypertonic to the cytoplasm, so they draw water out of the cytoplasm. This allows the concentration of the cytoplasm to stay consistent, while the water is continually moved into the cell. As turgor pressure builds in the vacuole, it pushes out against the sides of the cell. Each cell is assembled so their cell walls are pushed together. In this way, each cell in a plant becomes a water filled brick. The cells can be stacked to great heights. Plants can even turn their leaves and stems toward the sun by modifying the turgor pressure in their cells. If water is let out of the vacuole, the cell deflates. Even a few brick collapsing in a building allow gravity to pull the rest down. In the same way, by changing the turgor pressure of a small group of cells, plants can rotate, lift and otherwise move their parts.
To keep their turgor pressure, plants and fungi must keep their internal cells in a hypotonic environment. The increased concentration of solutes on the inside of the cell helps water move up the organism, and reach all the cells. Once all cells have sufficient turgor pressure, water is usually evaporated out of the leaves or from the surface of the organism to keep the flow of nutrients from the roots established. Animal cells typically try to avoid turgor pressure, because they do not secrete a cell wall to protect their cells from over-expanding and lysing, or breaking apart. Animals typically rely on a series of mechanisms that keeps their blood isotonic compared to their cells.