Turgor Pressure

Have you ever eaten at a Chinese buffet, then a few hours later be more thirsty than normal? That is because all of the sodium in the food; it sucks the water right out of your cells. When a lack of water occurs for plants, they simply wilt until they are replenished. Against popular opinion, plants don't wilt because they are sad, but rather because of a force called Turgor pressure.

To understand Turgor pressure, we first need to understand the terms hypertonic, hypotonic, and isotonic as they relate a solution. In a hypertonic solution, their is a higher concentration of solutes outside of the cell than inside. Because of this, water will flow out from the cell to balance out the concentration, leaving the cell dry (what happens when you eat at a chinese buffet). Hypotonic solutions are the exact opposite, water flows into the cell because that is where the solute concentration is higher. Isotonic means perfect equilibrium, the solution concentrations are the same inside and out of the cell. Here is a diagram to help visualize what I am describing.



When a plant is watered, the roots absorb the liquid and a transport tissue called xylem carries the water and soluble nutrients through the rest of the plant. The water enters the cells through a process called osmosis. Osmosis occurs when the solution outside the cell is hypotonic, meaning water will flow into the cell to balance out the higher concentration inside. All of this water flowing into the cell fills the vacuole and causes the cell to swell, enlarging the cell membrane until it pushes against the cell wall. At this point the cell becomes rigid and it called turgid, hence Turgor pressure. Turgor pressure is simply the force that pushes the plasma membrane against the cell wall, causing the cell to be structurally rigid. This structural rigidity is what causes plants to stand up straight. When there is a lack of water the cells are not turgid, so the whole plant loses its form and wilts.















To test this yourself, get two stalks of celery (bottom cut off) and two glasses. Fill one glass with salt water and one with freshwater, and place a celery stalk in both. After a night in the solutions, the stalk in the salt water should be more pliable and wilted, while the one in the freshwater should be rigid and proud.