Turgor pressure is the internal force exerted by water within a plant cell against the rigid structure of its cell wall. This pressure provides plant cells with firmness and stability, enabling the plant to maintain its shape and structural integrity. Without this constant internal force, non-woody plant structures would collapse, demonstrating its importance for plant survival.
How Water Movement Creates Internal Pressure
The generation of this internal pressure begins with the movement of water across the cell membrane, a process known as osmosis. Plant cells are typically surrounded by water with a lower concentration of dissolved substances, or solutes, than the fluid inside the cell.
The majority of the water that enters the cell is directed into the large central vacuole, which stores cell sap. This influx of water causes the central vacuole to swell, acting like an inflating balloon. As the vacuole expands, it pushes the cell’s internal contents, the protoplast, outward against the plasma membrane.
The plasma membrane is then pressed firmly against the plant’s tough, outer cell wall. The rigid cell wall resists this outward push, preventing the cell from bursting. This counter-force exerted by the cell wall is the measurable turgor pressure, which can be substantial; in growing root cells, it can exceed the pressure found in an average car tire.
The Role of Turgor in Plant Structure and Movement
High turgor pressure provides the structural support that allows herbaceous (non-woody) plants to stand upright, functioning as a hydrostatic skeleton. This internal stiffness maintains the extended position of leaves, maximizing the surface area available for capturing sunlight.
This internal pressure is also a driving force behind cell growth, particularly cell elongation. The turgor pressure pushes the cell wall outward, allowing the wall to stretch and expand as new cell wall material is deposited. This pressure is especially important for the growth of root tips, enabling them to push through soil.
Turgor pressure also controls specific, localized movements in plants, such as the opening and closing of stomata, the tiny pores on leaf surfaces. Guard cells surrounding the stomata accumulate solutes, increasing their turgor pressure, which causes them to become bowed and opens the pore for gas exchange. Conversely, a loss of turgor in these cells causes them to collapse and seal the pore, conserving water.
Rapid plant movements, like the characteristic leaf folding of the sensitive plant (Mimosa pudica), are entirely driven by swift changes in turgor pressure within specialized motor cells. A change in the distribution of ions within these cells causes water to rush out, resulting in the sudden loss of pressure and immediate leaf collapse.
What Happens When Turgor Pressure is Lost
A significant loss of water from the plant environment, such as during a drought or excessive evaporation, causes the turgor pressure to drop. When the rate of water leaving the cells exceeds the rate of water entering, the internal hydrostatic skeleton fails. The visible result of this pressure loss is wilting, where the leaves and stems become limp and droop.
At the cellular level, continued water loss causes the cell to become flaccid, meaning the internal pressure is zero or near zero. In extreme cases, particularly when a plant is exposed to very high salt concentrations, the cellular process of plasmolysis occurs. During plasmolysis, so much water exits the cell that the protoplast shrinks substantially, pulling the plasma membrane completely away from the inside of the cell wall.
This separation of the cell membrane from the wall indicates severe dehydration and can lead to cell death if not reversed promptly. However, plant wilting is often a reversible state; upon rehydration, water re-enters the cells via osmosis. The central vacuoles refill, and the outward force restores turgor pressure, allowing the plant to regain its upright, firm structure.