Turgor pressure is the internal water pressure that pushes outward against the cell wall of a plant, fungal, or bacterial cell. It’s what keeps plants upright, drives their growth, and controls the tiny pores on their leaves. When you see a wilted houseplant perk back up after watering, you’re watching turgor pressure at work. In some cells, this pressure can reach 2 megapascals, roughly double the air pressure inside a car tire.
How Cells Build Pressure
Turgor pressure starts with osmosis. When the concentration of dissolved molecules inside a cell is higher than in the surrounding fluid, water flows in through the cell membrane to balance things out. As water fills the cell’s large central vacuole, the expanding volume pushes the membrane outward against the rigid cell wall. The cell wall pushes back, and pressure builds until the two forces reach equilibrium. At that balance point, no more water enters the cell.
Because the cell’s contents behave like a liquid, turgor pressure acts equally in all directions, much like air pressure inside a balloon. This is a critical difference from animal cells, which lack a rigid wall and would simply burst under the same conditions. The cell wall acts as a pressure vessel, containing the force and converting it into structural support.
Three things can shift this equilibrium and change turgor pressure: a drop in pressure itself (from water loss), an increase in dissolved molecules inside the cell (pulling more water in), or a decrease in dissolved molecules outside the cell. Plants manipulate all three to regulate their internal pressure in real time.
Typical Pressure Values
Turgor pressure varies widely depending on cell type and species, but most measurements fall between 0.1 and 0.9 megapascals. Soybean stems clock in around 0.46 MPa. Corn and wheat roots range from 0.4 to 0.6 MPa. Tomato root cells sit near 0.3 MPa. Arabidopsis, a small plant widely used in lab research, shows about 0.18 MPa in its leaves and 0.3 to 0.6 MPa in its root surface cells. Onion cells tend to run higher, at 0.8 to 0.9 MPa. These numbers come from a technique called pressure probing, where a tiny micropipette is inserted directly into a living cell.
Why It Matters for Plant Structure
Turgor pressure is the main reason non-woody plants stay upright. Herbaceous stems and leaves don’t have thick layers of wood for support. Instead, millions of pressurized cells press against each other, creating a collective rigidity similar to how a stack of inflated balloons holds its shape. Remove the air, and the whole structure collapses. Remove the water from plant cells, and the plant wilts.
This is also how plants grow. For a cell to get larger, its wall must stretch. The cell acidifies the wall, loosening the bonds between its structural fibers, and turgor pressure then pushes the softened wall outward. Water continues to flow in to fill the expanding volume, and new wall material is deposited to maintain strength. Without adequate turgor, growth stalls even if nutrients and light are plentiful.
How Stomata Open and Close
One of the most elegant uses of turgor pressure happens in guard cells, the paired cells that form tiny pores (stomata) on the surface of leaves. Plants open these pores to take in carbon dioxide for photosynthesis and close them to conserve water. The switch between open and closed is entirely driven by turgor changes in the guard cells.
When light hits a leaf, guard cells pump potassium ions inward. The rising concentration of dissolved ions draws water into the cells by osmosis, inflating them. Because of the way guard cell walls are built (thicker on the inner edge), they bow apart as they swell, opening the pore between them.
Closing works in reverse. During drought, a stress hormone triggers guard cells to release potassium and chloride ions outward. Water follows, the guard cells deflate, and the pore snaps shut. This response can happen within minutes, allowing plants to fine-tune water loss throughout the day. It’s one reason why leaves close their stomata during the hottest part of the afternoon, even if there’s still plenty of sunlight.
What Happens When Turgor Drops
When soil dries out or a cut flower sits without water, cells lose turgor and become flaccid. The cell membrane pulls away from the wall, a process called plasmolysis. At the point of “incipient plasmolysis,” the membrane has just barely separated from the wall and all turgor pressure has been lost. The cell still holds some water, but none of it is generating outward force.
In a living plant, this plays out as wilting. Leaves droop because they’ve lost the internal pressure that held them rigid. If the soil is rewatered in time, cells reabsorb water, turgor rebuilds, and the plant recovers. But if water deprivation continues past a certain threshold, known as the permanent wilting point, the soil holds its remaining moisture so tightly that roots can no longer extract it. At that stage, the damage becomes irreversible.
Turgor in Fungi and Bacteria
Turgor pressure isn’t exclusive to plants. Any cell with a rigid wall and a higher internal solute concentration will develop it, and that includes fungi and bacteria. In these organisms, turgor contributes to cell shape, structural integrity, and even invasion strategies.
Some pathogenic fungi exploit turgor pressure to break into their hosts. They build specialized structures called appressoria that generate remarkably high pressures, enough to physically puncture plant tissue or insect exoskeletons. High turgor also helps fungi maintain their shape in harsh environments like compressive soil, decaying plant material, or biofilms. Across plants, fungi, bacteria, and even some animal embryos, turgor pressure can approach megapascal values, making it one of the most universal mechanical forces in cellular biology.
The Water Potential Equation
Biologists describe water movement between cells using a concept called water potential, which combines all the forces acting on water in one value. Turgor pressure is one component of this equation. The others include solute potential (the pull created by dissolved molecules), gravity (relevant in tall trees), and matric potential (the attraction of water to surfaces like soil particles or cell walls).
Water always flows from regions of higher water potential to lower water potential. When you water a dry plant, the soil’s water potential jumps above the root cells’ water potential, and water rushes in. Turgor pressure is the component that resists further water entry. As turgor builds, it raises the cell’s total water potential until it matches the surroundings, and flow stops. This framework explains everything from why roots absorb water to why over-fertilizing can actually dehydrate a plant: adding too much solute to the soil drops its water potential below the root cells’, reversing the flow.