Turgor pressure (P) is the pressure exerted by the protoplast (the cell contents) pushing outwards against the cell wall. The wall pushes back on the contents with an equal and opposite pressure, sometimes called wall pressure. Turgor pressure is also called pressure potential, written ψp.How it builds up: when water enters a plant cell by osmosis, the vacuole and cytoplasm swell. The cellulose wall is strong and only slightly elastic, so it resists further expansion. The contents become pressurised, just as air pressure builds up in a tyre being pumped up.
Key facts about turgor pressure:
• In a living plant cell it normally has a positive value, for example +400 kPa.
• Pressure pushes water out of the cell, so turgor pressure raises the water potential of the cell, making it less negative.
• The more water a cell takes in, the higher the turgor pressure, until the wall stops it expanding further and the cell is fully turgid.
• In a flaccid cell, the protoplast does not press on the wall, so the turgor pressure is 0 kPa. This is also true at incipient plasmolysis and in plasmolysed cells.Why turgor pressure matters to plants:
• It provides support. In non-woody plants, turgid cells press against each other and keep stems and leaves firm. When turgor is lost the plant wilts.
• It drives cell expansion during growth, as the pressure stretches a softened wall.
• It controls stomata. When guard cells take in water and their turgor pressure rises, they bend apart and the stoma opens. When they lose turgor, the stoma closes.
Key facts about turgor pressure:• In a living plant cell it normally has a positive value, for example +400 kPa.
• Pressure pushes water out of the cell, so turgor pressure raises the water potential of the cell, making it less negative.
• The more water a cell takes in, the higher the turgor pressure, until the wall stops it expanding further and the cell is fully turgid.
• In a flaccid cell, the protoplast does not press on the wall, so the turgor pressure is 0 kPa. This is also true at incipient plasmolysis and in plasmolysed cells.Why turgor pressure matters to plants:
• It provides support. In non-woody plants, turgid cells press against each other and keep stems and leaves firm. When turgor is lost the plant wilts.
• It drives cell expansion during growth, as the pressure stretches a softened wall.
• It controls stomata. When guard cells take in water and their turgor pressure rises, they bend apart and the stoma opens. When they lose turgor, the stoma closes.