Cambridge O Level Biology · Syllabus 5090 · Movement Into and Out of Cells
Turgor Pressure
What is Turgor Pressure?
Turgor pressure is the outward pressure exerted by the cell contents against the cellulose cell wall of a plant cell as water enters by osmosis and the vacuole expands. A cell in which this pressure is high is described as turgid, and the combined turgor of many cells is what supports the stems and leaves of non-woody plants. When water is lost the pressure falls, the cell becomes flaccid, the tissue loses support and the plant wilts.
This definition is part of the Movement Into and Out of Cells chapter in Cambridge O Level Biology.
Turgor Pressure in context
Turgor pressure is the outward pressure of the cell contents against the cell wall. Water entering by osmosis expands the vacuole, which presses the cytoplasm and membrane against the rigid cellulose wall; the wall pushes back and the pressure rises until the cell is turgid. Lose that water and turgor pressure falls: the cell becomes flaccid, and with severe loss the cell membrane and cell contents pull away from the cell wall — the cell is plasmolysed. Because non-woody plants are held up by the turgor of their cells rather than by wood, losing turgor across a whole tissue makes the plant wilt.
Common mistakes with Turgor Pressure
- 5. “A plant cell bursts when water enters it.” Why wrongRigid cellulose walls resist the outward pressure, so the pressure inside rises instead of the volume increasing without limit. CorrectThe cell becomes turgid. Rising turgor pressure opposes further net entry of water, and the cell reaches a stable state. Say this“The cellulose cell wall resists the outward pressure, so the cell becomes turgid rather than bursting.” CheckWhich cell type bursts in distilled water, and why? (An animal cell — it has no cell wall.)
- 8. “Flaccid and plasmolysed are two words for the same thing.” Why wrongThey describe different amounts of water loss, and the diagrams are different. CorrectFlaccid: turgor pressure has fallen, the membrane still lies against the wall. Plasmolysed: so much water has gone that the membrane has separated from the wall. Say this“The cells are flaccid; if more water were lost they would become plasmolysed.” CheckA wilting plant is watered and recovers within an hour. Were its cells flaccid or plasmolysed? (Flaccid — plasmolysis is far more severe and may be irreversible.)
Questions students ask about Turgor Pressure
Why does a plant cell not burst when it takes in water?
Because it is enclosed by a rigid cellulose cell wall. As water enters by osmosis the vacuole expands and the cell contents press outwards on the wall, producing turgor pressure. The wall resists, so the pressure inside rises instead of the volume increasing without limit, and the cell becomes turgid. An animal cell has no cell wall, so in a solution of higher water potential it swells and may burst.
What is the difference between a flaccid cell and a plasmolysed cell?
They describe different amounts of water loss. A flaccid cell has lost enough water for its turgor pressure to fall, but the cell membrane still lies against the cell wall. A plasmolysed cell has lost so much water that the cell membrane and the cell contents have pulled away from the cell wall. The wall itself never moves or changes — only the living contents shrink — so "the cell wall becomes plasmolysed" is wrong.