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The force applied to the plasma membrane of a plant cell against its cell wall is called turgor pressure. Water travels within a cell and creates pressure within the cell wall. It is the characteristic feature of cells of plants, bacteria, and fungi. The pressure is maintained by the process called osmosis.
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The water potential, osmotic potential, and root water uptake determine the turgor pressure in the plants.
- It plays an important role in the growth of plants and cell volume expansion.
- When turgor pressure is low, the cell becomes flaccid.
Related Links:
- What is Turgor Pressure in Plants?
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- Mechanism of Turgor Pressure (How Turgor Pressure is Developed)
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- Things to Remember
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What is Turgor Pressure in Plants?
In plants, cells grow primarily by expansion, rather than by division, like in animals. The uptake of water by plants results in the expansion, pushing against the rigid cell, which ultimately leads to cell enlargement. The force that regulates the entire process of water uptake in plants is called turgor pressure.
- In plants, the importance of turgor pressure is evident in maintaining the erectness of leaves and stems, as well as in the opening and closing of stomata.
- Loss of turgor pressure leads to wilting, a common phenomenon observed in plants under water stress.
Key Terms Related to Turgor Pressure
When a plant cell absorbs water through osmosis, the vacuole swells, pushing the cytoplasm against the rigid cell wall, creating positive internal pressure known as turgor pressure. Some of the important terms related to turgor pressure in the NCERT Class 11 Biology Book are given below:
| Term | Meaning |
|---|---|
| Turgid cell | Cell with high turgor pressure; fully inflated and firm |
| Flaccid cell | Cell with low or zero turgor pressure |
| Plasmolysis | Shrinking of cytoplasm away from the cell wall due to loss of water in a hypertonic medium. |
| Wilting | Drooping of leaves and stems caused by loss of water due to turgor pressure |
Examples of Turgor Pressure in Daily Life
- Lack or absence of water in plants reduces the turgor pressure, resulting in wilting of leaves. This usually occurs when the concentration of solute is lower outside the cell than inside it.
- The crisp texture of fresh lettuce or cucumber is due to high turgor pressure in their cells.
- Changes in turgor pressure of guard cells control the opening and closing of stomata in the morning and at night.
- Touch-me-not plant (Mimosa pudica) folds its leaves rapidly due to the sudden loss of turgor pressure in pulvini cells.
Mechanism of Turgor Pressure (How Turgor Pressure is Developed)
The development of turgor pressure in plants occurs through the process of osmosis and depends on the water potential gradient.
Step-by-Step Mechanism
- Plant roots absorb water and minerals from the soil, creating a hypotonic environment outside root hair cells.
- Water moves into the cell by osmosis because the water potential inside the cell becomes lower than outside.
- The large central vacuole stores incoming water and dissolved solutes, increasing the volume of cell sap.
- The elastic plasma membrane expands slightly, but the rigid cell wall resists further expansion.
- This resistance creates an opposing force called wall pressure, and the net inward push of water is balanced by wall pressure, resulting in turgor pressure (TP =Ψp)
Therefore,
| OP (osmotic pressure) - TP = DPD (diffusion pressure deficit) (turgor pressure) |
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Relationship Between Water Potential, Turgor Pressure, and Solute Potential
The relationship between water potential, turgor pressure, and solute potential are explained below:
| Component | Relation |
|---|---|
| Water Potential (Ψw) | Ψw = Ψs + Ψp |
| Solute potential (Ψs) | Always negative; lowers water potential |
| Pressure potential (Ψp) | Usually positive in turgid cells; equals turgor pressure |
Note: In a fully turgid cell; Ψw (cell) = Ψw (pure water) = 0, therefore Ψp = –Ψs
Significance of Turgor Pressure in Plants
To better understand the significance of Turgor pressure, let's create two cell systems, System A and System B, to represent the plant cell.
- Cells in system A are densely packed with zero space, similar to brick walls, but cells in system B are loosely packed with space.
- When the turgor pressure of this system A cell is considered, its water concentration is greater than that of system B, which has a high turgor pressure.
- System B cells are comparably flaccid, implying low turgor pressure.
- High turgor pressure in plant cells leads to optimal form, which means the plant's body will be in a favourable position.
- Low turgor pressure causes cells to shrink inward (cell walls are less flexible), resulting in an intracellular space (voids between cells).
- The plant cell wilts when the intracellular space develops, causing the plant to wilt.
- This means healthy plant growth needs adequate water, and turgor is prominent.
- Turgor pressure is a phenomenon in plants, fungi, and bacteria induced by the osmotic movement of water.
Thus, turgor pressure plays a crucial role in maintaining the shape, size, and rigidity of plant cells. The role of Turgor Pressure in plants is to:
- Provide mechanical support and maintenance to the shape of herbaceous plants.
- Driving cell elongation and tissue expansion during growth (responsible for the rapid development of seedlings)
- Facilitating stomatal movements (seismonasty, photonasty) and nutrient transport.
- Acting as a counterforce against gravity, enabling plants to orient their leaves toward sunlight.
Things to Remember
- Turgor pressure refers to the movement of water inside the cell, which in turn develops a pressure within the cell wall.
- A net positive water flow into the cell causes turgidity in the cell.
- Excessive osmosis in animal cells may lead to cell bursting.
- Vacuoles maintain the majority of the turgor pressure within the cell.
- It also aids in the removal of toxic waste products from the cell.
FAQs
Ques: What is turgor pressure in biology?
Ans: Turgor pressure is the pressure exerted by water inside the vacuoles of plant cells (and some bacteria/fungi) against the cell wall. It is the force that pushes the plasma membrane against the rigid cell wall, keeping plant cells firm. Turgor pressure is created when water enters the cell via osmosis, causing the cell to become turgid (swollen and hard). It is responsible for the crispness of healthy leaves and stems.
Ques: What is turgor pressure and its importance in plants?
Ans: Turgor pressure is essential for plants because it:
- Maintains structural support and rigidity (keeps plants upright without a skeleton)
- Drives cell expansion and growth of plant cells
- Opens and closes stomata (guard cells become turgid to open stomata)
- Enables movements like leaf folding in Mimosa pudica or Venus flytrap snapping
- Keeps leaves and stems erect (loss of turgor = wilting)
Without adequate turgor pressure, plants wilt and cannot perform photosynthesis efficiently.
Ques: What is the formula for turgor pressure?
Ans: Turgor pressure (ψₚ) is calculated using the water potential equation:
ψ = ψₛ + ψₚ
Rearranged for turgor pressure:
ψₚ = ψ − ψₛ
Where:
- ψ = total water potential of the cell
- ψₛ = solute (osmotic) potential
- ψₚ = pressure potential (turgor pressure)
In fully turgid cells, the water potential (ψ) of the cell equals that of the external solution (usually O in pure water), so turgor pressure equals the negative of the solute potential.
Ques: What happens when turgor pressure decreases in plant cells?
Ans: When turgor pressure decreases (due to water loss or high external solute concentration):
- Plasmolysis occurs – plasma membrane pulls away from the cell wall
- Cells become Flaccid (limp)
- The plant Wilts
- Stomata close (reduces photosynthesis and transpiration)
- Growth stops
Extreme loss leads to cytorrhysis (collapse of the cell wall in some cases).
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