Turgidity is an essential concept in plant cells, as it helps maintain structure and function. When a plant cell absorbs water, it swells and becomes fully turgid. But what happens to the different pressure components inside the cell when this occurs?
One crucial aspect of cell turgor is pressure relationships, particularly turgor pressure, solute potential, and pressure potential. In this topic, we will explore which of these values becomes zero when a cell reaches full turgidity.
1. Understanding Turgor Pressure
Turgor pressure is the force exerted by the plasma membrane against the cell wall due to water uptake. It is a critical factor that helps:
✔ Maintain cell rigidity
✔ Support plant structure
✔ Regulate growth and movement
When a plant cell is placed in a hypotonic solution (one with lower solute concentration than the cell’s cytoplasm), water enters the cell via osmosis, increasing turgor pressure.
2. Osmosis and Water Potential
Osmosis is the movement of water from an area of higher water potential to an area of lower water potential across a semi-permeable membrane. The key components of water potential ( Psi ) include:
✔ Solute potential ( Psi_s ) – Also called osmotic potential, it decreases as solute concentration increases.
✔ Pressure potential ( Psi_p ) – The physical pressure exerted by the cell wall on the cytoplasm.
✔ Gravity potential ( Psi_g ) – Significant in tall plants but usually negligible in individual cells.
The equation for water potential is:
Water moves into a plant cell until the internal pressure balances the external environment, causing the cell to become fully turgid.
3. What Happens When a Cell Becomes Fully Turgid?
When a plant cell reaches full turgidity:
✔ The turgor pressure ( Psi_p ) is at its maximum
✔ The cell wall prevents further expansion
✔ Water potential inside the cell equals the surrounding water potential
This results in no net water movement, but which pressure component reaches zero?
4. The Component That Becomes Zero: Net Water Potential Difference
The correct answer is net water potential difference. When a cell is fully turgid:
✔ Solute potential ( Psi_s ) remains negative, as solutes are present inside the cell.
✔ Pressure potential ( Psi_p ) is positive, balancing the solute potential.
✔ Water potential ( Psi ) equals the surrounding water potential, meaning there is no net movement of water.
Since water moves only when there is a difference in water potential, this value effectively becomes zero, stopping further water intake.
5. Why Doesn’t Turgor Pressure or Solute Potential Become Zero?
Solute Potential Remains Negative
Solute potential ( Psi_s ) is always negative because the presence of solutes lowers the water potential. Even in a fully turgid cell, solutes still exist inside the cytoplasm and vacuole.
Turgor Pressure Remains Positive
Turgor pressure ( Psi_p ) is a positive force exerted by the expanding cytoplasm against the cell wall. It does not reach zero unless the cell loses water and becomes flaccid.
6. Importance of Turgidity in Plants
Turgidity is crucial for plant survival because it:
✔ Maintains structural integrity – Prevents wilting by keeping cells firm.
✔ Supports growth – Helps in cell expansion and elongation.
✔ Facilitates movement – Enables stomatal function for gas exchange.
If a plant loses turgor pressure due to water loss, it becomes flaccid, leading to wilting and reduced growth.
7. What Happens in Different Solutions?
Hypotonic Solution (Water Moves In)
✔ Water enters the cell via osmosis.
✔ Turgor pressure increases.
✔ The cell becomes fully turgid.
Isotonic Solution (No Net Movement of Water)
✔ Water moves in and out at equal rates.
✔ The cell maintains its shape but is not fully turgid.
Hypertonic Solution (Water Moves Out)
✔ Water leaves the cell, causing it to shrink.
✔ The plasma membrane pulls away from the cell wall (plasmolysis).
✔ The cell loses turgidity and becomes flaccid.
When a plant cell becomes fully turgid, its net water potential difference becomes zero, meaning there is no further movement of water. However, turgor pressure remains positive, and solute potential remains negative.
This state is essential for maintaining plant structure, growth, and survival, ensuring that plants remain upright and function properly. Understanding turgor pressure and water potential helps explain how plants regulate hydration and respond to their environment.