Difference Between Turgidity and Flaccidity

Main Difference – Turgidity vs Flaccidity

The mechanism by which water molecules move in and out of the cell through its plasma membrane is known as osmosis. The two types of osmosis are endosmosis and exosmosis. Endosmosis is the process by which water enters the cell. On the other hand, exosmosis is the movement of water out of the cell. The tonicity of the cytoplasm and surrounding solution determines the direction of the water movement. Tonicity is the effective osmotic pressure gradient defined by the water potential of the two solutions. Hypertonic, isotonic, and hypotonic are the three types of solutions that cause different tonicities. Turgidity and flaccidity are two conditions that occur depending on the tonicity. The main difference between turgidity and flaccidity is that turgidity is caused by the movement of water into the cell by endosmosis when it is placed in a hypotonic solution whereas flaccidity is caused by the movement of water out of the cell by both endosmosis and exosmosis when it is placed in an isotonic solution.

Key Areas Covered

1. What is Turgidity
     – Definition, Mechanism, Importance
2. What is Flaccidity
    – Definition, Mechanism, Importance
3. What are the Similarities Between Turgidity and Flaccidity
     – Outline of Common Features
4. What is the Difference Between Turgidity and Flaccidity
     – Comparison of Key Differences

Key Terms: Endosmosis, Exosmosis, Hypertonic Solution, Isotonic Solution, Osmosis, Plasma Membrane, Tonicity

What is Turgidity

Turgidity refers to the state of being turgid or swollen due to high fluid content inside the cell. The cell is in its fully expanded condition during turgidity. Turgidity occurs when a cell is placed in a hypotonic solution that contains low solute concentration than that of the cytoplasm. Here, water enters the cell by endosmosis. The water pressure generates the turgor pressure that pushes the cell membrane against the cell wall. Turgidity is an important factor for plant cells. The opening and closing of stomata are determined by the turgidity of the guard cells. When guard cells are in their turgid state, the stomata open. The opening and closing of stomata maintained by the turgidity of the stomata are shown in figure 1.

Figure 1: Opening and Closing of the Stomata
Opened stomata (left) Closed stomata (right)

Cells enlarge in plume due to turgidity. The dispersion of spores and seeds also occurs by the turgidity of the cells in sporangia and fruits. Most aquatic plants get rigidity due to the turgidity of cells. Therefore, turgidity provides mechanical support to plants. Germination also occurs due to the turgidity of the cells in the embryo. Thus, the embryo comes out from the seed, diming the germination. Nutrient solutions move from one cell to another cell due to the turgidity. Therefore, turgidity is important to save plants from wilting.

What is Flaccidity

Flaccidity refers to the state between turgidity and plasmolysis where the plasma membrane is not pushed against the cell wall. It occurs when a turgid cell is placed in an isotonic solution. Since the water potential inside the cell is higher than in the surrounding solution, water molecules from the cytoplasm move out of the cell by exosmosis. This reduces the turgor pressure to some extent. The movement of water continues until the water potential at both sides of the plasma membrane becomes equal. Thus, in the flaccid state of a cell, there is no net movement of water across the plasma membrane. Both endosmosis and exosmosis occur at the same rate. The plasmolyzed, flaccid, and turgid states of a plant cell are shown in figure 2.

Figure 2:  Plasmolyzed, Flaccid, and Turgid States of a Plant Cell

The more extreme state of flaccidity is known as plasmolysis in which the persisting release of water molecules occurs from the cytoplasm when the cells are placed in a hypertonic solution.

Similarities Between Turgidity and Flaccidity

• Both turgidity and flaccidity are two conditions of cells that occur depending on the tonicity.
• The movement of water molecules occurs across the cell membrane, causing both turgidity and flaccidity.
• The movement of water into the cell occurs in both turgidity and flaccidity.

Difference Between Turgidity and Flaccidity

Definition

Turgidity: Turgidity refers to the state of being turgid or swollen due to high fluid content inside the cell.

Flaccidity: Flaccidity refers to the state between turgidity and plasmolysis in which the plasma membrane is not pushed against the cell wall.

Type of Solution

Turgidity: Turgidity occurs when cells are placed in a hypotonic solution.

Flaccidity: Flaccidity occurs when cells are placed in an isotonic solution.

Type of Osmosis

Turgidity: Turgidity is caused by endosmosis.

Flaccidity: Flaccidity is caused by both endosmosis and exosmosis.

Water Movement

Turgidity: Turgidity is caused by the movement of water into the cell.

Flaccidity: Flaccidity is caused by the movement of water out of the cell.

Net Movement of Water

Turgidity: No more water enters the cell at turgidity.

Flaccidity: No net movement of water occurs in flaccidity. The rate of endosmosis and exosmosis is equal.

Water Potential

Turgidity: Turgidity has the highest water potential that can occur in a cell.

Flaccidity: Flaccidity has a less water potential than the turgidity.

Plasma Membrane

Turgidity: Plasma membrane is pushed against the cell wall by the turgidity.

Flaccidity: Plasma membrane is not pushed tightly against the cell wall in flaccidity.

Conclusion

Turgidity and flaccidity are two states of a cell caused by differential movement of water molecules between cytoplasm and surrounding solutions across the plasma membrane. Turgidity occurs by endosmosis when cells are placed in hypotonic solutions. On the other hand, flaccidity occurs when cells are placed in isotonic solutions. Water potential of the cytoplasm is equal to that of the surrounding solutions in flaccidity. Therefore, no net movement of water occurs. The main difference between turgidity and flaccidity is the water potential inside the cytoplasm.

Reference:

1. “Turgidity: Definition and Importance.” QS Study, 21 Aug. 2017, Available here.
2. “Flaccidity.” Flaccidity – Biology As Poetry, Available here.

Image Courtesy:

1. “Stomata opened and closed unlabelled” By domdomegg – Own work (CC BY 4.0) via Commons WikmediaWikmedia
2. “Turgor pressure on plant cells diagram” By LadyofHats  (Public Domain) via Commons Wikimedia