Main Difference – Osmosis vs Plasmolysis
Passive diffusion is the movement of molecules from a higher concentration to a lower concentration. Osmosis and plasmolysis are two events that occur due to the movement of water molecules. Water is considered as the universal solvent in cells that dissolves polar molecules. The main difference between osmosis and plasmolysis is that osmosis is the movement of water molecules from high water potential to a lower water potential across a semipermeable membrane whereas plasmolysis is the shrinkage of a cell due to the persisting movement of the water molecules out of the cell. Plasma membrane serves as the semipermeable membrane during osmosis. The two types of osmosis are endosmosis and exosmosis. Plasmolysis occurs due to persisting exosmosis.
Key Areas Covered
1. What is Osmosis
– Definition, Types, Importance
2. What is Plasmolysis
– Definition, Types, Importance
3. What are the Similarities Between Osmosis and Plasmolysis
– Outline of Common Features
4. What is the Difference Between Osmosis and Plasmolysis
– Comparison of Key Differences
Key Terms: Concave Plasmolysis, Convex Plasmolysis, Crenation, Deplasmolysis, Endosmosis, Exosmosis, Osmosis, Plasma Membrane, Plasmolysis, Tonicity, Water
What is Osmosis
Osmosis refers to a type of passive diffusion in which the movement of water molecules occurs through a potential gradient across the plasma membrane. It does not require cellular energy since the movement of molecules occurs through a concentration gradient. Plasma membrane does not allow the movement of solutes in the cytoplasm to move outside. Therefore, only the water molecules move across the cell membrane. The movement persists until the water potential between the cytoplasm and surrounding solution is equal. Osmosis across a semipermeable membrane is shown in figure 1.
Figure 1: Osmosis
Endosmosis and exosmosis are the two types of osmosis. Endosmosis is the movement of water molecules into the cell when the cell is placed in a hypotonic solution. The hypotonic solution contains the higher water potential than that of the cytoplasm. Thus, water molecules move into the cells, swelling the protoplasm of the cell. This swelling increases the turgor pressure, pushing the plasma membrane against the cell wall in plant cells. This state of a plant cell is known as turgidity. In plants, the absorption of capillary water from the soil by roots and the entrance of water into the xylem vessels occur by endosmosis.
Exosmosis is the movement of water molecules out of the cell when the cell is placed in a hypertonic solution. The hypertonic solution contains a low water potential than that of the cytoplasm. So, water molecules move out of the cell into the solution, shrinking the cytoplasm. The movement of water from the root hair cells to the cortical cells of the root occurs by exosmosis.
What is Plasmolysis
Plasmolysis refers to the loss of water by cells in hypertonic solutions. The shrinking causes the separation of the cell membrane from the cell wall. Hypertonic solutions contain low water potential than that of the cytoplasm. Thus, water molecules move out of the cell by exosmosis. This leads to the loss of turgor pressure. At some point, the cell peels away from the cell wall due to the persisting loss of turgor pressure. This leaves gaps between the cell wall and the cell membrane, shrinking and crumpling the cell. Two types of plasmolysis may occur depending on the type of cells and the viscosity of the cytoplasm. They are convex plasmolysis and concave plasmolysis.
Figure 2: Cells in Different Tonicities
In convex plasmolysis, the cytoplasm is rounded up to form convex ends. In concave plasmolysis, the separation of the cytoplasm produces concave pockets. Plasmolysis is a reversible process that can be replaced by placing the cell in a hypotonic solution. The persisting plasmolysis may lead to cytorrhysis – the complete collapse of the cell wall, leading to the death of the cell. The fate of the cells when they are placed in different solutions with variable tonicity is shown in figure 2.
The plasmolysis of animal cells is known as crenation. The reverse process of the plasmolysis is known as deplasmolysis; this establishes the normal size of the cell.
Similarities Between Osmosis and Plasmolysis
- The movement of water molecules across the plasma membrane occurs in both osmosis and plasmolysis.
- Water potential gradient or tonicity leads to both osmosis and plasmolysis.
Difference Between Osmosis and Plasmolysis
Definition
Osmosis: Osmosis is a type of passive diffusion in which the movement of water molecules occurs through a potential gradient across the plasma membrane.
Plasmolysis: Plasmolysis is the loss of water by cells in hypertonic solutions.
Significance
Osmosis: Osmosis is the movement of water molecules through a potential gradient across the plasma membrane.
Plasmolysis: Plasmolysis is a state of the cell that occurs due to the persisting movement of water out of the cell.
Types
Osmosis: The two types of osmosis are endosmosis and exosmosis.
Plasmolysis: The two types of plasmolysis are concave plasmolysis and convex plasmolysis.
Types of Solutions
Osmosis: Osmosis occurs in hypertonic, isotonic, and hypotonic solutions.
Plasmolysis: Plasmolysis occurs in extreme hypertonic solutions.
Conclusion
Osmosis and plasmolysis are two events that occur due to the movement of water across the plasma membrane. Osmosis is the movement of water through its potential gradient across a semipermeable membrane. The inward movement of water towards the cytoplasm is known as endosmosis. On the other hand, the outward movement of water out from the cytoplasm is known as exosmosis. Plasmolysis is caused by the persisting exosmosis. The main difference between osmosis and plasmolysis is the fate of the cell by each type of water movement.
Reference:
1.“Brent Cornell.” Osmosis | BioNinja, ib. Available here.
2.“Plasmolysis.” Wikipedia, Wikimedia Foundation, 11 Dec. 2017, Available here.
Image Courtesy:
1. “0307 Osmosis” By OpenStax (CC BY 4.0) via Commons Wikimedia
2. “OSC Microbio 03 03 Plasmolysi” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
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