Plasmolysis in onion epidermal cells refers to the shrinking of the cell's protoplast (the living part of the cell inside the cell wall) away from the cell wall due to water loss when the cell is placed in a hypertonic solution.
Understanding Plasmolysis
Here's a breakdown of plasmolysis in onion cells:
- Initial State: Before plasmolysis, onion cells are typically in an isotonic environment, meaning the concentration of water inside and outside the cell is balanced.
- Hypertonic Environment: When onion epidermal cells are exposed to a hypertonic solution, such as salt water, the water concentration outside the cell is lower than inside.
- Water Loss: This concentration difference leads to osmosis – the movement of water from a higher concentration to a lower concentration. Water moves out of the onion cell towards the surrounding hypertonic solution.
- Cell Shrinkage: As water leaves, the cell's cytoplasm and vacuole shrink, pulling the plasma membrane away from the rigid cell wall. The cell's protoplast contracts and the cell is said to be plasmolyzed.
Key Aspects of Plasmolysis
| Aspect | Description |
|---|---|
| Environment | Hypertonic solution (e.g., salt water). |
| Process | Osmosis – water moves from inside the cell to the surrounding solution. |
| Cellular Change | Protoplast shrinks and pulls away from the cell wall. |
| Visual Outcome | The cell appears smaller, and the plasma membrane is no longer in contact with the cell wall. |
| Reversibility | Plasmolysis can be reversed if the cell is placed back into a hypotonic or isotonic solution, allowing water to re-enter. |
Practical Implications
- Microscopy: Plasmolysis is a common phenomenon observed under microscopes when plant cells are subjected to salt or sugar solutions.
- Preservation: This process is the underlying principle in methods like pickling, where high salt or sugar concentrations dehydrate microorganisms, inhibiting their growth and preserving the food.
- Plant Physiology: It is a basic concept for understanding how plants respond to water stress.
Example
The reference provided describes how red onion cells, initially in an isotonic solution, undergo plasmolysis when exposed to salt water. The salt water is a hypertonic solution. This causes water to move out of the cells, resulting in the shrinkage that defines plasmolysis.
