Plasmolysis in a plant cell is defined as shrinkage of cytoplasm in a hypertonic medium.

Plasmolysis is a phenomenon that occurs in plant cells when they are placed in a hypertonic solution. In a hypertonic solution, the concentration of solutes outside the cell is higher than inside the cell. As a result, water from inside the cell moves out to the surrounding solution through osmosis. This loss of water causes the cytoplasm to shrink away from the cell wall, resulting in plasmolysis.

Explanation:

1. Hypertonic medium:
- Plasmolysis occurs in a hypertonic medium where the concentration of solutes outside the cell is higher than inside the cell.
- The hypertonic medium creates a concentration gradient that causes water to move out of the cell.

2. Shrinkage of cytoplasm:
- As water moves out of the cell, the cytoplasm loses water and shrinks away from the cell wall.
- This shrinkage of the cytoplasm is a characteristic feature of plasmolysis.

3. Cell wall:
- The cell wall remains intact during plasmolysis and does not shrink.
- This is because the cell wall provides structural support and prevents the cell from collapsing completely.

4. Plasma membrane:
- The plasma membrane also contracts and pulls away from the cell wall as the cytoplasm shrinks.
- However, it does not break down (lysis) during plasmolysis in a hypertonic medium.

5. Reversibility:
- Plasmolysis is a reversible process. When a plasmolyzed cell is placed in a hypotonic medium (where the concentration of solutes is lower outside the cell), water moves back into the cell through osmosis, and the cell regains its turgidity.

In conclusion, plasmolysis in a plant cell is defined as the shrinkage of the cytoplasm in a hypertonic medium. It occurs when water moves out of the cell, causing the cytoplasm to shrink away from the cell wall. The plasma membrane contracts but does not break down during plasmolysis. Plasmolysis is a reversible process and the cell can regain its turgidity when placed in a hypotonic medium.