A cell dipped in 0.5M sucrose solution do decrease in size but this takes time. The process of shrinking of the protoplasm of cell due to movement of water from its higher concentration (inside cell) to the region where its concentration is low (in sucrose or salt solution) is known as plasmolysis. Plasmolysis takes more time when the cell is placed in the solutions of low molarity. When the same cell is placed in 0.5M NaCl solution, the decrease in cell size is seen immediately because though the molarity is same but the solution contains salt as a solute and is an electrolye solute. Thus, it causes the rate of plasmolysis to increase.

The 0.5 M NaCl solution is hypertonic to the cell. So, water will move from cell cytoplasm (lower concentration) to the outside (higher concentration due to NaCl hypertonicity) through the semi permeable membrane of the cell, this is called plasmolysis. Thus, the cell get plasmolysed.

Plasmolysis in a cell dipped in 0.5 M NaCl:
Plasmolysis is the process in which a plant cell loses water and shrinks due to the loss of water from the cell's cytoplasm. It occurs when a cell is placed in a hypertonic solution, where the solute concentration outside the cell is higher than inside the cell. In this case, when the cell is dipped in 0.5 M NaCl, the cell will undergo plasmolysis.

Explanation:
To understand why the cell undergoes plasmolysis in 0.5 M NaCl, let's compare the osmolarity of the cell and the solution it is dipped in.

- The cell's cytoplasm has a certain concentration of solutes, including NaCl.
- The 0.5 M NaCl solution has a higher concentration of NaCl compared to the cell's cytoplasm.

When the cell is placed in the NaCl solution, water molecules from inside the cell tend to move out of the cell towards the higher solute concentration in the external solution. This process is called osmosis.

Osmosis:
Osmosis is the movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration. In this case, the water molecules move from the cell's cytoplasm (lower solute concentration) to the NaCl solution (higher solute concentration).

Effects of Osmosis:
As a result of osmosis, the cell loses water and undergoes plasmolysis. Plasmolysis causes the cell membrane to detach from the cell wall, resulting in the shrinkage of the cell. This shrinkage is due to the loss of water and the collapse of the cell's cytoplasm.

Comparing with sucrose solution:
In contrast, when the cell is dipped in 0.5 M sucrose solution, sucrose molecules cannot easily cross the cell membrane. Therefore, there is no significant movement of water molecules across the membrane. As a result, the cell neither gains nor loses water, and there is no change in the cell's size or shape.

Conclusion:
In summary, when a cell is dipped in a hypertonic solution like 0.5 M NaCl, the cell undergoes plasmolysis due to the movement of water molecules out of the cell. However, in a solution like 0.5 M sucrose, there is no significant movement of water, and the cell remains unaffected.