• Diffusion: The movement of molecules from a region of their high concentration to a region of their lower concentration is known as diffusion.
• Osmosis: Osmosis is the movement of a solvent (generally water) across a semi-permeable membrane.
• Cytolysis (osmotic lysis) occurs when a cell bursts due to an osmotic imbalance that has caused excess water to diffuse into the cell.
• Plasmolysis is defined as the process of contraction or shrinkage of the protoplasm of a cell.

Concept of Osmosis:

Diffusion: The movement of molecules from a region of their high concentration to a region of their lower concentration is known as diffusion.

Osmosis: Osmosis is the movement of a solvent (generally water) across a semi-permeable membrane. Osmosis is a selective process since the membrane does not allow all molecules to pass through it.

1. If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.
   
   
2. If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.
   
   
3. If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.