Permeability, Diffusion, Osmosis, Plasmolysis

Permeability

Permeability is the property of a membrane to allow the passage of the substances through it. The plant cell wall is permeable because it allows both solvent and solute molecules to pass through it. All biological membranes (cell membrane, mitochondrial membrane, nuclear membrane) are selectively permeable as they allow penetration of only solvent molecules but not the solute molecules.

Diffusion

Diffusion is the intermingling of molecules of the same or different substances as a result of their random movement. It is dependent on the difference in concentration of molecules of different substances in the adjacent areas and this difference is called diffusion gradient. For example, the fragrance of incense sticks (agarbatti) spreads from one corner of the room to the other due to diffusion.

Diffusion is an effective method of transport of matter over short distances. For diffusion to take place no membrane is required. If a membrane is present, it should be fully permeable.

Osmosis

Osmosis is a special kind of diffusion of water molecules from a region of their high concentration to their region of low concentration through a semipermeable membrane. In osmosis, the water molecules move, and the presence of a semipermeable membrane is essential.

Plasmolysis

When a cell is placed in a solution, it will either shrink, swell or will remain unchanged depending upon the concentration of the bathing solution or the solution in which the cell is placed.

When a cell is placed in a hypertonic solution i.e. when the concentration of the outer solution is higher than the cell sap, water from the cell move out resulting in shrinkage of the protoplasm in the centre of the cell. This phenomenon is known as plasmolysis. The space between the cell wall and the protoplast is occupied by the bathing solution as the cell wall being dead, is permeable to the outer solution.