Absorption Transport and Water Loss in Plants: Four Basic Phenomena-Permeability, Diffusion, Osmosis and Plasmolysis (For CBSE, ICSE, IAS, NET, NRA 2022)

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Permeability is the ability of a membrane to allow passage of gases, liquids and liquid dissolved substances through it. A permeable membrane may be fully permeable which allows passage of solvent and solutes (e. g. cell wall) , semipermeable which allows passage of solvent but no allow the solutes (e. g. collodion, cellophane) , selective permeable which allow passage of some selective solutes to pass through them with the solvent molecules (e. g. cell membrane, tonoplast, mitochondrial membrane) and impermeable membrane (e. g. cork wall, cuticle layer) .


The exchange of gases like CO2 and O2 between aerial organs of a plant and atmosphere and also movement of molecules of liquids, gases, or solutes from region of higher concentration to region f low concentration until molecules are evenly distributed, takes place by diffusion. Diffusion may define as the movement of molecules from the region of their higher free energy to a region of lower free energy. The free energy of a substance depends upon three factors; temperature, pressure and number of molecules present per unit volume. It is also known as chemical potential or kinetic energy of the substance. Thus, the rate of diffusion of a substance is dependent upon its kinetic energy or chemical potential. An example takes half-filled water beaker and placed a few crystals of copper sulphate at the base of the beaker. The copper sulphate molecules (blue in color) diffuse through the water molecules spread uniformly giving light blue color to the water.

Image Showing Diffusion of Copper Sulphate (CuSO4) in Water


When a plant cell is placed in a solution of higher osmotic concentration (hypertonic solution) , exosmosis occurs. Because of exosmosis, water begins to move out of the cell. It results in the shrinkage of the protoplasm. This shrinkage of protoplasm is known as plasmolysis. Plasmolysis is a reversible process. When pure water or hypotonic solution is added to the same plasmolysed cell, the cell grains water by endosmosis and the cell membrane and the cytoplasm come back to their normal position. Such a phenomenon of bringing the plasmolysed cell back to normal is known as deplasmolysis. The cell becomes fully turgid. When a cell is placed in an isotonic solution or a solution with similar concentration as the cell sap, there is no change in the shape of the protoplasm or the cell. Plasmolysis is a physical phenomenon. Plasmolysis is a kind of defense mechanism against adverse (stress) conditions such as hypertonic soil solution.

Image Showing Stages in Plasmolysis of a Plant Cell


Osmosis was discovered by Abbe Nollet. The movement of molecules across a semipermeable or selectively permeable membrane is known as osmosis. Osmosis can be regarded as a special kind of diffusion of water molecules from a region of their high concentration to their low concentration i.e.. from a dilute solution to a concentrated solution through a semipermeable membrane. In a physical system, osmosis depends upon three factors: temperature, pressure and number of solute molecules.

Image Showing Osmosis - Movement of Water Molecules through …