The plant and the animal bodies both are complex systems, and more developed the organism of study, more is its complexity increased. For example, in single-celled organisms, all the important biological and biochemical processes take place in the cell, however, in higher organisms with tissue or organ level organization, the aspect of communication within the cells have a very important role. The processes of osmosis and diffusion play a pivotal function in this regard, establishing transport systems across cells, which is necessary for the distribution of nutrients and oxygen to the cells and the elimination of toxic waste products from it.
Even though a basic concept of osmotic flow has been in existence since ancient times, it was in 1748 when the French clergyman and physicist, Jean-Antoine Nollet, was the first to document the observation. The term was established by René Joachim Henri Dutrochet, a French physician, from the words ‘endosmose’ and ‘exosmosis’. The process of osmosis or reverse diffusion can be defined as the movement of a solvent across a semipermeable membrane to a region of higher concentration of solute and a lower concentration of solvent from a region of higher concentration of solvent and a lower concentration of solute. In biological systems, the solvent is usually water, but osmosis can take place for other liquids, supercritical liquids, and even gases if the conditions are right and a semipermeable membrane is present.
There are two different kinds of osmosis- endosmosis and exosmosis. The key difference between the two processes is that in endosmosis the movement of water inside the cell while in the case of exosmosis the elimination of water out of it. In this regard, we must introduce the concept of hypertonic, hypotonic and isotonic solutions. If a cell is placed in a solvent with a solute concentration higher than that in the cell then the solution is called hypertonic to the cell and the cell sap is hypotonic to the solvent. If the solvent and the cell sap have the same solute concentration, they are termed as isotonic to each other. Endosmosis takes place if the cells are placed in hypotonic solutions because of which, the cells may swell because of endosmosis. Exosmosis occurs if the cells are in hypertonic solutions due to which the cells shrink. In isotonic solutions, no exosmosis or endosmosis takes place.
Fate of RBCs when placed in Hypotonic, Hypertonic, and Isotonic Solutions
Endosmosis: Endosmosis can be defined as a type of osmosis in which the flow of the solvent is towards the inside of a vessel or cell. The phenomenon takes place when the water potential in the outside of the cell is higher than in the cytosol. Therefore, the concentration of the solute in the solution surrounding the cell is less than the solute concentration in the cytoplasm. Molecules of water move inside the cell via the cell membrane (which is semipermeable in nature) in endosmosis and cells are swelled up because of the water entering into them and causing it to become turgid. An example of endosmosis occurring in nature can be given when the absorption of capillary water by roots from the soil takes place in plants and the water enters the xylem.
Exosmosis: The process of exosmosis denotes the type of osmosis where the direction of the water or solvent is outside of a vessel or cell. It takes place if the water potential in the outside of the cell is lesser than inside the cells. Hence, the concentration of the solute in the solution surrounding the cell is greater than the solute concentration in the cytoplasm. The molecules of water flow outwards from the cell through the semipermeable cell membrane during exocytosis. Because of this, the cells shrink causing plasmolysis.