What is osmosis?
Osmosis is a physicochemical phenomenon of large biological implications and interesting practical applications, which despite its unusual name, is very present in our daily lives. Osmosis is the passive diffusion of a solvent through a semi-permeable or selective permeability membrane, which allows the passage of the solvent but not that of the solute, from the most dilute to the most concentrated solution.
It is said to be a passive process because it occurs without energy expenditure, and what causes it is the tendency for solute concentrations to equalize on both sides of the membrane. The selectivity is given by the size of the pores, which allow the small molecules to pass through and not the largest.
Osmosis in animals
Osmosis through the plasma membrane of the animal cell is the most classic example of this process, essential for our metabolism. As we know, water is a fundamental compound for life and is the most abundant molecule in the cell. The concentration of nutrients inside cells is regulated in part by the osmosis mechanism.
Cells harbor mineral salts and organic molecules as solutes. If the concentration of solutes increases in the extracellular liquids, the external environment will become hypertonic with respect to the cells, and as a consequence, the outflow of water will occur, which in extreme cases can lead to cellular plasmolysis.
On the contrary, if the extracellular fluid is diluted, it becomes hypotonic with respect to the cells, which will make the water tend to enter the cells, with which the cells will swell, that is, they will increase their turgor. In this sense, there is a certain difference between plant and animal cells.
Osmosis in plants
Under normal conditions, plants live in hypotonic media with respect to the internal environment of their cells, which makes them absorb water from the roots and distribute them in all tissues. With the entry of water into the cells, the turgor rises; the presence of the cell wall in the plant cell modulates the expansion and helps to prevent cell bursting.
When exposed to hypertonic media (eg, water with excessive salt content), plants are likely to “wilt”, precisely because of the loss of water associated with osmosis. Halophyte plants are those that are adapted to live in saline environments and therefore hypertonic.
Animal cells are subject to the same phenomena, but, lacking a cell wall, they would become more sensitive to the explosion if it were not for the fact that there are active mechanisms with energy expenditure that allow the passage of water and ions to be controlled, preventing this from happening.
The reverse osmosis occurs when exogenously increases the pressure on the side of the membrane hyperosmotic, causing the water out of said compartment rather than enter it.
Reverse osmosis provides the basis for a series of industrial processes, such as the removal of salts and hardness from water, nitrates, microorganisms and other particles in suspension, etc. (as illustrated in the figure).