Osmosis, what is it and what function does it have?

Osmosis is a phenomenon that occurs naturally in living beings and that humans have learned to use to apply it in technologies such as water desalination, in this case being reverse osmosis.

In this article we are going to explain what osmosis is with examples, how it works in living beings (and why it is important, and also the reverse osmosis used by humans in industry.

What is osmosis?

Osmosis is the phenomenon that occurs when two solutions with different concentrations are separated by a semipermeable membrane and the solvent diffuses through the membrane from the liquid of lower concentration to the liquid of higher until the concentrations balance. This phenomenon occurs spontaneously without energy expenditure and is therefore a passive diffusion phenomenon.

Osmosis is the mechanism where water passes through a semipermeable membrane, from a hypotonic solution to a hypertonic one.


In other words, if, for example, we had two solutions of water and salt separated by a semipermeable membrane (that is, one that only allows water to pass through); The water would move from the solution with the lowest concentration to the one with the highest concentration without the need to provide energy thanks to the phenomenon of osmosis.

Aqueous media can have different concentrations of one or more solutes. The concentration of solvents and solutes (for example, water would be the solvent and salt the solute in the previous example) allows aqueous media to be classified by comparison with respect to another into:

    • Hypotonic medium: when the solute concentration is lower compared to the medium with which it is compared
    • Hypertonic medium: when the solute concentration is higher compared to the medium with which it is compared.
    • Isotonic medium: when both media have the same concentration.

The pressure exerted by the solvent (water) on the side of the membrane where there is lower concentration towards the compartment with higher concentration is called osmotic pressure. Continuing with the previous terminology, the pressure produced on the side of the membrane from the hypotonic medium to the hypertonic medium is the osmotic pressure.

If you prefer the video explanation, we leave it here:

Osmosis in living beings

In living beings, osmosis is a fundamental process since for the survival of cells it is essential to maintain what is called the osmotic balance necessary for the cell to perform its functions.

Osmosis is a process that affects both internally and externally. Externally, it is very important for living beings that are exposed to environments with salinity and high osmotic pressure, such as those that live in the ocean or in salt marshes.

    • Salinity and osmotic pressure in nature

For this reason, living beings have developed osmoregulation systems that allow them to live in different environments, from the most extreme to the least aggressive in this regard.

Osmosis in the animal cell

Cell membranes are semipermeable so osmosis is a phenomenon that occurs naturally. Thus, if animals do not have mechanisms to adequately balance the concentration in the cells, two phenomena can occur due to osmotic pressure:

    • Cytolysis: the cell is found in a hypotonic medium and tends to absorb water to reach isotonic equilibrium; In this case the cell can burst, leading to cytolysis.
    • Crenation: occurs when the cell is in a hypertonic medium and water tends to escape. This can lead to dehydration, which can lead to cell death. This phenomenon is called creation.

The mechanisms to regulate osmotic balance in living beings can be found in this article on osmoregulation and excretion.

Among the mechanisms that can be found in this article are examples such as the kidney, the gills in fish, the salt glands in birds as far as vertebrates are concerned. But the osmoregulation mechanisms of amphibians, insects, reptiles, larvae… are also described in this article.

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