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The osmosis is a phenomenon in which the movement of water occurs (solvent) from a solution of lower concentration (hypotonic) to another higher concentration (hypertonic) when both are separated by a semipermeable membrane (the water passes, but not the solutes dissolved in it), until the two solutions reach the same concentration (isotonic).


By OpenStax [CC BY 4.0], via Wikimedia Commons

Water is displaced by the osmotic pressure difference between a concentrated (higher) solution that attracts the solvent from a more dilute solution (lower osmotic pressure). The osmotic pressure is directly proportional to the concentration of the solution.

The cell membranes may be considered semipermeable. If we compare two solutions, they can be isotonic if they have the same concentration or anisotonic if the concentrations are different, one is hypotonic and the other hypertonic.

The water will pass from the hypotonic to the hypertonic media, exerting a pressure on the membrane called osmotic pressure.

When a cell is introduced into a hypotonic environment, water will enter its interior and the cell will burst. On the contrary, if it is introduced into a hypertonic environment, the cell will lose water, wrinkle and dehydrate. It is evident that these situations are not compatible with life, so osmotic pressure homeostasis, osmoregulation , is another vital factor for the maintenance of life.

Throughout evolution, living things have adapted by developing strategies to avoid osmotic catastrophes. Plant cellsmonerafungi, and many protoctists have developed a rigid cell wall, strong enough to resist osmotic pressure and prevent the cell from exploding when in hypotonic environments. Therefore, the cell only swells reaching a state called turgor. The mechanical stiffness of the plants is due to turgor.

When a plant cell is introduced into a hypertonic environment, such as sea water, they lose water and the cell membrane that was attached to the cell wall contracts but some fragments remain attached to the cell wall, fracturing the cell. If the saline concentration is lower inside the cell than the extracellular environment, it loses water and dies, a phenomenon known as plasmolysis. This is the cause that the cells of the root of a plant lose water and, therefore, the plant dies if we plant it in a very saline environment.

Conversely, the animal cells in a hypotonic medium exploit irremissibly not having cellular wall phenomenon called cytolysis (hemolysis, in the case of blood cells). If the medium is hypertonic, cells lose water, become dehydrated, and die (crenation).

For this reason, distilled water cannot be directly introduced into the blood, as it would cause the turgidity of the red blood cells and the subsequent rupture of the membrane and, therefore, its destruction.

As a summary, we review these concepts:

Osmosis in a plant cell

  • In a hypertonic environment, the cell loses water and the volume of the vacuole decreases, causing the plasma membrane to detach from the cell wall, causing plasmolysis.
  • In an isotonic medium, there is a dynamic equilibrium.
  • In a hypotonic environment, the cell takes in water and its vacuoles are filled by increasing the turgor pressure, giving rise to turgor.

Osmosis in an animal cell

  • In a hypertonic environment, the cell loses water, becomes wrinkled, becoming dehydrated and dies, this is called creation.
  • In an isotonic environment, there is a dynamic equilibrium, that is, the constant passage of water.
  • In a hypotonic environment, the cell absorbs water, swelling and to the point where it can burst, giving rise to cytolysis. If it is a red blood cell, its cytolysis is called hemolysis.

Animation: Osmosis.

Question of reflection

What is the reason why foods such as meat and fish are kept and do not spoil, outside the refrigerator, if they are covered in salt?

Question of reflection

In soils with high salt concentrations, only plants can grow that absorb and  contain higher concentrations of salts inside their cells than those of the soil. Justify  the need to maintain a high intracellular saline concentration taking into account the  water requirements of plants

Fundamental ideas about osmosis

Osmosis is a process in which a solvent (water) diffuses through a semi-permeable membrane from a more dilute solution (hypotonic) to one of higher concentration (hypertonic) until the concentrations equalize (isotonic).

  • If the medium is hypertonic (more concentration in the medium than in the cell), the water leaves the cell to try to equalize the concentrations.
    • In the plant cell the cell wall is peeled off and plasmolysis occurs.
    • In the animal cell, dehydration and cell death occurs. It is called a crenation.
  • If the medium is isotonic, a dynamic equilibrium occurs, water enters and leaves but maintaining the concentrations in both media.
  • If the medium is hypotonic (less concentration in the medium than in the cell), water enters the cell to try to equalize the concentrations.
    • In the plant cell, the vacuoles are filled and turgor occurs.
    • In the animal cell, the cell swells and can burst, giving rise to cytolysis.


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