Osmosis

Definition of osmosis

Osmosis is the spontaneous passage of a solvent (which in biological systems is usually water), from the solution in which the solutes are more diluted to that in which they are more concentrated; this movement - which occurs through a semipermeable membrane - continues until an equilibrium situation is reached, in which both solutions gain and maintain the same concentration.

Practical example

To better clarify the concept of osmosis, let's imagine that we have a container divided into two compartments of equal volume (A and B) by a semipermeable membrane (that is, permeable only to the solvent - in this case water - and not to the solute). compartment A there is an aqueous solution in which a tablespoon of glucose has been dissolved, while in part B we have an aqueous solution of equal volume in which three tablespoons of glucose have been dissolved (it is therefore more concentrated). This difference creates a gradient of concentration for glucose on the sides of the membrane and, since this sugar cannot cross it, equilibrium is reached with the passage of water from compartment A (where glucose is more diluted) towards compartment B (where it is more abundant). If you prefer, it can also be said that water passes by osmosis from the solution in which it is more concentrated (A) to that in which it is less concentrated (B).

Following this flow, the water level in B increases and decreases in A, creating a certain level difference between the two. This phenomenon ends when the two solutions reach the same concentration, then keeping it constant.

Hypotonic, isotonic and hypertonic solutions

Taking two solutions with different molar concentration (different number of particles dissolved in them), the solution with the lowest molar concentration is defined as hypotonic and the more concentrated one is hypertonic. Two solutions are instead isotonic (or equimolar) when they have the same concentration.

In the example just made, solution B is hypertonic (therefore it contains more solutes) than the other (defined as hypotonic); therefore, under normal conditions, the solvent moves by osmosis from the hypotonic to the hypertonic solution. We talked about standard conditions because, playing with the laws of physics, it is possible to overturn the very concept of osmosis and favor the passage of the solvent from the most diluted to the most concentrated concentration (reverse osmosis).

Osmotic pressure and reverse osmosis

As expressed up to now, the net flow of the solvent - generated by osmosis - continues until the two solutions have reached the same concentration. Well, this movement can be countered, stopped, or even reversed by applying pressure to the compartment with the highest concentration.

In the previous example it is sufficient to place a piston in compartment B (which we remember to have a higher concentration), and push it down with a certain force, to favor the passage of water towards A; in this case we speak of reverse osmosis.

Osmotic pressure is the pressure that exactly opposes the passage of the solvent through the semipermeable membrane; consequently it is the pressure necessary to counteract osmosis.

For what has been said so far, two isotonic solutions have the same osmotic pressure; it should be emphasized, therefore, that the osmotic pressure depends exclusively on the number of particles present in the solution and not on their nature.

Osmosis and the human body

The plasma membranes that surround the cells of the human body, in fact, are semi-permeable membranes, which allow the direct passage, through osmosis, of small molecules (such as water and urea), but not of those with greater molecular weight (such as proteins, amino acids and sugars). Osmotic balances in body fluids are therefore essential to guarantee the cells an optimal environment in which to live.

If we take a cell like a red blood cell and immerse it in a hypotonic solution, this - by osmosis - undergoes a swelling (given by the entry of water), which can even make it explode. On the contrary, if immersed in a hypertonic solution the cell it undergoes, due to the passage of water towards the outside, a severe dehydration which causes it to wrinkle. Fortunately, in the human organism cells are immersed in isotonic solutions with respect to their internal environment, and there are various systems to keep these liquids in osmotic equilibrium.

Osmotic pressure and food storage

Let's think for a moment of a homemade jam ... sugar is added in abundance not only to improve its flavor, but also and above all to increase its shelf life. Still, sugar is an important element for the life of many microorganisms involved in the degradation of the product. This apparent contradiction is dismantled by the very concept of osmosis.

If we apply this law to jam, in fact, since its osmotic pressure is much higher, the bacterial cells present in the jar lose water by osmosis, wrinkling and dying (or at least inactivating). The use of hypertonic solutions, therefore, increases the storage times of food, because it reduces the availability of water for life and the proliferation of microorganisms. The laws of osmosis are also exploited in brines (in which foods are immersed in hypertonic solutions where the solute is the common table salt). Other examples are given by capers (or other foods preserved in salt) and candied fruit. So, in case you were wondering why salt is added to steaks only when cooked, now you have the answer: its presence on raw meat favors the release of intra and extracellular juices, reducing their palatability; in the same way certain vegetables, such as aubergines, are sprinkled with salt and left to rest for a couple of hours, just to allow the osmosis to purge their water and bitter liquids.