What is osmosis?

We explain what osmosis is and the types that exist. Also, why it is important, what is biological diffusion and examples of osmosis.

1. What is osmosis?

Osmosis or osmosis is a physical phenomenon of matter exchange through a semipermeable membrane , from a less dense medium to a higher density one, without incurring an energy expenditure . It is a passive phenomenon, but vital for the cellular metabolism of living beings.

The mechanics of osmosis strive for a balance of concentrations between the two segments of a solution separated by the membrane, transmitting solvent from one side to the other to dilute the higher concentration end. This will generate a pressure change , known as osmotic pressure. This is what happens with the cell membrane, whose interior can be at a greater, equal or lower concentration than the outside, allowing the entry and exit of water , that is, osmoregulation, without energy cost.

Osmosis was discovered in 1877 in studies on the plant physiology of German Wilhelm Pfeffer , although similar studies already existed in the field and the term had been coined in 1854 (by the British Thomas Graham).

2. Types of osmosis

There are two forms of osmosis: direct and reverse.

• Direct osmosis It is the one that occurs in the cells of living beings, in which water enters or exits through the plasma membrane , allowing a balance with the environment, although in cases of hypertonic (huge concentration of solute) or hypotonic ( minimum concentration of solute) can cause dehydration or explosion by cell accumulation, respectively.
• Inverse osmosis. It is an identical mechanism but in the opposite direction, which allows the flow of water or solvent from the point of greatest concentration to the lowest concentration of solute, which is ideal for purification or retention of solute. For this to happen, a pressure that overcomes the natural osmotic pressure must be applied (that is, it requires an energy cost).

3. Importance of osmosis

Osmosis is vital for cellular metabolism, since it is a form of transport of matter between the inside and outside of the cell that does not carry any energy expenditure, that is, that occurs passively, without consuming ATP . This principle is also fundamental to explain the origin of life , given that in the first forms of cell life there would not yet be active metabolic mechanisms.

On the other hand, the principles of osmosis can be replicated in everyday situations and allow, for example, water filtration (reverse osmosis), among other practical procedures such as the manufacture of catalysts or facilitate industrial refrigeration processes .

4. Biological diffusion

It is known as simple diffusion to an osmosis-like process, from the point of view in which it involves the transit of particles from one medium (such as the cellular interior) to another (such as the extracellular environment) through a semipermeable membrane, moving from the medium with the highest concentration to the one with the lowest concentration (that is, following the concentration gradient). This takes place passively, that is, without added energy consumption.

Accordingly, the biological diffusion is what takes place in the cells, allowing the entry or egress of molecules through the plasma membrane , according to the concentration gradient. It is thus, for example, that oxygen enters the blood into the red blood cells, where hemoglobin can capture them for transport. This single example denotes the vital importance of this mechanism for life.

5. Osmosis Examples

Some simple examples of osmosis are:

• Water purification. To remove its impurities from water, the principle of reverse osmosis can be applied, to separate the contents dissolved in it by a semipermeable membrane.
• The hydration of an egg. The shell of an egg operates like an osmotic membrane, allowing the entry of water into its interior (more concentrated), so you can put an egg soaked without breaking its shell.
• The osmosis cell. Part of the cell transport mechanisms that allow the exchange (entry or exit) of matter between the cytoplasm and the environment without consuming ATP in the process.