OSMOSIS: PROCESS, TYPES, DIFFERENCES WITH DIFFUSION AND EXAMPLES - BIOLOGY - 2025

The osmosis is a passive phenomenon displacement water through a membrane. This can be a cell membrane, an epithelium or an artificial membrane. Water moves from a region of low osmotic pressure (or where water is more abundant) to the region with higher osmotic pressures (or where water is less abundant).

This process is of biological relevance and orchestrates a series of physiological processes, both in animals and in plants.

Source: OpenStax

The first researcher to report the osmotic phenomenon was Abbé Jean Antoine Nollet. In the year 1748, Nollet was working with animal cell membranes and noticed that when pure water was placed on one side of the membrane and a solution with dilute electrolytes on the other side, the water moved into the region with solutes.

Thus, the passage of water in favor of its concentration gradient was described and it was called osmosis. The term comes from the Greek roots osmos, which means to push.

In 1877, Wilhelm Pfeller made the first studies on osmotic pressure. His experimental design involved the use of a copper ferrocyanide "membrane" on the surface of a porous clay cup, giving rise to a membrane that allowed the passage of water molecules.

Pfeller's artificial membranes were strong enough to withstand significant osmotic pressures and not collapse. This researcher was able to conclude that the osmotic pressure is proportional to the solute concentration.

Process

The movement of water through a membrane from an area of ​​low concentration to an area of ​​high concentration is called osmosis. This process occurs from an area with the lowest osmotic pressure to the highest osmotic pressure.

At first, this statement can be confusing - and even contradictory. We are used to passive "high to low" movement. For example, heat can be from high to low temperatures, glucose diffuses from regions of high concentration to less concentrated areas, and so on.

As we mentioned, the water that experiences the phenomenon of osmosis moves from low pressures to high pressures. This occurs because water is more abundant per unit volume where solute is less abundant.

That is, during osmosis the water moves where it (the water) is more abundant to where it is less abundant. Therefore, the phenomenon must be understood from the perspective of water.

It is important to remember that osmosis governs the movement of water through the membranes and does not affect the movement of solutes directly. When solutes diffuse, they do so by following gradients of their own chemical concentration. Only water follows the concentration gradient of osmotic pressure.

Osmotic pressure

Pressures?

One of the most confusing aspects when it comes to understanding the osmosis process is the use of the word pressures. To avoid confusion, it is important to clarify that a solution by itself does not exert a hydrostatic pressure due to its osmotic pressure.

For example, a 1 M glucose solution has an osmotic pressure of 22 atm. However, the solution does not "explode" glass bottles and can be stored in the same way as pure water because an isolated solution does not translate into hydrostatic pressure.

The term pressures is used only because of a historical accident, since the first scientists to study these phenomena were physical and chemical.

Thus, if two solutions that differ in their osmotic pressures are separated by a membrane, a hydrostatic pressure will be created.

Osmotic and hydrostatic pressures

The osmosis process leads to the formation of a hydrostatic pressure. The pressure difference leads to a rise in level of the more concentrated solution, as the water diffuses into it. The rise in the water level continues until the net rate of water movement equals zero.

A net flow is achieved when the hydrostatic pressure in compartment II is sufficient to force the water molecules back into behavior I, at the same rate that osmosis causes the molecules to move from compartment I to II.

The pressure of the water that causes the particles to retreat (from compartments I to II) is called the osmotic pressure of the solution in compartment II.

How is the flow of water in cells controlled?

Thanks to the osmotic phenomenon, water can passively move through cell membranes. Historically, it is known that animals lack an active water transport system to control the flow of this substance.

However, active solute transport systems can change the direction of water movement in a favorable direction. In this way, active solute transport is one way by which animals use their metabolic energy to control the direction of water transport.

Quantification

There are mathematical formulas that allow measurement of the rate at which the water will cross the membranes by osmosis. The equation to calculate it is the following:

Osmotic transport rate of water = K (Π ₁ –Π ₂ / X). Where Π ₁ and Π ₂ are the osmotic pressures of the solutions on both sides of the membrane and X is the distance that separates them.

The relationship (Π ₁ –Π ₂ / X) is known as the osmotic pressure gradient or osmotic gradient.

The last term in the equation is K is the coefficient of proportionality that depends on the temperature and the permeability of the membrane.

Differences with diffusion

What is broadcasting?

Diffusion occurs by the random thermal movement of dissolved or suspended molecules, which causes their dispersion from the regions of high concentrations to the lowest. The diffusion rate can be calculated by means of the Fick equation.

It is an exergonic process due to the increase in entropy represented by the random distribution of the molecules.

In the event that the substance is an electrolytic, the total difference in charge between the two compartments must be taken into account - in addition to the concentrations.

Osmosis is a particular case of diffusion

Diffusion and osmosis are not opposing terms, much less mutually exclusive concepts.

Water molecules have the ability to move rapidly through cell membranes. As we explained, they diffuse from a region of low solute concentration to one of high concentration in a process called osmosis.

It seems strange to us to speak of “water concentration”, but this substance behaves like any other substance. That is, it diffuses in favor of its concentration gradient.

However, some authors use the term "water diffusion" as a synonym for osmosis. Applying it literally to biological systems can be wrong, since it has been shown that the rate of osmosis through biological membranes is higher than what would be expected by a simple diffusion process.

In some biological systems, water passes by simple diffusion through the cell membrane. However, some cells have special channels for the passage of water. The most important are called aquaporins, increasing the speed of water flow through the membrane.

Examples

Within biological systems, the movement of water through cell membranes is crucial to understanding dozens of physiological processes. Some examples are:

Osmotic exchange in freshwater fish

An interesting example of the role of osmosis in animals is the water exchange that occurs in fish that live in fresh waters.

Animals that inhabit bodies of fresh water are in a constant intake of water from the river or pond where they live into their bodies, since the concentration of blood plasma and other body fluids have a much higher concentration than that of water..

The Carassius auratus species of fish lives in freshwater environments. An individual who has a mass of 100 grams can gain about 30 grams of water per day thanks to the movement of water inside his body. The fish have systems - energetically expensive - to continually get rid of excess water.

Reabsorption of liquids

In the gastrointestinal system of animals, the phenomenon of osmosis must occur for it to function properly. The digestive tract secretes a significant amount of fluid (in the order of liters) that must be reabsorbed by osmosis by the cells that line the intestines.

In case this system does not carry out its work, severe diarrhea events may occur. Prolongation of this malfunction can lead to dehydration of the patient.

Turgor in plants

The volume of water inside cells depends on the concentration of both the internal and external environment, and the flow is orchestrated by the phenomena of diffusion and osmosis.

If an animal cell (such as an erythrocyte) is placed in a medium that encourages the entry of water, it could burst. In contrast, plant cells have a wall that protects them from osmotic stress.

In fact, non-woody plants take advantage of this pressure generated by the passive entry of water. This pressure helps to keep different plant organs, such as leaves, turgid. As soon as water begins to flow out of the cells, the cell loses its turgidity and withers.

References

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