OSMOREGULATION: WHAT IS IT, IN PLANTS, IN ANIMALS, EXAMPLES - BIOLOGY - 2025

The osmoregulation is a process that is responsible for maintaining the homeostasis of fluids in a body by actively regulating its internal osmotic pressure. Its purpose is to maintain adequate volumes and osmolar concentrations of the different biological compartments, which is essential for the proper functioning of organisms.

Biological water can be considered distributed in compartments that include the cell interior (intracellular compartment) and, in the case of multicellular organisms, the fluid that surrounds the cells (extracellular or interstitial compartment).

Movement of water and ions in freshwater telostus fish (Source: Raver, Duane; modified by Biezl (Own work), undefined Translated to spanish by –Cristina busch (talk) 20:53, 1 September 2014 (UTC) via Wikimedia Commons)

There is also, in more complex organisms, an intravascular compartment that brings intra and extracellular fluid into contact with the external environment. These three compartments are separated by selective permeability biological membranes that allow the free passage of water and restrict, to a greater or lesser extent, the passage of particles that are in solution in that liquid.

Both water and some small particles can move freely through pores in the membrane, by diffusion and following their concentration gradients. Others, larger or electrically charged, can only pass from one side to the other using other molecules that serve as means of transport.

Osmotic processes have to do with the movement of water from one place to another following its concentration gradient. That is, it moves from the compartment in which she is most concentrated to the one where her concentration is less.

Water is more concentrated in the place where the osmolar concentration (concentration of osmotically active particles) is lower and vice versa. The water is then said to move from a site with a low osmolar concentration to another with a higher osmolar concentration.

Living beings have developed complex mechanisms to control the osmotic balance in their interior and regulate the processes of water entry and exit, regulating the entry and / or exit of solutes, and this is what osmoregulation refers to.

What is osmoregulation?

The main objective of osmotic regulation is to adjust the inlet and outlet of water and solutes so that both the volume and composition of the liquid compartments remain constant.

In this sense, two aspects can be considered, one the exchange between the organism and the environment and the other the exchange between the different compartments of the body.

The entry and exit of water and solutes occurs by different mechanisms:

-In the case of higher vertebrate animals, for example, income is regulated by the intake of water and solutes, an issue that in turn depends on the activity of the nervous and endocrine systems, which also intervene in the regulation of the renal excretion of these substances.

-In the case of vascular plants, the absorption of water and solutes occurs thanks to the evapotranspiration processes that take place in the leaves. These processes "pull" the water column and drive its upward movement through the plant from the roots, which has to do with the water potential.

The exchange and balance between the different compartments of the organism occurs by accumulation of solutes in one or another compartment through their active transport. For example, the increase in solutes inside cells determines the movement of water towards them and the increase in their volume.

The balance, in this case, consists of maintaining an intracellular osmolar concentration that is adequate to maintain a constant cell volume and this is achieved thanks to the participation of proteins with different transport activities, among which ATPase pumps and other transporters stand out..

Osmoregulation in plants

Plants need water to live to the same extent as animals and other single-celled organisms. In them, as in any living being, water is essential to carry out all the metabolic reactions related to growth and development, which have to do with maintaining the shape and turgor of their cells.

During their life they are exposed to variable hydric conditions that depend on the environment that surrounds them, specifically on atmospheric humidity and on the levels of solar radiation.

In plant organisms, osmoregulation fulfills the function of maintaining the turgor potential through the accumulation or decrease of solutes in response to water stress, which allows them to continue growing.

Movement of water in root cells (simplastic transport and apoplastic transport) (Source: Dylan W. Schwilk via Wikimedia Commons)

The water found between the root hairs and the endodermis flows between the root cells through an extracellular compartment known as the apoplast (apoplastic transport) or through cytoplasmic connections (simplstic transport), until it is filtered along with ions and minerals into the cells of the endodermis and then travels to the vascular bundles.

As water and mineral nutrients are transported from the soil by the root to the aerial organs, the cells of the different tissues of the body "take in" the volumes of water and the amounts of solutes necessary for the fulfillment of their functions.

In plants, as in many higher organisms, the processes of entry and expulsion of water are regulated by growth regulating substances (phytohormones) that modulate responses to different environmental conditions and other intrinsic factors.

- Water potential and pressure potential

Since the intracellular concentration of solutes in plant cells is higher than that of their environment, water tends to diffuse by osmosis towards the interior until the pressure potential exerted by the cell wall allows it and this is what makes the cells cells are firm or turgid.

The water potential is one of the factors involved in the water exchange of both plants with their environment and the cells of their tissues with each other.

It has to do with the measurement of the direction of the water flow between two compartments and comprises the sum of the osmotic potential with the pressure potential exerted by the cell wall.

In plants, since the intracellular solute concentration is usually higher than that of the extracellular environment, the osmotic potential is a negative number; while the pressure potential is usually positive.

The lower the osmotic potential, the more negative the water potential. If it is considered a cell, then it is said that the water will enter it following its potential gradient.

Osmoregulation in animals

Multicellular vertebrates and invertebrates use different systems to maintain internal homeostasis, this in strict dependence on the habitat they occupy; that is, the adaptive mechanisms are different between saltwater, freshwater, and terrestrial animals.

The different adaptations often depend on specialized organs for osmoregulation. In nature, the most common are known as nephridial organs, which are specialized excretory structures that function as a system of tubes that open to the outside through pores called nephridiopores.

Flatworms have such structures known as protonephridiums, while annelids and mollusks have metanephridia. Insects and spiders have a version of nephridial organs called Malpighi Tubules.

In vertebrate animals, an osmoregulatory and excretory system is achieved, mainly composed of the kidneys, but the nervous and endocrine systems, the digestive system, the lungs (or gills) and the skin also participate in this process of maintaining water balance.

- Aquatic animals

Marine invertebrates are considered osmo-adaptive organisms, since their bodies are in osmotic equilibrium with the water that surrounds them. Water and salts enter and leave by diffusion when external concentrations change.

Invertebrates that live in estuaries where the saline concentration shows significant fluctuations are known as osmoregulatory organisms, since they have more complex regulation mechanisms due to the fact that the concentration of salts in their interior is different from that of the water where they live.

Freshwater fish have a saline concentration in their interior that is much higher than that of the water that surrounds them, so a lot of water enters their interior by osmosis, but this is excreted in the form of diluted urine.

In addition, some species of fish have gill cells for the entry of salt.

Marine vertebrates, whose salt concentration is lower than that of their environment, obtain water by drinking it from the sea and expel excess salt in their urine. Many marine birds and reptiles have "salt glands" that they use to release excess salt that they get after drinking seawater.

Most marine mammals ingest salt water when they feed, but their interior usually has a lower concentration of salt. The mechanism used to maintain homeostasis is the production of urine with a high concentration of salts and ammonia.

Difference in osmoregulation between plants and animals

The ideal state of a plant cell differs considerably from that of an animal cell, a fact that is related to the presence of the cell wall that prevents the excessive expansion of the cell due to water ingress.

In animals, the intracellular space is in osmotic equilibrium with extracellular fluids and osmoregulation processes are responsible for maintaining this state.

Plant cells, on the other hand, require turgor, which they achieve by keeping the intracellular fluid more concentrated than its environment, which is why water tends to enter them.

Examples

In addition to all the cases discussed above, a good example of osmoregulation systems is the one found in the human body:

In humans, maintaining the normal volume and osmolarity of body fluids involves a balance between the input and output of water and solutes, that is, an equilibrium where the input equals the output.

Since the main extracellular solute is sodium, regulation of the volume and osmolarity of the extracellular fluid depends almost exclusively on the balances between water and sodium.

Water enters the body through food and liquids consumed (the regulation of which depends on the mechanisms of thirst) and is produced internally as a result of the oxidation processes of food (metabolic water).

The exit of the water occurs by insensible losses, by sweat, feces and urine. The volume of urine excreted is regulated by the plasma level of antidiuretic hormone (ADH).

Sodium enters the body through ingested food and fluids. It is lost through sweat, feces, and urine. Its loss through the urine is one of the mechanisms for regulating the body's sodium content and depends on the intrinsic function of the kidney, regulated by the hormone aldosterone.

References

1. Alberts, B., Dennis, B., Hopkin, K., Johnson, A., Lewis, J., Raff, M.,… Walter, P. (2004). Essential Cell Biology. Abingdon: Garland Science, Taylor & Francis Group.
2. Cushman, J. (2001). Osmoregulation in Plants: Implications for Agriculture. Amer. Zool., 41, 758–769.
3. Morgan, JM (1984). Osmoregulation and water stress in higher plants. Ann. Rev. Plant Physiol., 35, 299-319.
4. Nabors, M. (2004). Introduction to Botany (1st ed.). Pearson Education.
5. Solomon, E., Berg, L., & Martin, D. (1999). Biology (5th ed.). Philadelphia, Pennsylvania: Saunders College Publishing.
6. West, J. (1998). Physiological Bases of Medical Practice (12th ed.). Mexico DF: Editorial Médica Panamericana.