Osmoregulation, Osmotic Balance and Osmosis in Different Organisms


Osmoregulation is one of the homeostasis processes which body does to maintain the concentration of salts and solutions. The diffusion of water across a selectively permeable membrane from an area of higher concentration to lower concentration is osmosis.

Each cell of living organism is adjusted to a defined amount of water and salt concentration so by homeostasis cell maintains the osmotic balance with respect to extracellular environment.

Plants are classified into different categories depending on their environment and method of osmoregulation. These are hydrophytes – the freshwater plants, xerophytes – the desert plants, halophytes – the sea plants and mesophytes – the terrestrial plants with moderate supply of water.

In animals, there are osmoregulators which necessarily need osmoregulation for their survival and others are osmoconformers that do not need osmoregulation actively as they are isotonic to external environment.


Osmosis is the diffusion of water across a membrane in reaction to osmotic pressure caused by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance (osmotic balance) across membranes within the body’s fluids, which are composed of water plus electrolytes and non-electrolytes.

An electrolyte is a solute that dissociates into ions when dissolved in water. A non-electrolyte, on the other hand, does not dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body’s fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the areas in between cells and tissues of the body.

The membranes of the body (such as the pleural, serous, and cell membranes) are semi-permeable: they allow passage of specific kinds of solutes and water, however not others. Solutions on two sides of a semi-permeable membrane tend to adjust in solute concentration by the movement of solutes and/or water across the membrane.


Osmotic Balance


Water is the solvent of the solutes in the cell. Each cell has been adjusted to a defined amount of water in relation to salts in it to perform its functions. Homeostatic systems typically maintain this concentration. Cells consistently encounter altering extracellular environment.

It may be of the diluted solution compared to the cell concentration, thus designated as the hypotonic environment. The more concentrated external environment is termed as hypertonic which resembles the internal solution is isotonic.

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The hypotonic environment osmotically causes entry of water into the cell and renders the cell solution diluted. The cell likewise ends up being turgid. Hence it may be damaged. The hypertonic environment, on the other hand; renders the cell solution concentrated and shrinks the cell due to loss of water. To prevent these situations cells osmoregulate themselves to keep water and salt balance in plants and animals.



Osmoregulation in Plants

Plants are distributed in different habitats of water, moderate and badly dry terrestrial nature, thus called hydrophytes, mesophytes, and xerophytes, respectively.


They have adjustments to remove the flooding of its cells in freshwater. In this type, the surface area of leaves is huge to transpire water excessively. Extensive stomata are present on the upper surface area facing the environment to promote loss of water.
Examples: Water lily, water lettuce.




They have moderate water accessibility. In the sufficient supply of water, stomata are kept open to promote loss of excess water, nevertheless, in restricted supply stomata close to prevent the loss.
Examples: Brassica, increased, mango, etc.




They have adaptations for a minimized rate of transpiration. Many xerophytes have small, thick leaves to limit water loss by reducing the area proportional to the volume. Their cuticle is thick, waxy, and leathery. Stomata are on the lower surface area of leaves and situated in depression. Some as cacti, throughout the driest season, shed their leaves to limit transpiration completely, therefore stems are the photosynthetic organs. In the rainy season, the stem stores water for use in dry conditions.
Examples: Cactus, Euphorbia, Opuntia, etc.


Osmoregulation in Animals


Animal cells require a more critical balance of water and solutes in the body as they cannot survive a net water gain or loss. Water constantly leaves and goes into the cells; however, the quantity of the water and the solutes is kept in balance. There are two methods for keeping this balance.

1. Osmoconformers

Animal body fluids are kept isotonic to the external environment even for the marine saltwater environments. These animals thus do not require actively to change their internal osmotic state, so are referred to as osmoconformers.
Example: Lobsters, Jellyfish, and so on.

2. Osmoregulators

The animals whose body fluid concentrations differ noticeably from the outside environment actively regulate to release excess water in hypotonic and excrete salts in hypertonic conditions for that reason are called as osmoregulators. Animals living in different environments have unique adaptations to control osmotic balance, e.g. marine, fresh water, and terrestrial environments.
Example: Humans, etc.

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Osmoregulation in Bacteria

Bacteria utilize a transport mechanism to absorb electrolytes when osmolarity around it increases. The osmotic tension activates certain genes in bacteria that synthesize osmoprotectants.

Osmoregulation in Humans

The kidney is the main organ responsible for osmoregulation in humans. Water, amino acids, and glucose are reabsorbed by the kidneys. When the water level in the body is high, it releases a large amount of hypotonic urine. When the water level is low, it retains water and produces a low amount of hypertonic urine. Therefore, the kidneys keep the electrolytic balance of the body.

Aldosterone, angiotensin II, and antidiuretic hormones control the absorption process. Some water and electrolytes are also lost by perspiration. Osmoreceptors in the hypothalamus of the brain control the thirst and secretion of ADH. ADH opens the water channels of aquaporins allowing the water to flow. Therefore, the kidneys keep absorbing water until the pituitary gland stops releasing ADH.



Osmoregulation in Different Environments



Marine Environment

A lot of marine invertebrates are osmoconformers. Among the vertebrate’shagfishes are isotonic with the surrounding sea’s water. The majority of cartilaginous fishes preserve lower, internal salt concentration than that of seawater. Their kidneys for osmoregulation excrete salts through gills and also have salt excreting organs such as rectal glands.

These utilize an active transport system to remove salt against the osmotic gradient. Some fishes have relatively low salts in body fluids but have rendered these hypertonic to that of seawater by maintaining urea in adequate concentration. Because urea in high concentration is damaging so these fishes keep another chemical trimethylamine oxide(TMAO) for defense against urea.

Bony fishes, the descendants of freshwater ancestors but later on ended up being marine constantly lose water from their hypotonic body fluids to hypertonic environments. These fishes have adjusted themselves to consume large quantities of seawater and excrete concentrated urine leading to maximum salt excretion and minimum water loss.



Fresh Water Environment

Freshwater animals are continuously dealing with the osmotic flooding of body fluids and loss of salts. Freshwater protozoa, Amoeba, and Paramecium drain excess water by special structures called contractile vacuoles. Numerous freshwater animals including fishes remove excess water by producing large volumes of very dilute urine. The loss of salts is compensated by the preference for salt-containing food and by the active uptake of salts by gills and skin.

Terrestrial Environment

The evaporative loss of water leading to dehydration is the major problem for terrestrial life. Arthropods and vertebrates have actually successfully adapted to the terrestrial mode of life. Terrestrial animals are covered by body surface area, which prevents loss of water as the waxy exoskeletons of insects and multi-layered dead, keratinized skin cells of most terrestrial vertebrates. Consuming and eating moist foods compensate for the loss of water. These animals likewise have metabolic and behavioral adjustments.

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Terrestrial animals can tolerate dehydration and it differs in different animals. This characteristic is referred to as anhydrobiosis.


Some desert mammals e.g. kangaroo rats make it through without drinking water by feeding upon seeds of desert plants containing more carbohydrates, which produce water on the metabolic process.

Terrestrial animals produce concentrate urine in their kidneys that reabsorb most filtered water in the process of excretion.