How do unicellular organisms maintain homeostasis


Osmoregulation, the ability of all living beings with a metabolism to control the concentrations of osmotically active substances, either to avoid osmotic stress (e.g. by polymerizing glucose to the osmotically neutral macromolecules starch or glycogen), or to reduce the osmotic potential, which yes always occurs on membranes to benefit, e.g. for transport processes. Important ions here are K+ (Potassium ion), approx2+ (Calcium ion) and H+ (Proton).

plants are able to react to a changed qualitative or quantitative supply of water via O. In addition to the specialized halophytes, the glycophytes also counter the stress of increased or changed salt concentrations in the substrate with specific actions to a certain extent; this includes ion uptake into the plasma or transmission into the vacuole, as well as the synthesis of osmotically effective proteins. A lack of water can be temporarily compensated by reducing the diffusion at the stomata and by lowering the turgor or by leaking water from the vacuoles (turgor).

At Animals and the People the O. serves to maintain a stable internal environment that shields the organs and tissues from fluctuations in the external medium. The physiological mechanisms required for this are usually active, energy-consuming transport processes that regulate the ion balance (electrolytes) on the one hand and (and cannot be separated from) the water balance and run between the body wall and the external medium as well as between the cell and the interstitial or extracellular fluid space. Generally, between poikilosmotic (Osmoconformer) and homoiosmotic (Osmoregulators, osmoregulators) organisms can be distinguished. Osmoconformer adapt passively to changing salt concentrations of the surrounding (poikilohaline) milieu, their body fluids are iso-osmotic (osmolarity) compared to the environment; however, they have a low salt tolerance, i.e. they are stenohaline. This includes most of the marine invertebrates that live in a very constant external (homoiohaline) milieu. If such animals are experimentally transferred into aqueous solutions with different salinity, they also show different abilities for volume and ion regulation. In response to osmotic stress, the concentrations of organic molecules, in particular amino acids, are often changed (e.g. reduced in a low-salt environment, which leads to a reduction in the influx of water). In animals that live in brackish water or migrate from the sea into rivers, for example, the salt tolerance range of the cells is much larger; they are euryhaline, e.g. the woolly crab (Eriocheir).

To the Osmoregulators belong to vertebrates and invertebrates whose area of ​​life is fresh or salt water. The body fluids of animals in fresh water are hypertonic to the surrounding medium; hyperosmotic regulation must therefore prevent them from increasing in volume due to the influx of water (and thus reducing the ion concentration of the extracellular space) and compensate for the loss of salt to the external medium. Even with protozoa living in freshwater, there are devices with the contractile vacuoles that enable volume regulation. According to the conditions in salt water, crustaceans and aquatic insects can keep their inner milieu constant by varying the amino acid concentration and ion transport, especially via chloride cells and transport epithelia on the gills or in diptera larvae via the anal papillae. The brine shrimp (Artemia salina) can survive in almost concentrated salt water as well as in salt water. It constantly absorbs water (an amount equivalent to around 3% of its own body weight per hour). Sodium chloride (table salt, NaCl) is absorbed from the gastrointestinal tract, and water passively follows into the tissue. The remaining salts are excreted through the intestines. The excess NaCl, on the other hand, is excreted via the gills, while the required NaCl is absorbed from the medium via chloride cells and ion-absorbing epithelia on the gills.

The O. of vertebrates in freshwater consists of increased urine production (volume regulation), increased salt absorption in the renal tubules (kidneys), so that very dilute urine is excreted, and active salt transport through the skin (frog) or gills (freshwater fish) into the Inside of the body. A low permeability of the body surface (fish) supports the O. The situation in the marine habitat is mastered in different ways: The body fluids of marine bony fish (which have migrated from fresh water) are hypotonic to seawater. An O. must therefore counteract the loss of water (especially via the gills) and the penetration of salts. Marine Teleosteer therefore drink seawater, actively excrete ions via the gills, actively secrete bivalent cations into the renal tubules with low urine production and in some cases have aglomerular kidneys as a morphological adaptation (hypoosmotic regulation). Marine cartilaginous fish, on the other hand, do not drink salt water, but achieve water retention through high concentrations of urea and trimethylamine, which they can maintain in the blood; they have devices in the rectal glands for active salt release.

Animals whose food reservoir is the sea, such as marine reptiles and birds, are also particularly confronted with the problem of the O. You can drink salt water and excrete the "superfluous" ions in strongly hyperosmotic fluid via special salt glands. Marine mammals are adapted to an increased excretion of salts without the need for increased water intake due to particularly well-developed Henle loops, which can produce highly concentrated urine. Land-dwelling birds and mammals who have to save water (desert animals, e.g. dromedaries, camels) have developed corresponding adaptations. Catadromous and anadromous fish, such as the eels that spawn in salt water and salmon that spawn in fresh water, are particularly capable of O; Depending on their current habitat, they can regulate hyperosmotically as well as hypoosmotically, with the possibility of reversing ion transport processes making up the main part of osmoregulation. (Excretion, excretory organs, homeostasis, water and mineral balance)