Osmoregulation
As osmoregulation in which is biology the regulation of the osmotic pressure of the body fluids of an organism referred to. Every organism must prevent the concentration of dissolved substances in its cells from becoming too high or too low. The osmoregulation ensures a tolerable water content, and in many animals it also ensures an almost constant state in the body ( homeostasis ).
principle
If two solutions are separated by a semipermeable membrane and have different water potentials, water moves by osmosis from the higher to the lower water potential (pure water has the highest potential). The more dissolved particles there are in the solution, the higher the osmotic value .
In all environments, whether in water ( aquatic ) or on land ( terrestrial ), organisms must maintain the concentration of dissolved substances and the water content of the body fluids in a range suitable for them. This requires the excretion (excretion) of excess metabolic products and salts , which would have a toxic effect if the concentration was too high . All processes and mechanisms in this context are part of osmoregulation.
Osmotic conformers and osmotic regulators
Osmoregulation distinguishes between two main types of organisms: osmoconformer and osmoregulator.
Osmokonformer (or osmotic conformer , Eng. Osmotic conformers , even Osmokonforme called) adjust the osmolarity of their body tissues to their environment, they are poikilosmotisch . This can be done either passively (without additional energy expenditure) or actively (with energy consumption). Most marine invertebrates are conformers. Also hagfish and elasmobranchii ( sharks and rays ) are conformer, but their soaked electrolyte composition of the sea water off. In particular, due to the Donnan effect , which occurs in the presence of non-permeable substances (e.g. proteins ) inside the organism, passive conformers are always weakly hyperosmotic towards the external medium.
Are more widespread in the animal kingdom, the Osmoregulierer (or osmotic regulator , Eng. Osmotic regulators ). They keep the osmolarity of the organism within narrow, almost constant limits and regulate the salt content of their body fluids independently of the salt content of the environment. Such organisms are called homoiosmotic .
- Freshwater fish are hyperosmotic regulators . They have a higher osmotic value inside the body than their surroundings and therefore actively absorb electrolytes from the surrounding water with their gills . The excess water absorbed in the process is excreted in the urine, which is therefore very dilute.
- In seawater living organisms are hypoosmotic regulators . They have a lower osmotic value inside the body than their surroundings, which without counter-regulation would lead to constant water loss. They therefore actively excrete salts through the gills.
Most species of fish are restricted to fresh or salt water environments ( stenohalin ). Species whose osmoregulation enables life in a wide salinity range are called euryhaline . For example, salmon , barramundi , bull sharks and diamond turtles have the ability to tolerate fresh water as well as brackish and salt water.
Osmoregulation
With plants
In general, plants need to absorb water to make up for the loss. Higher plants evaporate water mainly on the surfaces of the leaves and through their stomata , which are used to absorb the CO 2 , which is important in photosynthesis . Many plants show adaptations of their leaves that keep the water loss through transpiration low. These include needle-shaped leaves, sunken stomata and a thickened cuticle like that of the pines . The beach grass has rolled up leaves with internal stomata. Other species have developed water storage methods to take in water when it is abundant for use in drought conditions. As Xerophyte plants are dry habitats referred to the extensive drought periods can resist. Succulent plants like cacti store water in extensive parenchymal tissues.
Water release and uptake are determined by the internal and external influences that affect the transpiration of the plant. Most higher plants do not have specific organs for osmoregulation, with the exception of the salt glands in mangrove trees and some pioneer plants , including salt plants . These can absorb the salty water that prevails in their environment.
With single cells
Some unicellular organisms such as paramecia , amoebas or the alga Euglena have one or more contractile vacuoles that absorb excess water from the cell plasma through osmosis. The contents of the contractile vacuole can be removed from the cell either through a pore (paramecium) or exocytosis . The pulsation frequency of the contractile vacuoles is between 5 and 10 seconds ( Paramecium caudatum ) up to 30–40 minutes (in the ciliate Spirostomum ) , depending on the species, and is influenced by a number of external factors such as the ion concentration gradient and temperature .
In animals
The osmolarity of the extracellular fluid is the people critical of the sodium determined concentration, intracellular comes Potassium is highly important. The osmolarity is not kept constant by regulating the ion stocks, but by regulating the amount of water in which the particles are dissolved. Unlike salts, which can only cross membranes in a strictly regulated manner, water is freely distributed throughout the body and - driven by the osmotic pressure - balances out almost all differences in osmolarity. The regulation of the uptake and excretion of free water is suitable to keep the osmolarity in the target in the entire body so that cells do not experience any shrinkage or swelling. The volume regulation by controlling the sodium level ensures that the osmoregulation does not result in any undesired volume changes in the extracellular space.
The osmoregulation essentially consists of two control loops , the sensors of which in both cases are osmoreceptors in the hypothalamus . With hyperosmolarity (lack of water) these send signals so that
- in the neurohypophysis antidiuretic hormone (ADH) is released, which in the kidney the incorporation of aquaporin 2 in the collecting pipes stimulated, thus less water is eliminated, and
- Thirst is triggered, which leads to a behavioral change in the intake of water through drinking .
In both cases the water balance becomes positive, so that the hyperosmolarity is effectively counteracted ( negative feedback ). In the case of hypoosmolarity (excess water), the regulation is reversed.
Disorders of human osmoregulation
Osmotic disorders are on the one hand hyperosmolarity due to a relative lack of water and on the other hand hypoosmolarity due to a relative excess of water. Since the blood plasma is part of the extracellular fluid and its osmolarity is thus also primarily determined by the sodium concentration, hyperosmolarity can be equated with hypernatremia and hypoosmolarity with hyponatremia (an exception to this rule is hyperosmolar hyponatremia due to an excess of nonionic osmolytes such as glucose ).
Diabetes insipidus is an impressive disruption of osmoregulation . Because of the failure of the first control loop (lack of ADH secretion, lack of ADH effect due to defective receptor or defective aquaporin), there is permanent polyuria (excretion of large amounts of hypoosmolar urine ) and, as a result, hyperosmolarity. However, since the second control circuit is intact, thirst motivates those affected to ingest large amounts of fluid ( polydipsia ) so that a balanced water balance is achieved.
Elderly people often feel less thirsty. In the event of severe water loss, for example through sweating on hot days, this can lead to a lack of water, which is known as desiccosis . People with an intact sense of thirst can also develop desiccosis if they do not drink despite being thirsty.
The disease states described up to now due to isolated water deficiency can be summarized as hyperosmolar hypohydration . An isolated excess of water (hypoosmolar hyperhydration ) is hardly possible with intact osmoregulation, since excess water can be excreted very quickly; Possible causes are the intake of large amounts of hypoosmolar solutions (excessive drinking) - especially with impaired kidney function - as well as the Schwartz-Bartter syndrome or syndrome of inadequate ADH secretion .
See also
literature
- E. Solomon, L. Berg, D. Martin, Biology 6th edition. Brooks / Cole Publishing. 2002
- Robert Franz Schmidt , Florian Lang, Manfred Heckmann (eds.): Physiology of humans . 31st edition. Springer Medizin Verlag, Heidelberg 2010, ISBN 978-3-642-01650-9 , Chapter 30 Water and electrolyte balance .
Web links
- Article osmoregulation in the compact dictionary of biology
- Osmoregulation in Cyanobacteria
- Website Prof. Chuck Holliday Contains links on osmoregulation in crustaceans.
Individual evidence
- ^ Rolf Siewing (Ed.): Textbook of Zoology, Volume 1 General Zoology , 3rd Edition, p. 531. Gustav Fischer Verlag, 1985, ISBN 3-437-20223-5 .
- ↑ Herbert Remmert: About Poikiloosmotie and Isoosmotie . Z. see Physiologie 65 (1969), pp. 424-427.
- ↑ SimplyScience - Life in the Salt Stress. Retrieved May 28, 2020 .
- ^ Rolf Siewing (Ed.): Textbook of Zoology, Volume 1 General Zoology , 3rd Edition, p. 63. Gustav Fischer Verlag, 1985, ISBN 3-437-20223-5 .