Discuss the Need for Osmoregulation in animals, using specific examples and environments

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Jessica Beveridge                                                                                        Due 25/11/02

PoO supervisions: Dawn Muddyman

Discuss the Need for Osmoregulation in animals, using specific examples and environments

All external environments fluctuate frequently.  They can be relatively stable with small stochastic changes, or extremely random.  So, no matter in which habitat they live, all organisms have to have the ability to survive in changeable circumstances.  This is the fundamental basis of homeostasis, the process of maintaining a relatively constant internal environment.

        One of the major factors in homeostasis is the process of maintaining a constant osmotic pressure.  Osmotic pressure is “the pressure that can potentially be created by osmosis between two solutions separated by a semi-permeable membrane”(Eckert Animal Physiology).  This osmotic pressure is effectively the amount of pressure necessary to prevent osmotic flow between two solutions, and so it is important to maintain this value at a constant rate.  Water entering a cell with no cell wall, as in animals, at a high rate, can cause the cell to swell until it bursts.  This can be fatal.  By contrast, water leaving a cell down its concentration gradient can cause the cell to become flaccid and dehydrated to such an extent that it dies, because metabolic reactions cannot occur with the diluted solutes.

        Osmosis is the movement of water molecules only across a semi-permeable membrane down a concentration gradient.  The amount of water in a cell affects many variables that are dependent on it, because the volume of water in a solution affects its concentration.  Thus is water moves out of a cell, the concentration of the solutes in the cell increases because there is less water to dilute them.  This can have drastic consequences because it affects the ionic potential and thus the rate at which the ions move out of the cell as well.  This can cause problems because most tissues require an extracellular fluid high in sodium and chloride and low in potassium.  If this is not provided, errors can occur in neural transmission that occurs via a movement of these ions across the plasma membrane of neural cells.

        The composition of extracellular fluids in vertebrates is used as evidence for their freshwater origin.  This fluid is actually only a third of the concentration of seawater and some of the ions have been reduced in concentration of replaced by other similar ions, but in general, the two solutions are remarkably similar.  This has proved useful in experimental work, because a dummy interstitial fluid solution can easily be prepared for work on the nervous system.

        The body size of animals can contribute to the importance of their osmoregulatory systems.  Large animals, with a low body surface to volume ratio usually have one particular organ, usually the kidney, which can manage most of its osmolatory work.  Smaller animals, are more prone to dehydration effects because they have a larger surface area from which to lose water, and so may have a larger more general defence mechanism, such as the integument in amphibians, which varies in permeability.

                There are two main types of organism: osmoregulators and osmoconformers.  Osmoregulators are animals that maintain an internal osmolarity different from the medium in which they are immersed.  An animal that does not actively control the osmotic condition of its body fluids and instead conforms to the osmolarity of the medium which surrounds them is termed an osmoconformer.  Most vertebrates are osmoregulators, whereas marine invertebrates are often in osmotic balance with seawater and so are osmoconformers.  But what happens if the osmolarity of an animal decreases due to a lack of water? How do they increase their water supply again?  The most common way in osmoconformers is the use of osmolytes which are substances such as urea, and trimethylamine oxide, which by their presence in high concentrations, increase the intracellular osmolarity.  This reduces the problem that would be associated with changing the ionic composition of the cytosol rapidly.

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        There are two classes of environment commonly investigated in biology: aqueous (water-based) and terrestrial (land-based).  Aqueous animals can live in fresh or seawater, each of which gives rise to almost opposite adaptations.  Euryhalines can tolerate a wide range of salinities whereas, stenohalines can only tolerate a narrow range.

        Generally freshwater animals are hyperosmotic i.e a higher osmotic pressure, to their surrounding environment, and seawater animals are hypoosmotic.  There are two problems associated with freshwater habitation: the animals are subject to swelling due to an inward H2O movement, and a loss of salts to the surrounding environment.   These problems are overcome by the ...

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