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vitamin
Значение термина vitamin в knolik
vitamin - vitamin
vitamin - Organic substance which an organism must obtain from its environment, though it is necessary only in minute amounts. The essential amino-acids, which are similarly needed, but in larger amounts, are not included amongst vitamins. A vitamin plays an essential role in metabolism, probably often as part of an enzyme system (hence the small amounts required), and in so doing is broken down, like many other substances in the organism, and lost. What distinguishes a vitamin from most other compounds essential in metabolism is that the organism cannot replace its loss by synthesizing it, and so must obtain it from outside. Occasionally an animal may be able to synthesize part of its requirement (e.g. D by man), and often symbiotic organisms provide part or all of the requirement (e.g. B vitamins synthesized by gut bacteria of insects and vertebrates). Vitamins are required only by heterotrophic organisms; autotrophic organisms, such as most of the green plants, are by definition independent of an external supply of organic compounds. What is a vitamin for one heterotrophic organism may be synthesized in adequate amounts by, and is therefore not a vitamin for, another; or it may even take no part at all in the metabolism of another. There is no such thing as a vitamin in general, but only for specified organisms. There may be mutant strains within one species, some requiring a substance as a vitamin, others synthesizing it. Some (like those of the B complex) which are perhaps universal constituents of existing organisms are however required by a very wide range of organisms; others (like C) by very few. Every vitamin necessary for a given organism is synthesized by other organisms, otherwise a continuous supply will not be available. The vitamin requirements of the vast majority of organisms are quite unknown. Sometimes several different compounds can substitute for each other in satisfying a given requirement (e.g. See also: Vitamin D); either because the organism requires, not a specific molecule, but a specific chemical grouping which is present in, and available from, each of the alternative compounds; or because conversion of a few closely related groupings into the one required is possible within the organism. Deficiency of the vitamin reduces the rate of the (often unknown) metabolic process in which it is concerned, with widespread effects (symptoms of deficiency disease). A general effect of deficiency of most vitamins, which was important in the early history of their discovery, is that growth of young animals is stunted.
Рядом со словом vitamin в knolik
| vital staining | В начало буква "" буквосочетание "" | vitamin a |
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