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Home >>Biology Dictionary >> Villus - Vitamin
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Viable. Able to live. Of seeds or spores, able to germinate.
Vicariad. One of a group of closely related species whose distribution is allopatric.
Villus. A finger like projection. Intestinal villi (about 1 mm long in man), in enormous numbers, give lining of small intestine of many vertebrates velvet-like appearance.
Each villus is covered by absorptive epitheliums, contains blood vessels and a lacteal, and moves continually by smooth muscle within it. Absorptive surface of intestine is thus immensely increased. Chorionic villi: e.g. projections of chorion in mammalian placenta which increase area of contact between embryonic and maternal tissues.
Vinegar. A dilute impure acetic acid, made from beer or wine.
Virus. A particle that is too small to be seen with a light microscope or to be trapped by filters but is capable of independent metabolism and reproduction within a living cell. Outside its host cell a virus is completely inert. A mature virus (a virion) ranges in size from 20 to 400 nm in diameter. it consists of a core of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid).
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Some bear an outer envelope (enveloped viruses). Inside its host cell the virus initiates the synthesis of viral proteins and undergoes replication. The new virsions are released when the host cell disintegrates. Viruses are parasites of animals, plants, and some bacteria (see bacteriophage).
Viral disease of animals include the common cold, influenza, smallpox, hepatitis, polio, and rabies (see adenovirus; arbovirus; herpesvirus; picornavirus; poxvirus); some viruses are also implicated in the development of cancer (see oncogenic). Plantviral diseases include various forms of yellowing and blistering of leaves and stems (see tobacco mosaic virus). Antibiotics are ineffective against viral disease but vaccines provide good protection.
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Viscera. Organs within the craium, thorax and abdomen, especially the latter.
Visceral arches.
(1) The series of partitions on each side of pharynx between adjacent gill-slits, or between mouth and first gill-slit (spiracle) in fish, or between the corresponding gill-pouches or gill-slits of tetrapod embryos.
(2) The skeletal bars (cartilage or bone) lying in these partitions in fish.
Visceral Skeleton. Supporting framework of the jaws and gill arches and their derivative in the vertebrates.
Viscus. Any organ lying in the viscera.
Visual. Pertaining to sight.
Vitamin. Organic substance which an organism must obtain from its environment, though it is necessary only in little amounts. The essential amino acids, which are similarly needed, but in larger amounts, are not included amongst vitamins.
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A vitamin plays an important role in metabolism, probably often as part of an enzyme system, and in doing so broken down, like many other substance sin the organism, and lost. Occasionally an animal may be able to synthesize provide part or all of the requirement B. Vitamins are requirement only by neterotrophic organisms; autotrophic organisms, such as most of the green plant, are by definition independent of an external supply of organic compounds.
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 organism; others (like C) by very few.
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Every vitamin necessary for a given organisms is synthesized by other organisms, otherwise a continuous supply will not be available.
Sometimes several different compounds can substitute for each other in satisfying a given requirement; either because the organism requires, not a specific molecule, but a specific chemical grouping which is present in, and a 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 metabolic process in which it is concerned, with widespread effects. A general effect of deficiency of most vitamins, is that growth of young animals is stunted.
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