Текст I
VITAMINS AND PROVITAMINS
(1) Vitamin is any of several organic substances that usually are separated into water-soluble and fat-soluble groups. The substances commonly known as vitamins are diverse in chemical structure and function. Originally defined as organic compounds obtainable in a normal diet and capable of maintaining life, vitamins are distinct from carbohydrates, fats, and proteins in function, as well as in the quantities in which organisms require them. If a vitamin is absent from the diet, a specific deficiency disease may develop.
(2) The term “vitamin” originated from “vitamine,” a word first used in 1911 to designate a group of compounds considered vital for life; each was thought to have a nitrogen-containing component known as an amine. The final e of vitamine was dropped when it was discovered that not all of the vitamins contain nitrogen, and, therefore, not all are amines. The term “accessory food factor” sometimes is used instead of vitamin to refer to these substances.
(3) Since they generally cannot be synthesized by an animal and must be obtained from the diet or from some synthetic source, vitamins are called essential nutrients. Vitamin C can be synthesized by some organisms in sufficient amounts so that the dietary requirement is eliminated; vitamin C usually is considered a vitamin, however, because it must be included in the diet of man. Vitamins are distinct from many other compounds, which, although indispensable for proper animal functions, can be synthesized in adequate quantities.
(4) Provitamin is similar in structure to a specific vitamin and can be converted to it by a few metabolic reactions. Antivitamin is a compound that prevents the normal function of certain vitamins. Antivitamin may act by binding a vitamin, by destroying a vitamin, or by inhibiting the coenzyme function of a vitamin.
(Encyclopedia Britannica)
Укажите главный смысловой компонент в составе нижеследующего сочетания слов:
... water-soluble and fat-soluble groups ... .
