STRUCTURE AND ACTIVITIES OF CELLS 



quantities but are produced in the body. When a certain compound is stated 

 to be a vitamin, it is not to be inferred that all animals require it in their 

 food. Certain animals can synthesize a compound which is a specific require- 

 ment for maintenance or growth, for example, or they may have bacteria 

 living in their digestive tracts which can synthesize it. Other kinds of animals 

 must obtain this same substance ready-made in the food they eat; for them, 

 it is a vitamin. Vitamins are detected in studies of the specific nutritive re- 

 quirements of difTerent species of animals; rats are widely used in such labora- 

 tory studies. When these nutritional substances were first discovered, they 

 were classified on the basis of their solubility in fats or in water. Letters were 

 used for identification since their chemical structure was unknown. At pres- 

 ent, the chemical formulas of all the commonly known_ vitamins have been 

 determined. They may be designated now by names which either describe 

 the compound chemically or indicate its source or major function. 



The fat-soluble vitamins include those designated as A, D, E, and K. In 

 each, two or more closely related chemical compounds have been found to 

 give the eflfect originally assigned to a single vitamin. Deficiencies of the 

 fat-soluble vitamins may result either from an inadequate diet or from a dis- 

 turbance of the mechanism for fat absorption from the intestine. 



Vitamin A is found only in the animal kingdom. It may be obtained, pre- 

 formed, from milk, butter, egg yolk, and fish-liver oils. A precursor, or 

 provitamin, called beta-carotene, is present in green and yellow vegetables. 

 This compound can be converted to vitamin A by the animal. The physio- 

 logical activity of vitamin A is concerned with the maintenance of the 

 epithelia of exposed surfaces, such as epidermis of the skin and cornea of the 

 eye. It also participates in the synthesis of visual pigments such as rhodopsin, 

 or visual purple, a pigment of the retina of the eye necessary for vision, 

 especially in dim light (p. 107). Deficiency of vitamin A results in excessive 

 keratinization of the skin and cornea so that they become dry and horny, and 

 in varying degrees of night blindness. 



Vitamin D is a group name for at least ten different steroids, of which two, 

 D2 and D3, are especially important to man in the development of teeth and 

 bone. These vitamins increase the intestinal absorption of calcium and 

 phosphorus and are required for the actual deposition of these substances in 

 bone. Sources of this vitamin are fish-liver oils, butter, liver, and egg yolk. 

 The content of vitamin D in milk can be increased by exposure to ultraviolet 

 radiation which brings about the transformation of a precursor. The same 

 kind of reaction occurs in human skin when it is exposed to sunlight. A 

 deficiency of vitamin D results in rickets in children and in a similar disorder 

 in adults, especially older ones. In each age group the bone is inadequately 

 calcified, and deformities may result. 



Vitamin E, another group of compounds, is known as the antisterility vitamin 

 and is necessary for normal reproductive functions in the rat. The male be- 

 comes sterile in the absence of vitamin E; the female is unable to maintain 

 the placentas for the nourishment of the embryos, and they are aborted. An 



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