m 



Vitamins and Antivitamins 



It would be wrong to give the impression that living things contain 

 only proteins. An enormous number of smaller molecules have been 

 found, usually in rather small quantities, in living tissues. They are 

 the intermediates, the partly completed structures which will eventu- 

 ally enter into the larger aggregates, or the messengers which go to 

 and fro, carrying energy or otherwise facilitating the working of the 

 machinery of the cells. 



The organisms make many of these substances themselves, but 

 different organisms differ enormously in their ability to synthesize 

 compounds. Some, like plants, can create everything they need from 

 very simple nutrients — a few inorganic salts, ammonia or nitrate, 

 phosphate and carbon dioxide from the air. General speaking, the 

 higher an organism is in scale of complexity, the less is its synthetic 

 ability. Micro-organisms usually have much greater power of syn- 

 thesis than animals, but even they are found to differ greatly in their 

 nutritional requirements. Some are perhaps even less exacting than 

 the plants; for example, the nitrogen-fixing bacteria which exist in 

 nodules of leguminous plants have the ability to use the nitrogen 

 directly from the air. 



The basic food requirements of the higher animals are certain 

 inorganic materials like salt, phosphate, calcium, iron (the mineral 

 elements of diet), proteins or their constituent parts, from which they 

 are able to build new protein, and carbohydrates, and perhaps also 

 some fat as sources of energy. It was found, however, in many 

 laboratories engaged in nutritional studies that animals fed with 

 what appeared to be completely satisfactory diets, containing all 

 these constituents in a purified form but nothing else, failed to grow 

 and often developed severe symptoms of disease. 



One of the earliest experiments of this kind was made in 1912 by 

 Sir Frederick Gowland Hopkins. He found that young rats would 

 not grow properly on a diet made from the purified constituents of 



