CHEMICAL STRUCTURE AND METABOLISM 85 



Heterotrophic organisms vary in their dependence upon other organisms 

 for food. Many of them can manufacture all needed proteins, fats, vitamins, 

 and so on, if they are provided with a source of carbon other than the car- 

 bon dioxide of the air. That is, they do not possess the green plant's ability 

 to build carbohydrates from carbon dioxide and water but must obtain the 

 needed carbon from carbohydrates (sugars, starches) manufactured by 

 green plants. Most fungi are in this category. Given a source of carbon in 

 the decomposition of organic matter, fungi can manufacture for themselves 

 everything else they need. Thus the bread mold, Neurospora, can do this, 

 except that it cannot manufacture its own biotin, one of the B group of vita- 

 mins. Neurospora must obtain its biotin already formed. 



Animals are heterotrophic. With few exceptions they cannot manufac- 

 ture from carbon dioxide and water any considerable proportion of the 

 carbohydrates they need. Thus animals are "energy spenders" (Blum, 

 1 955 ) , obtaining their energy from carbon compounds built up, synthesized, 

 by green plants. Animals vary in ability to manufacture other necessary 

 materials, given a suitable source of carbon (in compounds furnished by 

 plants). Most mammals, for example, can manufacture their own vitamin 

 C (ascorbic acid), but the guinea pigs and the primates (monkeys, apes, 

 and man) cannot. 



Proteins are built up of organic compounds called amino acids. Amino 

 acids, in turn, are manufactured from carbohydrates and some source of ni- 

 trogen, such as ammonia. Given these ingredients animals can manufacture 

 some of the necessary amino acids and synthesize them into proteins. Green 

 plants can synthesize all their proteins in this way. Animals vary in their 

 ability to do so. There are about nineteen amino acids generally involved 

 in the synthesis of one or another of the animal proteins. Of these nine- 

 teen, man, for example, can manufacture eleven for himself in quantities 

 sufficient for his needs. The others must ordinarily be included, already 

 formed, in his diet. White rats, the lower mammals whose dietary require- 

 ments have been most intensively studied, cannot manufacture two amino 

 acids which man can manufacture. Despite differences, however, there is 

 surprising similarity among animals, even among such unlike forms as in- 

 sects, birds, and mammals, in the amino acids which can, and those which 

 cannot, be synthesized by the animal itself. Or looking at the matter the 

 other way around, there is surprising similarity among animals in dietary 

 requirements — in the nature of materials which must be supplied already 

 formed. Here again we see fundamental similarities indicating that all life 

 is one. 



But along with the resemblances we have noted differences. These ex- 



