CHAPTER 5 



Nutrition 



NiMALS DIFFER GREATLY in their Capacity to alter their diet, 

 to convert compounds of one class to another, and to syn- 

 thesize specific compounds which are essential for cellular 

 metabolism. What is a vitamin for one animal may not be a dietary require- 

 ment for another. It has frequently been pointed out that the amino acids 

 which constitute proteins are the same in all organisms, although the propor- 

 tions vary from one protein to another, and also that the coenzymes of gly- 

 colysis and cellular respiration, such as the phosphopyridine nucleotides, are 

 similar in all cells. There is variation, however, among animals as to which 

 amino acids and which coenzymes can be synthesized, and which must be 

 taken as food. Also some particular food requirements are associated with 

 physiological specializations of certain animals. Substances needed for specific 

 cellular reactions may be obtained by an animal in two ways: they may be 

 synthesized by the organism, or they may have to be taken in its diet. Some 

 substances may be synthesized by an animal but in insufficient amounts for 

 cellular needs; hence these are required in the diet of the organism. Some of 

 the specific mineral requirements of animals have been mentioned in Chapter 

 3, page 94. Salt mixtures have been worked out empirically for several 

 groups of animals, particularly for rats. These provide the essential elements 

 which must be added to the diet in nutrition studies. 



In this chapter we are concerned principally with the requirements for 

 organic compounds. No attempt will be made to encompass the vast litera- 

 ture regarding food preferences and regarding the biochemical action of specific 

 substances; these are regularly reviewed in the Annual Review of Biochemistry, 

 in Nutrition Reviews, in Vitamins and Hormones, and in other review series; 

 they are also summarized in nutrition texts. ^-^ In the following account 

 references are omitted for many statements which are taken froni nutrition 

 texts. 



ORIGIN OF NUTRITIVE TYPES 



Carbon compounds are stores of energy, and energy is required for their 

 synthesis. How such stores came to be established, their relation to the origin 

 of life, the primitive kinds of energy-yielding reactions, and the nature of the 

 carbon compounds which ultimately resulted in living organisms are of 

 fundamental importance. Certainly most important biochemical evolution 

 occurred prior to the existence of any organisms such as are now known. 



Organisms fall into three classes with respect to the source of energy used 

 for synthetic processes: (1) chemotrophic organisms (largely bacteria) get 



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