138 Comparative Annual Physiology 



phosphorus utiHzation, and of others in reproduction, might yield evidence 

 of their need in some invertebrate animals. In any case, these substances are 

 associated with specialized chemical reactions rather than with the general 

 cellular reactions in which many of the B vitamins participate. 



Several evolutionary trends are shown in nutrition. First, the methods of 

 obtaining carbon and nitrogen from inorganic and simple organic sources for 

 the synthesis of protoplasm evolved early, probably among flagellates, and 

 within that same group the methods of synthesis from simple sources were 

 lost so that complex organic sources of carbon and nitrogen are needed by all 

 other groups of animals. In a general way the kind of carbon source is related 

 to the nitrogen source, and heterotrophic animals (but not yeasts) lack the 

 synthetic enzymes of autotrophic organisms. The total caloric requirements 

 depend on levels of metabolism, which will be discussed in Chapter 8. Second, 

 there are many substances, amino acids and coenzymes, which are universal 

 cellular components, which can be synthesized in adequate amounts by some 

 unicellular organisms but not by the specialized Protozoa. Higher animals also 

 are often unable to synthesize sufficient supplies of these substances and de- 

 pend on food or symbiotic microorganisms for them. Finally, with increased 

 chemical specialization there appear needs for particular compounds which 

 are not universal requirements. The best examples of these are in fat metab- 

 olism, in the deposition of skeletal calcium, and in production of blood cells. 

 The fact that there are still unknown requirements prevents the culture in 

 solution of various animals, such as intracellular blood parasites, parasitic 

 helminths, planaria, fish, and others, which have so far been cultured only on 

 highly organized food, such as that found in mammalian liver. The compara- 

 tive approach to nutrition can be expected to lead to the discovery of many 

 specialized dietary requirements and may ultimately provide a partial descrip- 

 tion of the sequence of enzyme evolution. Such a goal will be attained only 

 when the cellular functions of the specific required compounds are known. 



REFERENCES 



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2. Addis, C. J., and Chandler, A. C, /. Parasitol. 30:229-236 (1944). Vitamin 

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3. Almquist, H. J., Fed. Proc. 1:269-273 (1942). Amino acid requirements of 

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4. Almquist, H. J., and Grau, C. R., /. Nutrit. 28:325-331 (1944). Amino acid 

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5. Almquist, H. J., and MEccm, E., /. Bro/. C^iem. 135:355-356(1940). Amino 

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6. Bahrs, a. M., and Wulzen, R., Physiol. Zool. 5:198-206 (1932). Kidney as 

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7. Barker, H. A., ]. Cell. & Comv. Physiol. 7:73-93 (1935). Metabolism of alga 

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8. Bergmann, W., and Low, E. M., /. Org. Chem. 12:67-75 (1947). Sterols in 

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