The Biochemical Function of Cells 



403 



must only be taken in a relative sense. It is scarcely possible to regard as scien- 

 tific the attempts of some workers to consider the enzymic fimctions of the 

 structural elements of cells without taking into account the external conditions 

 and the physiological state of the organism. Unfortunately such failure to take 

 account of the changeability associated with stages of growth and physiological 

 states is commonly met with in investigations of the structural elements of 

 animal cells. 



The localization of biochemical functions depends, not only on the type of 

 the structures, but also on the nature of the organism itself. This proposition 

 may be illustrated most strikingly by the example of the synthesis of proteins, 

 a biochemical function which is universal in all forms of life. 



It has been established that, in isolated systems, the individual structural 

 elements of the animal cell have unequal powers of incorporating radioactively 

 labelled substances into their proteins. In the works of Keller, Siekevitz, 



800 



65ZII 32ni IIX 8X 24-1 7-11 4-BZ'18IS: 



Dates of experiments 



Fig. I. Changes in the activity of the invertase in the plastids in the process of 

 development of the organism. 



I — Chloroplasts. II — Leucoplasts. 



Zamecnik [14-16] and others, it has been shown that when labelled amino acids 

 are introduced in vivo the greatest incorporation of label is found in the proteins 

 of the microsome fraction obtained from a homogenate by differential centri- 

 fugation, while, when the individual isolated fractions (nuclei, mitochondria and 

 microsomes) are incubated with labelled amino acids, this is not observed. 

 Incorporation is only observed when the individual fractions are incubated 

 together. In the experiments of Siekevitz [15] it was shown that maximal 

 incorporation is brought about by a combination of the microsomal and mito- 

 chondrial fractions imder conditions which allow oxidative phosphorylation to 

 take place. In such experiments the mitochondrial fraction may be successfully 

 replaced by a non-protein factor obtained from mitochondria after their pre- 

 liminary incubation under conditions suitable for oxidative phosphorylation. 

 In the experiments of Zamecnik and his colleagues [16] it was shown that, when 

 the individual cellular fractions are incubated with labelled amino acids imder 



