The Biochemical Function of Cells 415 



The mitochondria of plant cells can also carry out oxidative phosphorylation. 

 In the plant cell, however, many of the functions which are characteristic of the 

 mitochondria of the animal cell are carried out by the plastids. 



The plastids are similar to the mitochondria in their enzymic properties. 

 Oxidative enzymes, dehydrases, hydrolases, and esterases have been found in 

 them. In the general complexity of the plastids, about twenty enzymic systems 

 have been found, which take part in the various metabolic processes of the cell. 



In the plastids, in particular the chloroplasts, there occurs the formation of 

 peptide bonds and of phosphohpids, the synthesis and oxidation of fatty acids, 

 and photosynthetic phosphorylation. 



An important part in the synthesis of proteins in the animal cell is ascribed 

 to the microsomes, which are the smallest of the intracellular structures. Their 

 participation in the synthesis of protein has been demonstrated by many experi- 

 ments on the incorporation of labelled amino acids in vivo and in vitro. In such 

 in vitro experiments it has been shown that this requires the participation of 

 energy-giving processes which are brought about either in the mitochondria 

 imder conditions of oxidative phosphorylation or in the dissolved fraction 

 imder conditions of anaerobic glycolysis. 



In the plant cell the synthesis of the peptide bond occurs in both the mito- 

 chondria and the chloroplasts. 



Thus, the general activity and integration of the chemical processes of the 

 cell are secured by the interaction of polyenzymic systems of all the intracellular 

 structures and the soluble fraction. The role of the structural elements seems 

 to consist in the preferential localization on them of individual links in the total 

 chain of the single process of biological metabolism. The development and mani- 

 festation of a biochemical fimction in any particular given structure of the cell 

 is co-ordinated with the chemical nature and enzymic activity of the other 

 structures and intracellular components. 



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