56 THE BIOSYNTHESIS OF PROTEINS 



incorporating amino acids into proteins and most probably possess a com- 

 plete system for the synthesis of proteins. 



Summing Up 



From the results reviewed in the foregoing pages, it is clear that there are 

 many sites of protein synthesis in the living cell. Nuclei, mitochondria, 

 chloroplasts make their own protein constituents. In bacteria, the cell mem- 

 brane appears as a site of intense protein formation, but the ribosomes are 

 the most actively engaged in protein synthesis. The cytoplasmic ground 

 substance of higher organisms is the site of synthesis of the bulk of the 

 proteins of many cells. In the ground substance, protein appears first in the 

 ribonucleoprotein particles (ribosomes) which are responsible for the pro- 

 duction of the soluble cytoplasmic proteins. In secretory cells, the ergasto- 

 plasm has a remarkably elaborate structure which probably plays a part in 

 the process of secretion and with which many ribosomes are associated. 



Ribosomes are in all cases an essential part of the protein-making 

 machinery. 



