46 THE BIOSYNTHESIS OF PROTEINS 



the pancreas the finished proteins arise in the ribonucleoprotein particles 

 attached on the outer surface of the cisternae, and that these proteins pile 

 up as zymogen granules inside the cisternae (Siekevitz, 1959; Hirsch, 

 1960). The changes in intracellular distribution of pancreatic amylase 

 during the secretion cycle observed by Laird and Barton (1958) are in 

 agreement with this picture of the secretory process. 



The ergastoplasm of secretory cells can be visualized as a highly organ- 

 ized system for the manufacture, storage and delivery of the protein 

 secretion. 



It is important to realize that ribonucleoprotein particles are only one of 

 the constituents of microsomia! fractions, and of the ergatoplasm. In order 

 to avoid a very frequent confusion between 'microsomes' and 'ribonucleo- 

 protein particles', it has been proposed to reserve the name 'ribosomes' 

 for the discrete ribonucleoprotein particles (Roberts, 1958). This is the 

 more justified in that ribosomes seem to be a fundamental constituent of all 

 cells, whereas the structure of the ergastoplasm and the properties of the 

 'microsomal fraction' in vitro differ considerably from tissue to tissue, and 

 are not found in bacteria. 



2. Gefieral Occurrence of Ribosomes 



Electronmicroscopical studies on some forty types of mammalian tissues 

 (Palade, 1955) revealed the presence of these dense granules in all the cells 

 examined except mature red blood cells of the rat which, incidentally, 

 make no protein. Part of the ribosomes are associated with the ergasto- 

 plasmic lamellae, but part are free. The distribution of ribosomes is largely 

 similar to that of cytoplasmic basophilia, i.e. to the distribution of RNA. 

 As the size and composition of the ribosomes is very uniform, the general 

 relationship between the amount of RNA and the intensity of protein 

 synthesis, which was pointed out by Brachet and by Caspersson, can now 

 be expressed in newer terms as follows: In animal tissues, the intensity of 

 protein synthesis is related to the number of ribosomes. These are sites of 

 protein formation. 



Nucleoprotein particles similar to the Palade granules have been 

 isolated from the cytoplasm of plant tissues (Ts'O et ai, 1956; Ts'O, 1958) 

 and from micro-organisms. 



The association of part of the RNA of yeast with small particles has been 

 known for a long time (H. Chantrenne, 1943, 1944; Schachman et al., 

 1952). Yeast ribosomes were purified and studied more recently by physico- 

 chemical methods (Chao and Schachman, 1956). They make up a popula- 

 tion of nearly spherical particles with a sedimentation constant of about 

 80 S and a molecular weight in the range of 4-10^. About 40 per cent is 

 accounted for by RNA, the rest is protein. The stability of the particles 

 depends very much on salt concentration, and especially of the presence of 



