PALADE 45 



somes is found associated with their attached RNP particles. Further work will 

 show whether these enzymes are newly synthesized proteins still attached to 

 their sites of synthesis or enzymes released from other locations and absorbed 

 on RNP particles during tissue grinding. 



DIFFERENCES BETWEEN MICROSOMES AND RNP PARTICLES 



It follows from the results thus far summarized that the cytoplasm contains 

 more separable entities than were assumed a few years ago. It also follows that 

 Claude's microsomes are relatively large and complex structures in which mem- 

 branous and particulate components can be easily recognized. There is also a 

 microsomal content which, though usually amorphous, may occur as formed 

 granules ([57], cf. [58]) under certain conditions, thus increasing the com- 

 plexity of the structure. The term "microsomal particles," used in many com- 

 munications made at this meeting, can be properly applied to RNP particles 

 attached to the surface of microsomal vesicles. As already indicated, such par- 

 ticles can be detached by DOC treatment and subsequently collected in rela- 

 tively clean preparations. It is doubtful, however, that the same term can be 

 used to designate the free RNP particles isolated in postmicrosomal fractions 

 and especially RNP particles separated from bacterial cells, in which there is 

 no endoplasmic reticulum to start with, and from which no microsomes can be 

 obtained. Pellets obtained from bacteria are composed of much smaller and 

 simpler cell components which morphologically seem to correspond to the free 

 RNP particles of the pancreatic cell. In general, a morphological label is justi- 

 fied as long as there is no information on the chemistry and function of the 

 structure involved. Here, however, we know that we are dealing with ribonu- 

 cleoprotein particles, and consequently there is not too much sense in retaining 

 a morphological label, especially after realizing that it is misleading. 



More than accurate terminology is involved in this argument. What is known 

 so far about the fine structure of bacterial cells suggests that internal mem- 

 branous systems, like the endoplasmic reticulum, are not necessary for the 

 organization and function of a simple type of self-sustaining cell. Such mem- 

 branous systems appear in more elaborate cell forms and could therefore be 

 regarded as superstructures. We do not know what special problems are solved 

 by their introduction, but we may wonder whether they are not connected with 

 an increase in cell volume, subsequent difficulties in diffusion, and relative 

 decrease in available surface. At higher levels of biological organization, similar 

 problems are frequently encountered and usually solved by the invagination of 

 surface structures and by the concomitant interiorization of part of the sur- 

 rounding medium. There is therefore an important difference in organization 

 between bacterial cells on one side, and animal and plant cells on the other: 

 the superstructure that can be ground into microsomes appears only in the 

 latter. Terms such as "microsomes" or "microsomal particles" of bacterial 

 origin do not take this basic difference into account. 



