146 The Nature of Biological Diversity 



molecular composition and arrangement in the system itself rather 

 than hy an organization in the matrix, or continuous phase of the 

 cytoplasm. The latter seems to display too much randomness in its 

 behavior to support a patterned structure of this nature. Furthermore, 

 the fragments of the ER which compose the microsome fraction, as 

 isolated in sucrose, retain a form reminiscent of that of the intact 

 system and this in the absence of the cytoplasmic matrix (Palade and 

 Siekevitz, 1956). Hence it is reasoned that the system is to a degree 

 independently structured according to genetic information effective 

 in any particular cell. Except that the expanse of this membranous 

 system is greater than that encountered in mitochondria and chloro- 

 plasts, it is not more complicated or more incredible than the internal 

 membrane systems of these smaller organelles. The ER, based on the 

 nuclear envelope, pervades the whole cytoplasm; the cristae mito- 

 chondrial's, only the cavities of the mitochondria. 



Beside being constant for any single type of cell, these patterns are 

 found to repeat to some degree in cells performing similar functions. 

 It was in fact this tendency, brought to light by comparative cytology, 

 which gave the first clues to the functional properties of these struc- 

 tures. Thus it was observed that the ER in cells engaged in the syn- 

 thesis of protein for export is made up frequently of large, lamellar 

 cisternae which are usually in parallel array (Dalton, 1951; Bernhard 

 et al., 1952 ) . And these cisternae always have dense 150 A particles 

 attached to the outer or matrix surface of the limiting membranes 

 (Palade, 1955b). This has led to the practice of referring to this 

 form of the ER as rough or granular, as opposed to the smooth or 

 agranular forms to be considered in greater detail in a succeeding 

 section. 



The rough ER 



This form of the ER refers not so much to patterning as it does to 

 a particular characteristic of the component elements. In many in- 

 stances, it is true, the particle-studded cisternae are arranged in stacks 

 or are regularly spaced in parallel arrays which may occupy a large 

 part of the cytoplasm. This is the ergastoplasm of classical cytology 

 (Gamier, 1899). Thus one expects to find particles associated with 

 this organization and form of the ER vesicle. There are, however, in- 

 stances where parallel cisternae are particle-free (Porter and Yamada, 

 1960), as also there are instances where particle-studded elements 

 occur singly (Fig. 6). In general, however, the large flat cisternae of 

 the ER are reserved for particle association and tend to show a degree 



