NONCHROMOSOMAL GENES 245 



including the nuclear membrane, the plasma membrane, the Golgi mem- 

 branes, and the endoplasmic reticulum (ER). 



The nuclear membrane is a double envelope peppered with perfora- 

 tions which may or may not be fullv permeable to the passage of mate- 

 rials between nucleus and cytoplasm. The ER, a network of membrane- 

 bound canals and vesicles, which is profuse in some cell types and scant 

 in others, connects the plasma membrane at its outer limit with the 

 nuclear envelope at the cell center. When synthesis of a secretory 

 protein is in progress, as in the acinar cells of the pancreas, the ER is 

 seen to serve as a channel for passage of newly formed protein to storage 

 vacuoles and, ultimately, to the plasma membrane. Protein synthesis 

 occurs in association with the ribosomes, also called microsomal particles, 

 which in these pancreatic cells are attached in abundance to the mem- 

 branes. 



Both rough (ribosome-loaded) and smooth-surfaced membranes of the 

 ER have been described. The Golgi apparatus, consisting of a char- 

 acteristic arrangement of densely packed flattened vesicles near the cell 

 center, seems to be a reservoir of membranes, available for transport of 

 cell products or for other purposes. Connections have often been 

 described between smooth-surfaced Golgi membranes and membranes of 

 the ER. The apparent interconvertibility of cell membranes suggests 

 that they possess a common molecular structure, or else reversible 

 flexibility in incorporating or giving up particular structural constituents. 

 Examples of these structures are shown in Plates XI, XII and XIII. 



Ribosomes are dense spherical granules about 150 A in diameter, as 

 seen with the electron microscope; they contain most of the cytoplasmic 

 RNA. Found in profusion on the outer surface of the nuclear membrane 

 in most cell types, their concentration varies with cell function in other 

 parts of the cytoplasm. For example, in the acinar cells of the pan- 

 creas, during periods of rapid protein synthesis, ribosomes are present 

 in large numbers, located primarily on the membranes of the ER. In 

 dividing cells, for example in the alga Chlamydomonas, most of the 

 ribosomes are free in the cytoplasm. In other materials, one finds various 

 combinations of free and bound particles. The significance of this dis- 

 tribution is not known, but it seems likely that ihep particles are func- 

 tional in both conditions, whether bound or free. In bacteria, ribo- 

 somes are found throughout the cytoplasm in the free form. 



Four sizes of ribosomes have been identified in extracts of mammalian 

 tissues, higher plants, yeast, and bacteria. Although the precise sizes 

 vary somewhat with the species, the general picture is the same for all 

 (Plate IV). For example, the predominant particle found in rapidly 

 growing bacteria is identified by its sedimentation rate in the ultra- 



