142 KEITH R. PORTER 



one and naturally suggests a functional relationship. Particularly is this so 

 when, as in the azo dye experiments, a hypertrophy or abnormality in one 

 is accompanied by a diminution in the other. But whether the association 

 is important in glycogenesis and storage or in glycogenolysis (or both) is 

 not clearly evident. By way of resolving the problem one can cite certain 

 instances where glycogen is stored without apparent involvement of the 

 smooth ER. In diabetes mellitus, for example, the nuclei of liver cells show 

 large glycogen stores. The same have been reported in liver nuclei of young 

 frog tadpoles [41] and in the nuclei of cells of a chicken sarcoma [42], 

 where membranous elements of the ER are not normally found. Also in 

 chondrocytes, the cytoplasmic deposits of glycogen apparently put down 

 for future use of the cell, show no prominent membrane components. If 

 one excludes the possibility that the fixation or staining (or even the 

 timing of the fixation) have not been appropriate in these instances to 

 preserve this form of the ER, one is left with the conclusion, also supported 

 by recent report of Revel et al. [43], that the ER is not an obligate associate 

 of glycogen synthesis. 



This encourages one to focus on the depletion side of the metabolism 

 cycle, in which the liver cell has a more unique function to perform. As is 

 well known, it has to export glucose on demand and keep it unavailable to 

 the general carbohydrate metabolism of the liver cell itself. It is important, 

 therefore, to have it sequestered near the site of storage or availability and 

 thence transported to the cell surface. If functional in this manner, the 

 smooth ER in the liver cell would be performing a role closely analogous 

 to that of the system in other types of cells described earlier in this report. 



It is not difficult to find in the pictorial data, evidence to support this 

 concept. The morphology of the ER relative to the glycogen is identical 

 during the depletion phases in fasting animals and after glucagon. At such 

 times the association of vesicle and glycogen is more intimate. The vesicles 

 show a very low density suggesting that these contents are soluble. And 

 it is usual to find the glycogen complexes close to the cell margins with 

 many of the smooth surfaced vesicles up against the adjacent plasma 

 membrane, possibly preparatory to discharging their contents, assumed to 

 be glucose. Also in this regard, the association of gIucose-6-phosphatase 

 with microsomal membranes (presumably agranular) is not to be over- 

 looked. Since, however, these observations constitute the only information 

 we have, it is clear that the final chapters in this investigation of the smooth 

 ER and glycogen have yet to be written. 



Endoplasmic reticulum in plant cells 



Thus far in this review the observations we have considered have been 

 derived from animal cells. Certain generalizations have been presented 



