154 KEITH R. PORTER 



48. Becker, W. A., Bot. Rev. 4, 446 (1938). 



49. Godman, G. C, and Porter, K. R.,^ biophys. hiochem. Cytol. 8, 719 (i960). 



50. Porter, K. R., and Bruni, C, Atiat. Rec. 136, 260 (i960). 



Discussion 



Mazia: Do you know what Dr. Palade meant when he entitled his talk "The 

 endosplasmic reticulum; its good fortune during the last 5 years" ? 



Porter : I think he intended to show how well or fortunately the system had 

 lent itself to investigation. 



Davis: Would the available information exclude the possibility that a major 

 function, perhaps more important than the synthetic function, would be that of 

 transport ? In lipid deposition, for example, I presume that at a time when the 

 animal is fed, glucose must get from the exterior to the sites of formation and 

 deposition, while in starvation it moves in the other direction. Is it not possible 

 that the endoplasmic tubes furnish means of communication, the enzymes then 

 being in the proper regions for the process but not necessarily part of the tubes? 



Porter : I think it very unlikely that glucose or glucose-6-phosphate finds its 

 way into the cavities of the P3R on the uptake side of metabolism. There is, in any 

 case, no evidence of any continuity between pinocytotic vesicles and the ER. 

 Transport of lipid and of fully formed proteins is indicated and presumably the 

 limiting membrane functions in the sequestration of these if not in their synthesis. 



Allfrey : I am no morphologist but I am brash enough to think that one can 

 answer this question on the penetration of substances into cells by the type of 

 experiment that we have been doing with frog ovocytes. If you take a frog ovocyte 

 and put it into a medium containing radioactive sodium and leucine and then do 

 an autoradiograph, you find that the substance penetrates the nucleus without an 

 apparent prior accumulation in the cytoplasm. We have checked this at a very early 

 time, and it suggests that there is a direct connection between the medium and the 

 membrane system around the nucleus, I would take the argument for transport 

 very seriously. 



Porter : There are others who feel as you do about this, but where the examina- 

 tion of uptake has been studied with particulate materials which could be visualized 

 afterwards in electron micrographs, no evidence could be found for the migration 

 of particles from the outside into the cavities of the ER. It is conceivable that in 

 this particular material, which Dr. Allfrey knows better than I, there are infoldings 

 of the plasma membrane which reach to considerable depths into the cytoplasms 

 of these cells, and function in transport. It is also possible, I suppose, that the 

 metabolites he mentions move by unchannelled diffusion. If there is continuity 

 between the internal phase of the endoplasmic reticulum and the cell's exterior, 

 even intermittent continuity, and some uptake, it would mean that all manner of 

 materials of the environment would be carried to the cell's interior and even to the 

 surface of the nucleus. This would give the cell a substantial problem in sorting 

 out the useful and rejecting the unuseful. Furthermore it seems to me as rather 

 unlikely that a system obviously used for segregation and sequestration of products 



