602 PETER MITCHELL 



cytochrome system, and unlike the proteins universally accepted as enzymes, then 

 we would be justified in inventing a new name to describe them. But I would hesi- 

 tate to consider such special proteins until we have isolated at least one. 



Davis : I would like to ask Dr. Porter to comment on another aspect of this 

 problem. Dr. Mitchell has stressed the advantages of bacteria arising from their 

 smallness and simplicity, but of course we are all aware of disadvantages which 

 also arise from smallness, such as the difficulty of recognizing morphological sub- 

 units. Now electron microscopists until very recently have all agreed that there is 

 no endoplasmic reticulum in bacteria; but a few months ago Glauert published in 

 the Journal of Biochemical and Biophysical Cytology some pictures showing with 

 new methods of fixation what appears to be a very fine reticulum in parallel 

 lamellae in an actomycete (which is fundamentally an elongated bacterium). I 

 wonder whether Dr. Porter would care to comment on the generalization that 

 bacteria may or may not have such a reticulum. 



Porter: Audrey Glauert at the Strangeways Laboratories in Cambridge, 

 England, has taken and published very informative micrographs of complex mem- 

 brane systems in Streptomyces and some suggestion has been made that these may 

 be analagous to the endoplasmic reticulum of other cells. Actually the bacterial 

 cytoplasmic membranes seem to be infoldings of the membrane limiting the pro- 

 toplast and could represent an attempt on the part of the cell to increase a surface 

 available to diflfusible metabolites. Certain blue-green algae show a similar com- 

 plex infolding of the plasma membrane. They are evidently common to lower 

 forms which do not possess a nuclear envelope and associated ER. Whether such 

 complex infoldings of the surface membrane have evolved into the surface- 

 independent endoplasmic reticulum of higher cell forms is an interesting topic for 

 speculation. 



Davis: Well, 1 wonder, Dr. Mitchell, whether you have any evidence as to 

 whether or not certain activities which you observed in the centrifugable fraction, 

 particularly those of the cytochrome system and the TCA cycle, could be in small 

 particles attached to the membrane rather than in the substance of the membrane 

 itself. 



Mitchell: Yes, I think this query is a very difl^icult thing to resolve experi- 

 mentally. It is also difficult to speak about. The succinic dehydrogenase active 

 centre is certainly inside the osmotic barrier. If you think of the succinic oxidase 

 as a particle, you may imagine that this piece of the cytochrome, as well as the 

 succinic dehydrogenase attached to it, is under the osmotic barrier. You may, if 

 you are a biochemist, regard the plasma membrane as a hydrophobic sheet with 

 various activities attached to it as particles, which can be isolated and characterized 

 biochemically. But, if you are a cytologist, you will be impressed by the fact that 

 under certain conditions the plasma membrane complex behaves as a single 

 mechanical unit containing the enzyme activities of all its so-called particulate 

 constituents. I think that the work in Dr. Weibull's laboratory and in my own 

 laboratory has now established this fact in the case of Bacillus megaterium, Staphy- 

 lococcus aureus and Micrococcus lysodeikticus beyond any reasonable doubt. One 

 would presume that the residual bonding that is holding the various parts of the 

 membrane complex system together is stronger in some places than in others; so 

 that if you treat it very kindly, as you must do if you wish to isolate cytologically 



