THE GROWTH OF SACCHARIDE MACROMOLECULES 325 



and results in the liberation of a large cellulose molecule, we are attracted to suppose 

 that such molecules diffuse into the medium and there crystallize to yield fibrils. 



Mitchell: Could I try to clarify my question a little ? In these organisms, one 

 would presume that the cell wall, the outer stiff region, is a molecular sieve with 

 holes in it that would be quite big enough to let through, say, sucrose molecules 

 or an individual fibre, one sugar molecule in width, which is going to form a 

 cellulose fibril. But, the plasma membrane, which is underneath the cell wall and 

 is impermeable to glucose phosphates will also certainly be impermeable to glucose 

 and fructose. We have never done permeability determinations on the organism 

 that Dr. Hestrin has mainly been speaking about, but with many others, even 

 when glucose is rapidly metabolized, the membrane is nevertheless quite imper- 

 meable to free glucose in the normal sense. You therefore have the problem of 

 how the precursor comes through the plasma membrane and why it polymerizes 

 outside and not inside the protoplast. We would like to imagine that the trans- 

 location and the polymerization of the precursor is controlled by a concerted cata- 

 lytic process. 



Porter : As you may have guessed the other day I am fairly naive in this area 

 of wall formation but I have been impressed by the general facility with which cells 

 seem to shed their outer layer and then form a plasma membrane under the 

 shed cortex. I don't suppose this happens in the micro-organisms but I did want 

 to ask you if you thought the proposals made the other day that elements of the 

 ER might carry the catalysts for cellulose polymerization were at all feasible ? 



Hestrin : There does not appear to be any protoplasmic connection between 

 cellulose in the medium and a cell. One might speculate that the cellulose mole- 

 cules diffuse from the cell and crystallize in the medium at a distance from the cell. 

 As to the polymerization step leading to the cellulose molecule, we still do not know 

 whether it occurs in the cell wall, on the outer surface, or within the cell. 



