BIOLOGICAL TRANSPORT 



at 500,000 per cell, because he failed to detect any glucose binding, 

 this value representing the sensitivity limit of his measurements. 



An important approach is the search for end-group reagents 

 that will stably mark transport sites on intact cells, so that isolative 

 procedures will not dislodge the marker. Such reagents may per- 

 haps be discovered by looking for irreversible inhibitors of transport. 

 This approach has been illustrated above (page 20), by the report 

 of Stein (1958), of an amino-terminal histidine on the red blood 

 cell surface, whose reaction with phenylisothiocyanate is blocked 

 by the inhibitor of glycerol transport, 1,3-propanediol. Again, be- 

 cause of the sparsity of such sites, unusual procedures are called for 

 to make the marking sufficiently specific. 



One may also hope that the isolation of transport sites might 

 be monitored at each stage by examining for the specific binding- 

 properties corresponding to the pattern of solutes transported. Ad- 

 vantage would need to be taken of the aspects in which the charac- 

 teristic specificity of the carrier differs from the specificity of 

 known enzyme systems. 



Attempts at the isolation of the plasma membrane are currently 

 receiving substantial interest and effort. For the large cells of mam- 

 malian organisms, the amount of material comprising the plasma 

 membrane cannot be large; for example, perhaps 0.25 per cent of 

 the cell mass of the Ehrlich ascites tumor cell should represent the 

 plasma membrane, taking the estimate of the electron microscopists 

 (70 A) for the thickness of the membrane. A principal problem 

 in the isolation of plasma membranes of many cells is their separa- 

 tion from other membraneous elements, particularly those of the 

 endoplasmic reticulum. Some cytologists have interpreted parts of 

 the reticulum either to represent infoldings of the plasma membrane 

 or to be continuous with the plasma membrane. Palade wrote in 

 1956: 



Finally smooth surfaced profiles, similar to those belonging to 

 the endoplasmic reticulum are found in close contact with the 

 cell membrane in numerous cell types. Some of these profiles are 

 closed whereas others appear to be partially or completely open 

 at the level of the cell membrane. They can represent vesicles 

 of the endoplasmic reticulum establishing contact with the cell 

 surface, or small invaginations of the cell membrane forming 

 cytoplasmic vacuoles or vesicles. 



An earlier illustration (Figure 2) shows the vesicular bodies 

 in question. Under Palade's first interpretation, the cisternae formed 



7 2 



