APPROACHES TO THE ANALYSIS OF SPECIFIC MEMBRANE TRANSPORT 59 1 



further addition of 15% w./v. human serum albumin. The brightness of 

 the anoptral contrast image increases with the refractive index of the 

 object. If the serum albumin had penetrated into the periplasm, enlarged 

 by plasmolysis, there would be no more contrast between the periplasm 

 and the suspension medium in Fig. 2{b) than in Fig. 2{a). The fact that 

 the periplasm is darker than the suspension medium in Fig. 2{b) shows 



ia) (b) 



Fig. 2. Anoptral contrast micrographs ( x 3800) of Escherichia coli (ML 30) 

 plasmolyzed in 0-4 m NaCl {a), and the same with addition of 15% (w./v.) serum 

 albumin (b). The dark periplasm between cell wall and retracted protoplasts in (6) 

 shows impermeability of cell wall to serum albumin. 



that the serum albumin (M.W. 70 000) does not penetrate the cell wall. 

 It seems reasonable to conclude that the cell wall acts as a molecular sieve, 

 allowing entry of glucose 6-phosphate, but preventing the escape of 

 glucose 6-phosphatase from the surface of the protoplast. The glucose 6- 

 phosphatase is thus in a position to catalyze the first step in the metabolism 

 and uptake of glucose 6-phosphate in Escherichia culi. 



The facts established by this work have far-reaching implications tor 



