BIOLOGICAL TRANSPORT 



an ATP-generating system outside the barrier is not needed; further- 

 more, a secondary barrier is not needed to retain the carrier-solute 

 complex until it crosses the osmotic barrier. 



As we have already seen, Danielli (1954) suggested that a pore 

 formed with polar elements of protein structure might conduct 

 solutes through the membrane (see Figure 8). This arrangement 

 would produce only a simple facilitated diffusion if each stage 

 of the migration were fully reversible. But, if the transfer of a solute 

 occasioned a structural change at any stage in this sequence, inter- 

 fering with its reversibility, the phenomenon of flow driving coun- 

 terflow could be produced. Indeed, Danielli (1954) and Stein and 

 Danielli (1956) pointed out that uphill transport could arise utilizing 

 the energy that might be made available from structural changes 

 triggered by the passage of the solute molecule. The following ex- 

 tended passage represents an attempted synthesis of evidence and 

 ideas offered by Danielli in 1954: 



If the data and considerations presented above are brought 

 together to present a general picture of the plasma membrane 

 we must take into consideration: 



(a) the "sandwich" structure of the membrane; 



(b) its approximation to a homogeneous lipoid layer; 



(c) that abnormal permeabilities may be explained if in some 

 areas a polar structure extends right through the membrane; 



(d) that enzymes are present at the sites of transfer, as shown 

 by cytochemical methods; 



(e) that poisons for these same enzymes selectively block 

 transfer; 



(f) some enzymes are known to provide the mechanism 

 whereby chemical energy may be used to activate a contractile 

 protein mechanism; 



(g) to facilitate permeation of polar molecules, hydrogen 

 bonds between the molecules and water must be broken; this 

 can be done by supplying protons or alternative hydrogen-bond- 

 forming groups; 



(h) hydrolytic enzymes, such as phosphatases and esterases, 

 probably work by providing a stereochemically specific hydro- 

 gen-bonding proton-conducting surface (just as the non-specific 

 hydrolytic catalysis characteristic of ionic resins and ionic col- 

 loidal micelles is probably due to their non-specific proton- 

 conducting surfaces); 



(i) so far as can be seen, the specificity for certain molecules, 

 both of enzvmes and of transfer processes, must depend upon the 

 same organization of groups in space, both with respect to their 

 nature and their critical spacing and orientation. 



28 



