BIOLOGICAL TRANSPORT 



process is facilitated diffusion. But if the movement is deter- 

 mined by a contraction-expansion, or oscillation, impressed upon 

 a protein by the energy released by the enzymic action of that 

 protein, then we have active transfer. Thus Goldacre's (1952) 

 concept of the importance of contractile proteins in active 

 transport becomes logically connected with the mechanism de- 

 duced for facilitated diffusion. 



Other writers have suggested instead that continuous, spon- 

 taneous mechanical movements of the membrane might cause bind- 

 ing groups to function to produce uphill transport. Such a behavior, 

 although presumably wasteful in terms of energy, could probably 

 also permit flow to drive counterflow. 



The lesson taught by the close relationship between the medi- 

 ated transport that is not uphill, on the one hand, and the mediated 

 transport that functions uphill, on the other, is that we cannot afford 

 to limit our attention to active transports. Amino acid transport is 

 uphill in the reticulocyte (Riggs et ah, 1952), but apparently it 

 ceases to work uphill after the reticulum is lost (Riggs et al., 1952; 

 Winter, 1962). One should also remember that a transport capable 

 of generating a gradient does not wait until the 1 : 1 distribution 

 ratio is passed to begin to operate. The refined criterion of Ussing 

 (1949) for active transport takes account of this difficulty. This 

 criterion says that, when the flux ratio exceeds the ratio of electro- 

 chemical gradients, an active transport must be occurring. 



Occasionally the term active transport is applied incorrectly 

 when only a dependence on a metabolic, energy-yielding process 

 has been shown. This cannot be considered a reliable indication 

 that the transport is uphill. Perhaps the most convincing lines of 

 evidence that the uptake by cells is active are two: first, the gradients 

 in some cases may exceed the plausible limits for binding agents. 

 For example, the muscles of some marine crustaceans contain up- 

 ward of a \-M level of amino acids collectively, including taurine. 

 The Ehrlich tumor cell may attain gradients of 60 to 160 mA/ for a 

 single amino acid. Few possible binding sites are present at such 

 levels. Second, water movements can often be demonstrated in 

 approximately direct relationship to solute uptake. This relationship 

 was shown for glycine uptake by the Ehrlich cell (Christensen, 

 1955) as illustrated in Figure 11, and for lactose uptake by E. col'i 

 spheroplasts in Figure 12 (Sistrom, 1958). 



A more elaborate demonstration that the activity of the Ehrlich 



3° 



