BIOLOGICAL TRANSPORT 



site by an apolar bond. The direction of this bond is probably not 

 especially critical, since hydrocarbon groups of various lengths and 

 branchings can serve. Perhaps the small but definite affinity shown 

 by the d isomers (about one-tenth that shown by their l enantio- 

 morphs) arises from a degree of flexibility in the direction of this 

 third point of attachment. Conceivably, the large hydrocarbon 

 groups, rather than serving statically as a third point of attach- 

 ment, could facilitate the passage or creeping of the solute mole- 

 cules from one polar site to another, by bonding transiently to 

 corresponding hydrocarbon sites in the membrane. 



As Figure 20 shows, however, the whole story of the uptake of 

 neutral amino acids by the Ehrlich cell is more complex than the two 

 preceding paragraphs may have suggested. The curve marked 

 Leucine shows how quickly the amino acids with large hydrocarbon 



20 



20 40 60 



Time, min 



Figure 20 Time course of the uptake of four amino acids by the 

 Ehrlich ascites tumor cell. The curve for a-aminoisobutyric acid uptake 

 resembles that for glycine except that higher distribution ratios are 

 reached. "Cycloleucine" designates 1-aminocyclopentanecarboxylic acid. 

 [From Christ ens en et al. (1962), in J. F. Holto?i (ed.), Conference on 

 Free Amino Acids, Elsevier, Amsterdam; with permission.] 



56 



