90 NITROGEN METABOLISM 



proposed that such a distribution in a system in equilibrium 

 is the natural outcome of the presence of intracellular non- 

 diffusible ionic substances such as proteins [2]. 



The experiments with Strep, faecalis, Staph, aureus and 

 Sac. cerevisiae have shown that certain amino-acids can 

 pass into the organisms against the concentration gradient 

 and at equilibrium their internal concentration may be, and 

 often is, greater than their concentration in the external 

 environment. Furthermore, two distinct mechanisms appear 

 to be operating in the absorption of amino-acids, one 

 involving simple diffusion, e.g. the uptake of lysine by 

 Strep, faecalis and Staph, aureus, whilst the other involves 

 the utilization of metabolic energy, e.g. the uptake of glu- 

 tamic acid. These facts may be accounted for by one of four 

 explanations: 



(i) That as a result of electrostatic attraction, the amino- 

 acid becomes associated inside the cell with a non- 

 diffusible ion of opposite charge, thus establishing 

 a type of Donnan equilibrium [2]. 



(ii) That once inside the cell, the amino-acid is con- 

 verted into a compound which cannot itself pass 

 through the cell wall. 



(iii) That the cell is permeable, not to the amino-acid 

 itself, but to a derivative whose synthesis is 

 endergonic. 



(iv) That the migration of the amino-acid results from 

 the movement of another ion whose formation or 

 transport is endergonic. 



Mechanism of lysine absorption 



Najjar and Gale suggested that the absorption and accu- 

 mulation of lysine in Strep, faecalis is due to the establish- 

 ment of a type of Donnan equilibrium [16]. In physio- 

 logical conditions of pH lysine carries an overall positive 

 charge, consequently if only lysine is absorbed it is reason- 

 able to suggest that this is accompanied either by the excre- 

 tion of an equivalent amount of another cation such as H 

 or K"*", or by the absorption of an equivalent amount of 



