ABSORPTION OF AMINO-ACIDS 89 



addition of glucose completely suppressed this outward 

 migration. On the other hand, there was no outward migra- 

 tion of glutamic acid from Strep, faecalis unless glucose was 

 present [5]. 



The uptake of histidine and aspartic acid by Strep, faecalis 

 is also dependent on metabolic energy [5] and in Sac. 

 cerevisiae this applied to all of the amino-acids investigated 

 [19, 6]. 



Mechanism of amino-acid absorption 



Whilst there are unaccountable differences even between 

 somewhat similar organisms such as Strep, faecalis and 

 Staph, aureus (cf. the action of inhibitors in 6, 9), there is 

 sufficient data to warrant a discussion of the mechanisms 

 which may be operative in the absorption of amino-acids. 

 This topic is of particular interest since the physical and 

 chemical structure of the barrier separating the interior of the 

 bacterial cell from the external environment is now being 

 actively studied by several workers [15, 17, 21]. Moreover, 

 amino-acids exist in solution as ions, consequently their 

 mode of absorption may be only one aspect of the general 

 problem of ion transport across cellular membranes [cf. 20]. 



The normal electrochemical properties of a cell will be 

 markedly altered by the absorption of ions of one species 

 unless there is simultaneously an equivalent migration of 

 ions either of opposite charge into the cell or of like charge 

 out of the cell. Many experiments with plant and animal 

 cells show that the uptake of one type of ion usually involves 

 the concurrent transport of other ions, and it is therefore 

 legitimate to expect that the uptake of amino-acids is like- 

 wise associated with the movement of other ions. Whether 

 this does in fact happen has not yet been investigated, and 

 indeed little is known about the migration of ions into and 

 out of micro-organisms. Several plant and animal cells con- 

 tain higher concentrations of particular inorganic ions than 

 the extracellular fluids, e.g. the K"^ content of mammalian 

 cells is greater than that of the plasma. One explanation of 

 the unequal distribution of diffusible ions between cells and 

 their environment has been advanced by Donnan, who 



