ABSORPTION OF AMINO-ACIDS 83 



towards amino-acids. Cells which initially contain negligible 

 amounts of amino-acids (i.e. 'amino-acid deficient cells') are 

 especially suitable for this type of experiment and they were 

 obtained from cultures grown on a liquid medium in which 

 the concentration of amino-acids was just sufficient for 

 growth. Such cultures were harvested near to the cessation 

 of active cell division since the ability to absorb amino- 

 acids was observed to decline appreciably in the stationary 

 phase. The amino-acid deficient cells were suspended in an 

 inorganic salt medium (pH 7-2) to which other substances 

 (amino-acids, glucose, etc.) were added as required. At the 

 end of the experimental period the cells were collected by 

 centrifuging, washed, made into a thick suspension and 

 the internal concentration of the amino-acid determined. 

 Unless stated to the contrary the results given below apply 

 to Strep, faecalis. 



Uptake of lysine [5] 



As soon as lysine was added to the experimental system 

 it began to pass into amino-acid deficient cells of Strep, 

 faecalis, and after about 15 minutes equilibrium was reached 

 and there was no further increase in the internal concen- 

 tration. This uptake of lysine occurred rapidly at 37° C. and 

 was still appreciable at 4° C. The rate of appearance of 

 lysine inside the cells w^as approximately directly propor- 

 tional to its concentration in the external medium (Fig. 6.1), 

 and the Q^q over the range of 20°-30° C. was i -4. From these 

 facts it may be deduced that lysine is entering the cells by 

 a process of diffusion. However, this uptake did not repre- 

 sent simple equilibration betw^een lysine-deficient cells and 

 a lysine-rich environment, since at equilibrium the internal 

 concentration of the amino-acid was from three to twenty 

 times greater than the external concentration, the ratio of 

 internal to external concentration being inversely related 

 to the latter. In other words, the cells were accumulating 

 lysine against a concentration gradient (Fig. 6.2). The rate 

 of lysine absorption increased as the hydrogen-ion concen- 

 tration of the salt medium was decreased to pH 9-5, and 

 since the isoelectric point of lysine is 9*47, it is feasible that 



