98 MINERAL SALTS ABSORPTION IN PLANTS 



cytoplasmic surface in Escherichia coli is freely permeable to sodium 

 and potassium ions. The amount of sodium entering cells transferred 

 from distilled water to a solution of salt was found to be about equal 

 to that predicted on the assumption that equality of concentration 

 inside and outside the cell was attained; no metabolic control of 

 sodium uptake was observed. A similar, freely diffusible component 

 of potassium absorption was demonstrated, but in addition a 

 considerable metabolic binding of potassium ions to cell constituents 

 occurred. Unbound sodium and potassium was rapidly lost to the 

 medium when cells were placed in distilled water; bound potassium 

 was also lost, but at a much slower rate. Cowie et al. (1950) found 

 that sulphate is taken up passively by resting cells o^ Escherichia coli 

 into a water-free space occupying 75 per cent of the cell volume. 

 Negligible metabolic binding occurred, as was shown by the ease 

 with which the sulphate absorbed was subsequently washed out 

 again. In growing cells, on the other hand, sulphate becomes 

 incorporated into organic substances to an extent which is pro- 

 portional to the increase in cell mass. In contrast to passive 

 penetration, metabolic incorporation of sulphate depends on the 

 presence of glucose, and a nitrogen source; and it is affected by 

 temperature and aeration. A yeast, Torulopsis utilis, was found to 

 differ from Escherichia coli by its ability to incorporate sulphate into 

 organic sulphur compounds even without growth. A water-free 

 space was detected comparable to that found in the bacterium. 

 Similar experiments with phosphate indicated that in both 

 Escherichia coli and Torulopsis utilis, passive penetration occurs into 

 a free space which occupies an appreciable part of the cell volume, 

 and metabolic incorporation takes place, even under conditions 

 which do not permit growth. 



There is evidently as yet no general agreement on the passive 

 permeability of the outer cytoplasmic membrane in micro-organisms. 

 Possibly the situation varies with the physiological state of the cells, 

 but variation in the techniques employed by different investigators 

 may account for some of the discrepancies observed (Rothstein, 

 1959). 



Epstein (1955) estimated that the "outer space" of excised 

 barley roots for a variety of salts is about 23 per cent of the tissue 

 volume. The measurement was unaffected by external concentration 

 of solute, pH, or by the presence of other ions in the medium. The 



