HYDROGEN-ION CONCENTRATION 1G7 



exist at the limiting alkaline pH values, except that it is the adsorption of 

 essential anions which would be hindered by hydroxyl ions. 



A satisfactory explanation of all the phenomena involved in cell perme- 

 ability is lacking. It is known that the external pll affects the absorption 

 of various compounds, particularly those which ionize. The mycelium 

 of Aspergillus niger takes up acid dyes, such as light green and methyl 

 orange, when the external pH is 3.1 or less. Basic dyes such as methylene 

 blue and neutral red are absorbed only when the external pH is greater 

 than 3.1 (Biinning, 193G). These dyes escaped from the cells only when 

 the external pH was in the same range in which these dyes were absorbed. 



Wyss ct al. (1944) found the utilization of p-aminobenzoic acid by a 

 deficient mutant of Neurospora crassa to be greatly increased in acidic 

 media. The ionization constant of p-aminobenzoic acid is about 

 2 X 10'"'' {pKa, about 4.8). Therefore, at pH 3.8 about 90 per cent of 

 the metabolite would exist in the form of the free acid, and at pH 5.8 only 

 10 per cent would be in this form. It was found that about eight times 

 as much of this vitamin was required at pH 6.0 as at pH 4.0 to support the 

 same amount of growth (see Fig. 41). On theoretical grounds, it is 

 probable that the pH of the medium would affect the utilization of other 

 vitamins which are weak acids (biotin, pantothenic and nicotinic acids). 



The external pH of the medium has been shown to affect the internal 

 pH of fungus cells. By changing the external pH and by using indica- 

 tors, Biinning (1936) found the internal pH of Aspergillus niger cells could 

 be changed between 4.2 and 5.0 without injuring the cells. Greater 

 changes in internal pH were possible, but injury and death ensued. 

 Armstrong (1929) crushed the fruit bodies of a number of fleshy fungi 

 and measured the pH of the expressed juice. The pH range of these 

 liquids was 5.9 to 6.2. At best these are but average values. 



It is well known that the external pH may affect certain processes 

 within the fungus cells. For example, growth of Sordaria fimicola in 

 glucose-casein hydrolysate medium was slow when the initial pH of the 

 medium was 4.0, but when the initial pH of the medium was 3.6 to 3.8, 

 normal development did not occur (Lilly and Barnett, 1947). This 

 failure to grow was traced to a thiamine deficiency, for when thiamine 

 was added to the medium (initial pH 3.6 to 3.8), normal growth and 

 perithecial formation took place. It appears possible that the low exter- 

 nal pH may have lowered the internal pH to such an extent that the 

 synthesis of thiamine was prevented (this fungus is self-sufficient for 

 thiamine when the pH of the medium is 4 or greater). These effects are 

 shown in Figs. 38 and 40. Additional evidence indicated that these 

 conclusions are correct, for pyruvic acid accumulated in the culture 

 medium when the initial pH was 3.6 to 3.8. On the addition of thiamine 

 this acid disappeared from the culture medium. 



