ACIDITY 329 



indicator, it is a mild acid. In the stomach, through successive 

 oxidations of the carboxyl groups, there are left alkaline residues 

 (with 0H~ radicals), so that what is sour or acid at first later 

 becomes alkaline. We can express the facts this way. Lemons 

 and oranges have an organic acid in them which tastes sour, is 

 acid, and may cause an irritation of the stomach and therefore 

 pain, but the final products of metabolism are carbon dioxide 

 and an alkaline carbonate which is used to help maintain the 

 correct balance between the acids and bases in the body. AJso, 

 citric (lemon) acid is a buffer; it may, therefore, function in this 

 capacity in the stomach and reduce acidity present there. 

 These two interpretations involve in a sense each other. Acids 

 of this type are metaholically alkaline. 



The foregoing agricultural and medical problems in acidity 

 and alkalinity are of great importance in our everyday life. 

 There are other pH problems which as yet are only of theoretical 

 interest, as were all originally. Among these are the pH values 

 of living protoplasm and of the sap of plant cells. 



All organisms appear to possess gradients of a number of kinds, 

 in metabolic activity, in salt concentration, in acidity, and in 

 electric potential. Some of these gradients have been experi- 

 mentally determined. A pH gradient implies that the body 

 fluid of organisms is of different pH values at different points 

 along the individual. This is not always easily proved, but an 

 experimental indication of it can be given in a simple way by 

 determining the pH values of the leaves and the basal stems of a 

 plant. The leaves may have an average pH of 6.2; and the 

 lower stems, 5.8. 



In determining the pH of the living substance itself, the 

 greatest care and ingenuity must be exercised. The earliest 

 attempts at an acid-alkali determination of protoplasm involved 

 merely the use of litmus paper, which indicated a pH of 6.8 for 

 the protoplasm of a slime mold. Next came the experiment of 

 Schaede, who immersed epidermal cells of the onion in methyl 

 red and in a few minutes observed that the cell sap was red and 

 the protoplasm yellow, which indicated an acid condition of the 

 former and a basic condition of the latter. More recent experi- 

 ments are those of Needham, Chambers, and others who have 

 injected color indicators into the cell. The results give a pH 

 of 7.6 for the protoplasm of Amoeba and 7.1 for other one-celled 



