ACIDITY 331 



has a pH of about 7.4. The Hving cell thus appears to consist 

 of an alkaline nucleus in acid cytoplasm, bathed in an alkaline 

 medium. 



The pH values so far given for protoplasm were obtained by 

 the color method. These are all subject to the criticism of 

 Deutsch (page 322) that in colloidal systems (sols and gels), 

 owing to their great adsorptive surface, color changes take place 

 which differ from those obtained in homogeneous solutions for 

 which the color indicators were calibrated. (The error, how- 

 ever, may not always be very great. The pH values obtained 

 for protoplasm appear, from every point of view, to be fairly 

 accurate.) 



Ingenious attempts to measure the pH of cells by the electro- 

 metric method have been made. Taylor and Whitaker have 

 inserted, with the aid of a micromanipulator, delicate electrodes 

 into the cells of the green alga Nitella and obtained the electrical 

 potential and thus the pH of the cell sap. With a specially con- 

 structed microelectrode of hydrogen (a vessel holding but J^ cc. 

 of fluid), Bodine measured the yolk fluid from the Fundulus 

 egg and found it to have a pH value of 6.39. With the same 

 delicate electrode, he was able to determine the pH of insect blood 

 of which but a drop is available. 



The acid-alkali equilibrium of the living body is a very impor- 

 tant and constant one, but it is not the only significant one. 

 Calcium, potassium, phosphorus, and other elements play their 

 part, both individually and in establishing a physiologically 

 balanced solution; thus, the potassium ion is a very necessary 

 one for plants, and the calcium-phosphorus balance is the 

 determining one in bone formation. Still, we come to the con- 

 clusion that while in recent years overemphasis may have been 

 laid on the hydrogen ion, it yet remains the most important 

 ion in physiological reactions. 



