166 



PHYSIOLOGY OF THE FUNGI 



properties due to carboxyl and sulfhydryl groups and basic properties by 

 virtue of having amino and other basic groups. The protophismic mem- 

 brane, therefore, should form salt-hke compounds with both cations and 

 anions. 



Bacteria are considered by McCalla (1940) to act as ion-exchange sub- 

 stances, and fungus spores have been shown to act in the same manner. 

 McCalla investigated ion replacement by saturating cells of Escherichia 



60 

 Time in hours 



Fig. 28. The effect of hydrogen-ion concentration on the rate of diffusion of oxygen 

 into 1 per cent peptone in phosphate buffer. Leucomethylcne bkie was used as an 

 indicator. The rate of penetration of oxygen with time was followed by measuring the 

 depth of the blue zone. (Drawn from the data of Rahn and Richardson, Jour. Bad. 

 41 : 240, 1941. By permission of The Williams & Wilkins Company.) 



coli with magnesium ion and tested the replacing effects of other cations. 

 Sodium and potassium ions replaced only a little magnesium, while hydro- 

 gen and calcium ions were much more effective. 



From this viewpoint the relative amounts of the various cations 

 adsorbed from a medium would be a function of the concentration of the 

 ions present and the relative affinity of the membrane proteins for the 

 different cations. The concentrations of the hydrogen and hydroxyl ions 

 in a culture medium change during growth and may act to regulate the 

 adsorption of other ions. The pH of the culture medium may alter the 

 relative adsorption of other ions which are essential to nutrition or which 

 are toxic. At the lower pH limit the protoplasmic membrane may be so 

 thoroughly saturated with hydrogen ions that the essential cations are 

 unable to enter the cell in adequate amounts. The same situation would 



