262 PLANT BIOCHEMISTRY 



tion. However, for the same reason the root cells should not be able 

 to absorb molecules or ions from the more dilute soil solution. Never- 

 theless this anomalous movement of solutes into plants against a 

 concentration gradient does take place. Such a phenomenon requires 

 a source of energy. 



The apparently abnormal behavior of soil ions in solution is known 

 to have some connection with plant respiration. When plants die, the 

 movement of solutes into plant cells ceases. Even living plants no 

 longer absorb nutrients when placed in an atmosphere of nitrogen or 

 carbon dioxide. Respiration must take place if plant root cells are to 

 secure molecules and ions from the soil solution. 



In our study of respiration and biological oxidation we have seen 

 that the end products of this process are carbon dioxide, water, and 

 energy. Carbon dioxide is given off by plant roots and is known to 

 play an important part in plant-soil relationships. However, the 

 principal effect of respiration on nutrient absorption may not be due 

 to the action of carbon dioxide. It is more probable that the energy 

 resulting from biological oxidation makes possible the anomalous 

 diffusion of nutrients into root cells. Conceivably the energy produced 

 may affect absorption by altering the charge on the semipermeable 

 membranes of root cells. 



The most important ions present in the soil solution are the follow- 

 ing: 



Ions differ greatly in the rapidity with which they are absorbed by 

 the growing plant. Of the common cations, potassium is most readily, 

 and calciimi least readily, absorbed. Of the common anions, the 

 nitrate ion is absorbed most readily, and the sulfate ion appears to 

 be least readily absorbed. 



The absorption of a given ion by a plant is influenced by the pres- 

 ence of other ions. For example, the intake of potassium is more rapid 

 when supplied as the nitrate than when supplied as the sulfate. 



One step in the process by which plants obtain cations from the 



