86 MINERAL SALTS ABSORPTION IN PLANTS 



act as intermediates in energy transfer, it seems not unreasonable to 

 conclude that active transport, like other endergonic processes, 

 depends on energy derived more or less directly from such substances. 

 The effects of phosphorylation inhibitors, such as dinitrophenol, on 

 salt absorption support this conclusion. 



The efficiency of active transport by a phosphorylation 

 mechanism is not limited to the same maximum value as are electro- 

 chemical hypotheses. On the assumption that the energy from one 

 "high-energy" phosphate group (~P) is utilized in the transport of 

 one ion pair and that a net 38 ~P are produced in the complete 

 oxidation of 1 molecule of glucose, the maximum value for the ratio 

 of ion pairs absorbed to oxygen involved =^6-3. In fact, values of up 

 to about 18 have been observed under some conditions with both 

 animal and plant material, which suggests that at least three ion 

 pairs may in certain circumstances be transported per — P utilized. 



The precise manner in which phosphate group energy might be 

 utilized in active transport remains to be elucidated. Several 

 hypotheses have been proposed of which two are illustrated in 

 Fig. 31 (see also Fig. 12 d; e p. 43), but none of them has yet 

 received convincing experimental support. Evidence indicating a 

 close relationship between ATP-energized protein turnover and salt 

 absorption is presented below (pp. 88-9). 



b. Phosphate absorption. Particular interest attaches to the 

 uptake of phosphate by plants because of its unique function in 

 metabolism. There is a large amount of evidence, from a variety of 

 micro-organisms and higher plants, that absorption is closely 

 related to metabolic utilization. A number of reactions are known 

 in which inorganic phosphate is esterified during the course of 

 metabolism, for example, in the synthesis of glucose- 1 -phosphate 

 during the breakdown of starch by phosphorylase, and in the 

 conversion of 3-phosphoglyceraldehyde to 1 :3-diphosphoglyceric 

 acid during glycolysis. The nucleotides, adenosine di- and tri- 

 phosphate are also implicated in phosphate binding. During 

 electron transport, ADP is converted to ATP with utilization of 

 energy and absorption of inorganic phosphate. Gourley (1952) 

 found that radioactivity from ■'^P-labelled phosphate enters the ATP 

 in red blood cells faster than it reaches inorganic phosphate, and 

 suggested that phosphate may be transported as ATP. Loughman 

 and Russell (1957) showed similarly that phosphate is transferred 



