88 MINERAL SALTS ABSORPTION IN PLANTS 



rapidly to ATP in barley roots and only later appears in hexose 

 phosphates and nucleic acids. While it is clear that phosphate 

 became bound, at first temporarily, and later permanently, into cell 

 constituents as a result of metabolic processes, the relationship 

 between these reactions and absorption remain unclear. Phosphate 

 either diffuses passively to the sites of binding which may be at or 

 near the surface of the cytoplasm, or it is transported actively by 

 a carrier mechanism across an impermeable membrane before 

 reaching the metabolic sites. If the latter situation obtains the nature 

 of the specific carrier substances and their relationship to inter- 

 mediates of phosphate metabolism need to be elucidated. Mitchell 

 (1957) has proposed that transport of phosphate across the surface 

 membrane in bacteria depends upon a specific enzyme-like protein 

 (a "translocase") located in the membrane, which is capable of 

 combining with phosphate on one side and releasing it on the other. 

 The mechanism may either facilitate phosphate exchange, or cause 

 accumulation, depending upon the rate of transfer of ions in the 

 reverse direction. 



c. Active transport and protein synthesis. 



From his investigations of the factors affecting protein synthesis 

 and salt absorption in storage tissues. Steward was led to conclude 

 that a close relationship exists between the two processes. In an 

 attempt to explain this correlation, Steward and Street (1947) 

 suggested that a precursor of protein combines with both anions 

 and cations at the surface of protoplasm, and functions as an 

 amphoteric carrier. Within the cytoplasm, the carrier is utilized 

 and the ions are released to diffuse passively into the vacuole. This 

 hypothesis was further elaborated by Steward and Millar (1954) to 

 implicate nucleic acids in the mechanisms of protein synthesis and 

 salt absorption (Fig. 31a). 



It has been pointed out that ions can be accumulated in cells in 

 the absence of net protein synthesis or even when protein hydrolysis 

 is occurring (Ulrich, 1941; Humphries, 1951). However, the 

 breakdown and resynthesis of protein (protein turnover) is probably 

 a constant feature of living protoplasm, and it is not impossible that 

 this process could mediate active transport even in the absence of 

 net synthesis. The observation that accumulation is prevented by 

 chloramphenicol, a specific inhibitor of protein synthesis, at con- 



