ASPECTS OF SALT ABSORPTION IN CELLS 107 



tissues led Stanbury and Mudge (1953) and Bartley and Davies 

 (1954) to examine the ability of isolated mitochondria to accumulate 

 solutes. They found that actively metabolizing liver and kidney 

 mitochondria are able to maintain concentration gradients for both 

 inorganic ions and organic substances. Robertson et al. (1955) 

 demonstrated that an ability to accumulate salt in mitochondria 

 from red beet and carrot roots is correlated with metabolic activity. 

 They concluded that whereas the accumulation of cations might 

 be attributed at least partly to establishment of Donnan equihbria, 

 accumulation of anions is probably due to active transport. 



Robertson (1951) suggested that the primary act of salt accumu- 

 lation occurs across the mitochondrial membranes, and in fact this 

 is the only place at which the Lundegardh mechanism could operate 

 because all the cytochrome in non-green cells is located in mito- 

 chondria. The idea is supported by the capacity of isolated mito- 

 chondria to accumulate ions. It presupposes that the plasmalemma 

 is permeable to ions so that salts can move freely from the medium 

 into the cytoplasm. As already indicated it now seems unHkely that 

 this is the case. Salts accumulated by the mitochondria might, 

 according to Robertson, be transferred into vacuoles subsequently by 

 disintegration of the particles at the tonoplast. There is no direct 

 evidence to support this view and it seems more likely that mito- 

 chondria and vacuoles are independent accumulators of salts. If 

 ions are accumulated in cytoplasmic organelles prior to transference 

 into the central vacuole, these are more probably small vacuoles or 

 vesicles with which the cytoplasm abounds (Plate I). The large 

 central vacuole arises by coalescence of smaller vacuoles during 

 development, and it is not impossible that this process continues 

 even in mature cells. The energy for salt accumulation in vacuoles 

 is presumably derived from high energy phosphorus compounds 

 produced in mitochondria, as is the energy for other endergonic 

 processes such as protein synthesis. Ions accumulated by mito- 

 chondria themselves are presumably retained and assist in main- 

 tenance of the turgidity and structural integrity of the organelle (cf. 

 the function of salts in vacuoles). 



d. Chloroplasts. Chloroplasts consist of densely packed lipo- 

 protein lamellae and in comparison with mitochondria, their water 

 content is relatively low so that a relatively large proportion of the 

 salt they contain is likely to be bound to structural components. 



