ASPECTS OF SALT ABSORPTION IN CELLS 101 



A distinguishing feature of ions held in the free spaces of cells is 

 that they will exchange readily with salt from the external medium. 

 About 10 per cent of the potassium in excised barley roots and in 

 storage tissue slices is rapidly exchangeable (Broyer and Overstreet, 

 1940; Sutcliffe, 1954b), the remainder exchanges more slowly, and 

 is said to be located in the "non-free space" (NFS) which includes 

 vacuoles and probably at least a part of the cytoplasm. MacRobbie 

 and Dainty (1958a) distinguished three distinct phases in the 

 cellular alga, Nitellopsis obtusa, on the basis of ion-exchange- 

 ability : 



(i) a free space including cellulose wall and perhaps part of the 

 cytoplasm, 



(ii) a so-called protoplasmic non-free space, and 



(iii) a vacuolar non-free space. 



From measurements of the changes in radioactivity of an external 

 solution to which the plants were transferred after they had been 

 immersed in solutions containing radioactive isotopes, it was 

 possible to calculate the rates of movement of ions in both directions 

 across the membranes separating these phases under defined 

 conditions ("steady state fluxes"). The results are presented in Fig. 

 35a, together with values obtained for the potassium, sodium and 

 chloride concentrations in the sap. The ratio of sodium to potassium 

 ions in the protoplasmic non-free space was found to be the same 

 as in the vacuole and to be higher there than in the external medium. 

 MacRobbie and Dainty concluded that accumulation of these ions 

 in the sap might occur passively along an electrochemical gradient 

 created by active transport of chloride ions, although the relative 

 impermeability of the inner membrane to cations seems to make this 

 unlikely. The higher ratio of potassium to sodium in the sap and 

 protoplasm was attributed to active excretion of sodium from 

 protoplasm into the medium. More probably the protoplasm 

 contains small vacuoles and vesicles which hold the bulk of the 

 cations present in the protoplasmic non-free space, and which, like 

 the sap, accumulate potassium in preference to sodium (see below). 

 The concentration of chloride in the protoplasmic non-free space is 

 apparently lower than in either the medium or the sap, leading 

 MacRobbie and Dainty to suggest that this ion is actively transported 

 from the protoplasm into the sap. Whether or not active processes 



8 M.S.A.P. 



