Lecture 3 — 63 — Absorption 



This might seem to suggest a strictly one way- 

 transport or one way permeability. Before reaching 

 this conclusion it is necessary to ask what happens 

 when roots are placed not in distilled water but in a 

 salt solution. With the aid of the radioactive isotopes 

 the following kinds of observations have been made. 

 Some radioactive potassium or bromide ions move out 

 of the cell in exchange for non-radioactive ions of the 

 solution. Yet at the same time the root tissue may 

 show a net gain of salt (Broyer and Overstreet, 

 1940). As I have already pointed out, without the 

 use of the tagged ions we should know only about the 

 net change of salt content. Thus ions can leave the 

 cells by some sort of an exchange process, but the rate 

 of outward movement by ion exchange is slow in 

 comparison with the rate of entry, even against a 

 gradient, if cells have a high capacity for metabolic 

 salt accumulation. The total exchange of potassium 

 ions taking place over a considerable period of time 

 comprises a relatively small percentage of the total 

 potassium present in the roots. The possibility exists, 

 as far as the data on the roots are concerned, that 

 the exchange of potassium ions may be chiefly con- 

 cerned with ions held by cell wall or protoplasm rather 

 than the vacuole. This appears less probable for 

 bromide ions. In the early experiments on Nitella 

 cells marked chloride-bromide exchange was observed. 



The active absorption and accumulation of salt 

 against concentration or activity gradients has been 

 stressed, but the question remains whether or not salt 

 or its ions can enter the root cells when the gradient 

 is inward, in the absence of aerobic metabolism. Seek- 

 ing some definite information on this point, barley 

 roots were subjected to solutions of potassium bromide 

 of 50 to 60 milliequivalents per liter concentration 

 (HOAGLAND and Broyer, 1942). 



The roots were of low-salt type, so that the con- 

 centration of potassium was initially higher outside 

 than in the sap. Despite this inward gradient and 



