8 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



which all the ions of a given element are moving randomly from the tissue 

 to the medium and likewise, at the same rate, from the medium to the 

 tissue. Such ions are said to be exchangeable. If the appropriate radio- 

 isotope is introduced into the medium, then the specific activity of the 

 tissue will increase with a concomitant decrease in that of the medium 

 until an equal value is reached at equilibrium. The relative specific 

 activities of the tissue and medium then give information as to the rate 

 of transport and, in cases where all the ions are not involved, also as to 

 what proportions are taking part in the process (see page 28 for kinetic 

 aspects). 



An example of this behavior has been reviewed by Sheppard (45) in a 

 discussion of potassium and cell physiology. Earlier, potassium was 

 regarded as a more or less passive cation, with its selective accumulation 

 in cells controlled by the permeability of the membranes. Radiopotas- 

 sium studies, however, have shown definitely that concentration gradients 

 in cells are not the result of membrane impermeability. Experiments 

 with such tissues and organisms as Escherichia coli, squid nerve, chicken 

 embryo muscle, and mammalian erythrocytes have indicated the con- 

 tinual movement of potassium ions between the tissue and the suspension 

 media. Cultures of E. coli showed a labile potassium fraction that 

 exchanged completely with the potassium of the suspension medium in 

 less than 5 min, and a tightly bound fraction that increased as metabolism 

 progressed. The entire cellular potassium of human erythrocytes was 

 found to exchange at a uniform rate under normal conditions of about 

 1.7 per cent of the cellular potassium per hour. Another example is the 

 behavior of calcium ions in the blood. Chemical calcium determinations 

 indicate that the plasma concentration of this element remains relatively 

 constant. However, measurement of the rate of removal of labeled cal- 

 cium introduced into the blood indicates that roughly 70 per cent of the 

 individual calcium ions are being removed and replaced per minute (46). 

 Similarly, in man, about 78 per cent of the plasma sodium and 105 per 

 cent of the plasma water are exchanged per minute with extravascular 

 sodium and water (47). This phenomenon is also important in plant- 

 physiology studies, as demonstrated by Overstreet and Broyer (48), who 

 reported on exchangeable potassium in the tissues of barley roots. 



The necessity for recognizing this behavior and taking it into considera- 

 tion in the design and interpretation of experiments is discussed in Chap. 2 

 under Exchange Reactions. 



A Substitute for Difficult Chemical Analysis. As already implied, if 

 an element is in equilibrium between two or more phases of a system and 

 its radioisotope is introduced into the system, then the specific activity 

 will be equal in each phase after the radioisotope has reached equilibrium 

 conditions. In other words, under such conditions the radioisotope will 



