86 Applied Biophysics 



excitation (Bohr) of protein molecules, following the absorption 

 of quanta of radiation, making the molecules capable of new 

 reactions. For example, an increase in intracellular osmotic pres- 

 sure may result from the formation of substances with smaller 

 molecular weights than the original substance. If the surrounding 

 fluids are not changed to the same extent, this would cause 

 swelling of cells. An increase in cell size after irradiation is 

 known to occur in certain instances,"'^* ^^' ^^•^' ^^"^ but is by no 

 means common to all cells affected by radiation. A suggestion 

 that radiation caused a rearrangement of colloid charges,^" which 

 was at first regarded as an alternative mechanism of action, can 

 now be fitted into Holthusen's photochemical theory by regard- 

 ing the change of charge as a photochemical process. 



Ionization rather than excitation became generally regarded 

 as the link between energy absorption and biological response, 

 and a hypothesis which has attracted a great deal of attention 

 was put forward, ^■*' ^"' ^^' ''*'• '- according to which there exists 

 in the cell a specially sensitive volume within which ionizations 

 are biologically effective, and these account for the changes sub- 

 sequently observed. More than one ionization may be required 

 to produce a biological effect, but any ionization which occurs 

 within the cell, but outside the sensitive volume, is ineffective. 

 This view of the mode of action of radiation has come to be 

 known as the target or "quantum hit" theory, and among its 

 supporters are many physicists. Differences in sensitivity to 

 radiation are explained by the chance distribution of ionizations 

 in the vital volume of the cell. Those who oppose the idea have, 

 perhaps, less well-defined views on radiation action, and are 

 united mainly in their opposition to the theory. As an alterna- 

 tive hypothesis they suggest that a chemical or metabolic change 

 is produced in the cell by irradiation, and they argue that the 

 biological results of physical as well as chemical agents can be 

 explained on the assumption that individual cells differ in their 

 reactions to the changes produced : the weakest succumb first, 

 then the less weak, and the strongest last of all. A great deal 

 of time and effort has been spent in attempts to prove and dis- 

 prove one or other theory, and most lively controversies have 



