764 RADIATION BIOLOGY 



ACTION OF HIGH-ENERGY 

 AS COMPARED WITH ULTRAVIOLET RADIATIONS 



The cytological effects of treatment with high-energy radiations are 

 probably the results of chemical and physical changes induced in certain 

 molecules by the release of energy inside the living cell. This assumedly 

 results directly or indirectly from ionizations induced by the radiation. 

 Through the selection of the kind of radiation one can control the manner 

 in which the energy is distributed within the cell. Gamma rays, /3 rays, 

 and short wave-length (hard) X rays, for example, which have a low 

 specific ionization in air, release their energy in relatively scattered rather 

 than clustered loci. The ionizing particles produced by long wave-length 

 (soft) X rays, however, give off energy in dense clusters, while a rays and 

 protons release their energy in very densely ionizing columns. The 

 quantity of energy absorbed by different structures in the cell depends on 

 the nature of the ionizing particle and not on the chemical composition of 

 the cell part affected. For this reason, different kinds of high-energy 

 radiation give biologically detectable effects that are quantitatively, but 

 not qualitatively, different. 



On the other hand, the different wave lengths of ultraviolet radiation of 

 interest to the biologist, i.e., those in the range of 2250-3650 A, induce 

 cytological changes that may be both quantitatively and qualitatively 

 different. Ultraviolet quanta are selectively absorbed by the different 

 molecules of the cell, a given molecule absorbing a certain quantum only 

 if it has a corresponding energy level. The energy of a given ultraviolet 

 wave length will, therefore, be absorbed with a resulting change in certain 

 types of molecules. Wave lengths around 2600 A, for example, will be 

 highly absorbed by the nucleic acids of the cell, while those around 

 2800 A will be more highly absorbed by certain proteins. The 

 absorbing molecules are altered in the process, and these changes may 

 be reflected in morphological and functional alterations of the cell 

 constituents. 



The kind of radiation determines the types of biological materials and 

 the methods of study that can be used. The greatest latitude is possible 

 with deeply penetrating radiations, such as y rays, neutrons, and hard X 

 rays. Beta rays, a rays, and soft X rays, however, impose definite 

 limitations, for their paths in tissue are very short. If the cells to be 

 studied for effect are single cells or if they are situated at the surface of a 

 tissue, the amount of energy reaching them can be determined with 

 reasonable accuracy. If deeper lying cells are to be studied, however, 

 more penetrating radiation must be used. An alternative is the method 

 used by Gray and Read (1942), wherein root tips were immersed in 

 radon solution, which readily penetrates the material and emits a par- 

 ticles within the cells. 



