BASIC RADIATION BIOCHEMISTRY 273 



RADIATION EFFECTS ON BIOLOGICAL SYSTEMS 



We now pass on from the discussion of radiation effects in the relatively 

 simple system of aqueous solutions to those in more complex systems 

 represented by cells, tissues, spores, bacteria, etc. There have been many 

 investigations of radiation effects on growth, cell division, mitosis, and 

 genetic changes. In all probability, some radiation-induced biochemical 

 reaction is involved in these cases. But to attempt an explanation of 

 these radiation effects on a biochemical basis would be futile since prac- 

 tically nothing is known of the normal biochemistry of such processes. 

 This narrows the discussion down to those, much fewer, cases in which 

 definite interference of a biochemical nature can be shown. ^ 



The important role which water plays as a solvent and converter of 

 radiation energy to active chemical agents will have become apparent in 

 the preceding sections. The question now arises whether and to what 

 extent it can play its part in living matter. Water is the preponderant 

 constituent, in tissues and cells, amounting to 80 per cent of their total 

 weight as the normal average and rising to 90 to 95 per cent in young tis- 

 sues and embryonic cells. The internal structure of cells is not homo- 

 geneous which leads to an uneven distribution of water, a point which 

 cannot be overemphasized. Apart from microscopically discernible den- 

 ser structures, e.g., the cell nucleus and granules in the protoplasm, the 

 basic matter is a highly hydrated colloid with submicroscopic channels 

 between the micelles as pathways for transport of solution and diffusion 

 of solutes. The denser structures themselves still have a considerable 

 water content. Thus the omnipresence of water as a source of active 

 chemical agents creates the conditions in which reaction with solutes 

 can take place by the processes outlined previously. 



Biologically active substances will also have a distribution varying 

 in place and time, though our knowledge of the state in which such sub- 

 stances are present is very limited. There are certain enzymes for 

 instance which are firmly linked to intracellular structures and cannot 

 be detached by artificial means, whereas other enzymes are easily extract- 

 able. The picture that can be drawn of the action of radiation is that 

 enzymes or other big molecules active in high dilution pass from their 

 storing place through the water channels between micelles to the region 

 where their presence is required, and, when subjected to radiation, would 

 fall victim to the attack by radicals. Thus the normal flow of supply 

 would decrease or cease, causing a disturbance of metabolism which would 

 last at least as long as irradiation continued. If this period is short, the 



2 We are here not concerned with one group of such definite biochemical radiation 

 effects which consists of biochemical mutants, e.g., a deficiency of growth factors or of 

 special enzyme systems as a result of radiation-induced mutations. The biochemical 

 change is of a secondary nature in these cases, being the sequence to a genetic change. 



