DAMAGE IN AEROBIC AND ANAEROBIC SYSTEMS 



39 



reflects the chemical properties and lifetimes of intermediates in 

 the reaction chains. 



On account of the technical difficulties referred to earlier, no 

 lifetimes liave yet been measured directly, but we may infer on 



o 



O 



ex. 



K 20 40 60 80 100 150 



Oxygen concentration fim/l 



1,350 



10 



30 



90 



-^ i- 



50 70 



mm Hq 

 Fig. 8. — Influence of oxygen on radiosensitivity 



760 



kinetic grounds (Howard-Flanders and Moore, 1958) that since 

 the influence of oxygen on sensitivity is represented by equation 

 (l)witliK ~ 5|Limoles/l (Deschner and Gray, 1959; Dewey, 1960a), 

 the species with which oxygen reacts probably have lifetimes of 

 the order of 100ju,sec when no oxygen is present. 



Although the sensitivity of a cell is apparently unrelated to its 

 respiratory activity at the time of irradiation, the sensitivity of 

 plant cells which have undergone a long period of anaerobiosis 

 (Beatty et al., 1956) or which are anaerobic for periods as short as 

 1 5 min after the end of a brief iiTadiation sometimes have a 

 different sensitivity from that of cells cultured aerobically through- 

 out. It is now clear that in this case respiration is important as a 

 source of energy for speciflc kinds of metabolism, which will be 

 briefly considered in the next paragrajih. 



