294 RADIATION BIOLOGY 



carbon dioxide into cell constituents. The question as to the role of the 

 light energy is thus specified. Indeed, various lines of research to be 

 discussed later lead to the concept that carbon dioxide is not directly 

 concerned with the action of light. 



The connections that have been recognized, however, between photo- 

 synthesis and other biochemical processes of carbon dioxide incorporation 

 have decreased neither the importance of the photosynthetic process in 

 nature nor its unique position in photobiology. It is well known that 

 photosynthesis constitutes the only path by which organized nature has 

 access to the solar energy. In fact, a very high percentage of the energy 

 mankind uses has its origin, directly or indirectly, in photosynthesis of 

 present or past time. On the other hand, even the most efficient crops 

 fix hardly more than 2 per cent of the photosynthetically utilizable solar 

 radiation falling on the cultivated area (Wassink, 1948a). Scientists have 

 recently turned attention to this situation and have raised the question 

 as to which limiting factors are operative in decreasing the efficiency of 

 the sunlight, since in direct laboratory measurements photosynthesis 

 shows a yield as great as 20-30 per cent. Cultivation of unicellular 

 algae has been suggested as possibly giving a larger gain of solar energy. 

 Higher yields might conceivably be obtained, since in relatively thick, 

 stirred suspensions of these organisms the incident radiation could be 

 more evenly distributed among the whole of assimilating tissue than in 

 a land plant, and an optimal carbon dioxide content might be easier to 

 maintain. These organisms, moreover, are usually more flexible in their 

 metabolism than higher plants are, so that it seems possible to cultivate 

 one and the same organism as carbohydrate or fat plant, depending on 

 the culture conditions. Important work in this respect has been done 

 especially by Spoehr and Milner (1949). In the laboratory of the pres- 

 ent writer, work on mass culturing of ChloreUa was started in connection 

 with an examination of the basic metabolic processes and energy rela- 

 tions. A more comprehensive treatment of related questions is to be 

 found in Chap. 4. 



Photosynthesis is distinguished from most other photobiological proc- 

 esses in that light absorption leads to a gain in energy in the system. 

 It is well to reaUze that in most, or possibly all, other biological processes 

 the intervention of light is not connected with any appreciable gain in 

 energy; i.e., light absorption initiates or directs only catabolic processes. 

 Another distinction, which is still not fully developed, is that in photo- 

 synthesis the effect expressed in energy change never surpasses the energy 

 of the absorbed light and mostly involves only a fraction of it, so that 

 appreciable effects arise only after the absorption of considerable amounts 

 of light. In many other cases, such as the various actions of ultraviolet 

 light and phototropism, the primary effect is exerted by 1 quantum or a 

 few quanta of light, finally leading to a reaction in Avhich large numbers 



