1090 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



The energy requirements are somewhat different if a "pyramidal" mechanism is 

 postulated instead of the "linear" reaction sequence represented in figure 29.2 {i.e., if it 

 is assumed that four light quanta produce four identical pairs of intermediates, which 

 then undergo dismutation by dark reactions, finally giving one pair of finished products, 

 [CH2O] + O2; cf. Vol. I, pages 156, 158 and 164). In this case, the four quanta re- 

 quired to reduce one molecule of carbon dioxide can be absorbed by four different chloro- 

 phyll molecules. (The same result can be achieved by other physical or chemical mech- 

 anisms permitting a "collection of quanta" absorbed by several pigment molecules in 

 one "reaction center." These "photosynthetic unit" theories will be presented in 

 chapter 32.) In this case, the total energy requirement is obtained by adding to the 

 accumulated energy (112 kcal/mole) approximately 20 kcal liberated in the formation 

 of the [CO2] complex, and the energy amounts liberated in the several dismutations (or 

 other "quanta-collecting" processes). It may be noted that one dismutation reaction 

 (dismutation of a peroxide, yielding an oxide and free oxygen) was included also in the 

 "linear" scheme. The dismutation of hydrogen peroxide liberates as much as 46 kcal 

 per mole O2 (Table ll.I, Vol. I); but dismutations of organic compounds, such as the 

 Cannizzaro reaction, are less exothermal (about 10 kcal/mole; cf. Table 9.III, Vol. I). 

 Even so, three such dismutations, together with one dismutation of a peroxide, will 

 bring the total energy requirement of the "pyramidal" reaction scheme up to the same 

 210 kcal, which were estimated above for the "linear" reaction sequence. 



For 16 years, the "4 quanta mechanism" of photosynthesis ^vas the ob- 

 ject of admiration and the source of headaches for those who approached 

 the problem of photosynthesis from the point of view of energy conversion. 

 During this time, no serious attempts were made to check the experimental 

 foundations of this mechanism, and the results of Warburg and Negelein 

 were considered final. 



We will see in the next section that even during this time some measure- 

 ments were made with the higher plants that gave considerably lower quan- 

 tum yields; but because of less suitable objects and less precise methods, 

 they were not considered to throw doubt on the validity of Warburg and 

 Negelein's results. Beginning in 1938, however, a series of investigations 

 appeared, in which the photosynthesis of the same algae as used by War- 

 burg was studied by several methods (gas analysis, polarography and calori- 

 metry) in the laboratories of the University of Wisconsin ; these measure- 

 ments gave rather widely scattered results, but the yields were invariably 

 much lower than 0.25. The maximum quantum yields observed in this 

 work (to be described in some detail in section 2) were of the order of 0.1. 

 These publications induced several investigators to repeat Warburg and 

 Negelein's determinations, adhering as closely as possible to the original 

 technique. 



Rieke (1949) used monochromatic light (mercury lines 546 and 578 

 m^) and an "integrating box" for the determination of light absorption. 

 The pretreatment of the algae (adaptation to weak light), the light intens- 

 ity (about 1000 erg/cm.- sec.) and the illumination periods (10 minutes) 



