Introduction 27 



in putting so many cells into the manometric vessels that the radiated light was 

 completely absorbed. But the great respiration of these many cells was inconve- 

 nient. Nowadays the respiration is compensated for by means of düfused blue- 

 green light before the incident light is allowed to irradiate the manometric vessel; 

 therefore, extensive respiration no longer presents an obstacle to the application 

 of the method of complete absorption. 



Incomplete absorptions we measured first by means of the quantum actinometer, 

 which we placed around the manometric vessel containing the cells. Later we 

 developed an Ulbricht sphere for measuring incomplete absorptions ; in the center 

 of the sphere the manometric vessels were moved about and irradiated as they were 

 in the yield determinations. If the photomultiplier built into the surface of the 

 sphere showed the deflection zD for white cells and i for green cells, the fraction of 

 irradiated light absorbed by the green cells was 



i 



a = —^ 



Thus, it is now possible to measure both very small and very large quan- 

 tities of absorbed light that is to determine the yields of photosynthesis for very 

 dilute as well as very dense suspensions of cells. The fact that we found almost 

 the same quantum yield for 5% light absorption as for 100" light absorption may 

 be considered a confirmation of the reliability of our methods of absorption 

 measurement. 



As may be gathered from all the aforesaid, two instruments are required for 

 measuring light absorption: one instrument to measure the incident quanta, 

 and another to measure the absorbed fraction of the incident quanta. A bolometer 

 and an Ulbricht sphere are examples of such instruments. Because both Calvin 

 and Daniels used the bolometer quite incorrectly, we should like to point out 

 that this instrument is not suitable for measuring light absorption (34) by light- 

 scattering suspensions. 



VI. Metallo-enzymes (2) 



The role of iron or copper in oxygen-transferring enzymes has been completely 

 explained from a chemical point of view by a valence change of these metals. 

 However, there is a second type of metallo-enzyme, in which the function of the 

 metal is not a valence change, e.g., when zinc is effective or when several metals 

 are interchangeable with one another. A metallo-enzyme of this type is yeast 

 zymohexase, which splits hexosediphosphate into triosephosphate and which is 

 activated by zinc, cobalt, or iron. 



The method we introduced in 1938 for the detection of functional heavy metals 

 in enzymes is not an analytical one, since the high molecular weights of the enzymes 

 make analytical results uncertain and since analysis does not give an explanation 

 of the function of metal. Instead, our method consists in removing the metal by 

 dialysis against complexing substances, e.g., phenanthroline, and in resynthesizing 

 the enzymes by addition of metals. Many functional heavy metals have been dis- 



