1088 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



atmosphere containing 5% CO2) offer a better promise of full photosynthe- 

 tic efficiency, and chose to use them instead of the "unphysiological" alkal- 

 ine buffers. Since only a small part of the carbon dioxide liberated in the 

 burst can be caught in pure water, the larger part must escape into the gas 

 space, and the consequent increase of pressure will be interpreted as in- 

 creased photosynthesis, unless a check on the photosynthetic quotient, 

 Qp, reveals that the liberated gas is mostly carbon dioxide and not oxygen. 

 It will be noted (c/. Vol. I, page 31) that we designate the photosynthetic 

 quotient as Qp and define it as the (positive) ratio — AO2/ ACO2, while 

 Warburg (and many others) designate the photosynthetic quotient as 7 

 (using the symbol ip for the quantum yield), and define it as the (negative) 

 ratio +ACO2/AO2. 



The discovery of a carbon dioxide gush by Emerson and Lewis has made 

 the interpretation of the results of Warburg and Negelein uncertain. The 

 latter's quantum yield values, calculated from net pressure changes in 10 

 minute exposures, with the assumption Qp = 1 (or more exactly, 1.09), 

 turned out to be close to 0.25 or V4- Offhand, this result seemed eminently 

 satisfactory in consideration of the fact that the reduction of carbon di- 

 oxide by water involves the transfer of four hydrogen atoms {cf. chapters 

 3 and 7, Vol. I). 



If the value M had not been so plausible chemically, the fact that this 

 high yield could be obtained only by following a specific schedule of experi- 

 ments, combined with special methods of cultivation of the algae, would 

 perhaps have attracted more attention. At first, using Chlorella cells 

 grown in full light, Warburg and Negelein obtained only quantum yields 

 of <0.06. Later they found much higher yields are obtainable with sus- 

 pensions adapted to weak light. These experiments were carried out in 

 yellow -f orange light; high values of 7 (up to 0.3) were obtained by extra- 

 polation to 7 = 0, since the light curves bent markedly even below 1000 

 erg/cm.2 sec. In a second paper (1923), in which monochromatic fight 

 was used, the curvature was less pronounced and Warburg and Negelein 

 calculated, without recourse to extrapolation, quantum yields ranging from 

 0.20 in blue, to 0.23 in red fight, corresponding to energy conversion factors 

 from 0.34 to 0.59. The least number of quanta of red fight containing suf- 

 ficient energy to cover the energy expenditure of the reaction CO2 + H2O — ^ 

 O2 + { H2CO I is three ; the lowest plausible number of elementary reaction 

 steps is four (corresponding to the transfer of four hydrogen atoms from 

 water to carbon dioxide). From Warburg and Negelein's results, it ap- 

 peared that plants are able to achieve photosynthesis with not more than 

 four photochemical steps, leaving only a small margin to cover losses of 

 energy by dissipation into heat, wh'n-h appear inovitaljle in a complicated 

 chemical process. 



