1136 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



tion). The abscissae in figure 29.8 are frequencies of absorption of light 

 ciuanta by each chlorophyll molecule (total number of (juanta absorbed by 

 the suspension in unit time, divided by the number of chlorophyll molecules 

 present). The slope of the Warburg curve, equated with 1/P'"'"'", indi- 

 cates a maximum production of one oxygen molecule per chlorophyll mole- 

 cule in about 10 minutes; the slope of the Emerson-Lewis curve, the pro- 

 duction of one oxygen molecule per chlorophyll molecule in about 1 minute. 



Table 28. ^' shows that the actual maximum yield in Chlorella in steady 

 light is of the ortler of one molecule oxj^gen per molecule chlorophyll in 30 

 seconds. A rapitl decline of the apparent quantum yield with / is consistent 

 with the assumption that Warburg's values were affected by the inclusion 

 of the carbon dioxide gush, since the relative importance of this gush must 

 decrease rapidly with increasing light intensity (as was suggested on 

 page 1103). 



Franck (1949) suggested that the rapid drop of I/70 in Warburg's 

 experiments with increasing light intensity, is an indication that they re- 

 flect a gradual repla(;ement of a 4-quanta process (half-way reversion of 

 respiration), by an 8-quanta process (true photosynthesis). 



On page 1 104, we described the more recent experiments of Warburg, 

 Burk and co-workers, who claimed that the highest quantum yields are 

 obtainable by intermittent illumination (which prevails in very rapidly 

 agitated, dense Chlorella suspensions). If this were true (it was men- 

 tioned on page 1106, that the results of experiments in flashing hght do not 

 support the contention of Warburg and Burk), then the relation.ship be- 

 tween quantum yield and the (average) light intensity must be more com- 

 plicated than was envisaged in the above derivations. 



(b) Quantum Yields in Strong Light 



It was suggested above that the yields of photosynthesis given for high 

 light intensities should not contradict the results obtained in quantum 

 yield measurements in weak light. What we meant can be illustrated by 

 the following examples taken from the work of Willstatter and Stoll (1918). 



According to figure 32.2, green leaves of Sambucus nigra reduce carbon 

 dioxide, at 6000 lux, at a rate of about 0.23 mg./cm.^ hr. or 1.44 X 10-'' 

 mole/cm. 2 sec. Since 1 lux corresponds roughly to a flux of 5 erg/cm. ^ sec. 

 in the region 400-700 mix {cf. chapter 25, page 838), and about 80% of this 

 flux is a])sorbed by a single leaf, we can calculate, for the energy conversion 

 factor : 



(1.44 X 10-' X 112 X 10^) cal 



((■) X 103 X 5 X 0.24 X 10-' X 0.8) cal 



0.28 



