THE ENERGETICS OF PHOTOSYNTHESIS 



79 



120 min lie;ht 

 180 min light 



300 min light 

 For the first two hours: 



For five hours: 



Large Vessel 

 Pressure Changes 



+ 4 mm 

 + 5. 5 mm 



+ 21 mm 



xo, = +33.2 m1 xv.o^ = -40.0 ^1 

 xo, = +75.5 Ml -vco, = -90.5 ^1 



+ 9.5 mm 



To determine the quantum requirement, the number of quanta of blue-green 

 hght absorbed must be multipHed by 0.5, as, according to Warburg and 

 Negelein (61), chlorophyll absorbs only 50% of light with a wave-length of 

 4700 A (see also § 26). It must also be considered that the Oo consumption 

 due to respiration of the cells at rest (100 /xl O., per 100 ^1 cells per hour) must 

 be added to the calculated value of Aq.,. As each vessel contains 3.5 /xl cells, 

 the respiration at rest is 3.5 ^1 O2 per hour. Thus, we find for the first two 

 hours 



120 X 24.8 X 0.033 + 0.5 X 120 X 0.95 X 0.2 



\/ip = — — —^ ^^ = 2./2 



33.2 + 2 X 3.5 



and for five hours 



300 X 24.8 X 0.033 + 0.5 X 300 X 0.95 X 0.2 . „, 



^'"^ 75.5 + 5 X 3.5 



If we had put in all instead of half the absorption of blue-green light, the 

 quantum requirement would have been 3.00 and 3.25, respectively. Cell 

 respiration is in reality greater than respiration at rest, so that in any case the 

 quantum requirement is even lower than the figures obtained. 



§ 31 The Nature of the Photosynthesis Enzyme 



Warburg (50, 56, 57) determined the catalytic action of eight blue-green 

 wave-lengths. The measured light had the wave-length 6450 A. Figure 34 

 shows the action spectrum of the photosynthesis enzyme with its sharp maxi- 

 mum at 4600 A. The absorption spectrum of a Chlorella suspension obtained 

 under identical conditions, with the aid of the Ulbricht sphere (Fig. 35), 

 is the absorption spectrum of all the pigments of living Chlorella and has a 

 shape quite different to the action spectrum of the enzyme. The carotenoids 

 and the flavins of Chlorella, like the enzyme, absorb mainly blue-green light, 

 but the spectra of the carotenoids, xanthophylls and flavines extracted from 

 Chlorella do not show the same picture as the action spectrum of the enzyme, 

 especially with respect to the blue part. The unknown pigment acting as an 

 enzyme must be a protein compound. It must be able to undergo a rapid 

 chemical change upon illumination and to return rapidly to its initial state 

 in the dark. Of the known pigments, only visual purple (rhodopsin) may 



