PROOKSSES IN TTTE PTTOTOSENSTTIVE COMPLEX 1020 



Comparing equation (28.31a) with equation (28.2G), we note that, in the 

 particular picture we are considering now, "half saturation" with light 

 corresponds to the equal distribution of chlorophyll between the forms 

 X-Chl-HZ and HX-ChlHZ: 

 (28.32) [HX-Chl-HZ] = Chlo/2 = [X-Chl-HZl 



If we assume that reaction (28.21a) has a quantum yield of unity, the 

 constant k* must be equal to the frequency of light absorptions by a single 

 chlorophyll molecule. We recall that / is the intensity of light actually 

 reaching the volume under consideration, and that light absorption within 

 this \'olume is supposed to be so small that the rate of absorption is practi- 

 cally the same everywhere within it. Under these conditions: 

 (2S.r>3) k* = a In 10 X 10-^ = 2.3 X 10^ « (mole/cm. 2) 



where a is the average molar absorption coefficient for the incident light. 

 This equation implies that / is measured in number of einsteins of light 

 falling per second on a square centimeter and Chlo is expressed in moles per 

 liter. According to page 838, for white light, 1 lux ^ 1.4 X 10' ^ quanta/ 

 sec. cm. 2 = 2.3 X 10"'- einstein/sec. cm.^ We thus have: 



(28.34) k* = 5.3 X IQ-^ a (mole/cm. 2) 



applicable when I is expressed in lux (meter candles). Average absorption 

 coefficient of chlorophyll for visible light is of the order of 10^; this means 

 /^* i^ 5 X 10-^ or one photochemical act per second per chlorophyll mole- 

 cule in Hght of 20,000 lux. 



Experiments indicate that i//o= is of the order of 5 X 10^ lux (cf. Table 

 28.1) ; assuming ^^ ~ 5 X IQ-^ and Ao ~ 5 X 10"^ m./l. (roughly the con- 

 centration of chlorophyll in the plastids; cf. Vol. I, page 411), we obtain 

 for kr a value of 5. When both [CO2] and / are very large, the rate ac- 

 cording to equation (28.28) approaches the "absolute ceiling": 



(28.35) PS: = nivChloAo (= nA:*Chlo ,/,/co) 



which is the maximum possible rate of reaction (28.21c) reached when a/i 

 chlorophyll is in the state HX-Chl-HZ, and all acceptor is occupied by 

 carbon dioxide. 



For low light intensities and ample supply of carbon dioxide, the light 

 curves defined by equation (28.28) approach asymptotically the limiting 

 straight line : 



(28.35a) P'/ = o = nA-*Chb/ 



The simplifying assumption that the carbon dioxide acceptor is, even 

 during intense photosynthesis, in equilil^rium with external cai'bon dioxide 

 could be dropped, and the more general expression (27.15) for [A-C02] 



