1030 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



could be introduced into equation (28.27) instead of the equilibrium expres- 

 sion (27.3). However, the resulting equations, embodying the effects of 

 carbon dioxide supply limitations together with the limitation due to light 

 supply, would become too unwieldy for practical use. 



One may ask whether equation (28.35) provides a sufficient explanation 

 of the "absolute saturation" of photosynthesis, i. e., of the maximum yields 

 discussed in section 5 of this chapter. We can obtain from equation 



(28.35) the follomng estimate of the maximum possible yield of photosyn- 

 thesis per chlorophyll molecule per second : 



(28.36) PSS:/Chlo = nkrAo ( = nk* yl^) ^ 0.025 



or, for "assimilation time," T^: 



(28.36a) Ta = Chlo/PS!S: ^ 40 sec. 



The experimental values of "assimilation time" (c/. Table 28.V) actually 

 range from 10 to 100 sec. (with the exception of the remarkably smaller 

 values found for aurea varieties). It thus appears as if chlorophyll could 

 be the component of the photosynthetic apparatus the slow rate of restor- 

 ation of which imposes an "absolute ceiling" on the rate of photosynthesis. 

 However, this is not the only adequate answer to the problem of absolute 

 saturation, since flashing light experiments reveal the existence of a "finish- 

 ing" catalyst (Franck's "catalyst B"), which appears to be present in 

 an amount equivalent to about one two-thousandth of that of chlorophyll, 

 and to have a "working time" of the order of 0.02 sec. at room temperature; 

 the "ceiling" imposed by this catalyst is of the same order of magnitude 

 as the one derived in (28.36), since 2 X 10^ X 2 X 10 ~^ is the same as 

 1 X 40. 



It is useful to show why the approximate agreement of the value (28.36) 

 with the experimental results is not a significant confirmation of the model 

 used in the derivation of this equation. 



We will see below (page 1043) that in a rectangular hyperbola, the three 

 parameters which in general specify a hyperbola — such as (a) the initial 

 slope, (6) the abscissa corresponding to half-saturation and (c) the satura- 

 tion value of the ordinate — are not independent of each other, but related 

 by equation (28.48C). What we did above was to insert the approximate 

 experimental value of two of these parameters (n ^^ 0.1 and i/J ^^ 10^ lux) 

 into equation (28.28), Avhich represents a rectangular hyperbola (of. 

 equation 28.29; the equation in parentheses in 28.36 is a special case of 

 28.48C) ; and to derive in this way, an approximately correct value of the 

 third parameter, p™^'^-. This result merely shows that the light curves (or, 

 at least, the limiting light curve at high [CO2]) do approximate rec- 

 tangular hyperbolae. Any kinetic mechanism which leads to light curves 



