Chapter 32 



THE PIGMENT FACTOR* 



1. Relation between Light Absorption and Pigment Content 



The Hght energy used in photosynthesis is taken up by the pigments. 

 In the reaction kinetics of photosynthesis, the "pigment factor" is there- 

 fore closely related to the "light intensity factor." The rate of absorption 

 (number of quanta absorbed per unit time in unit volume) is affected by 

 changes in the concentration of the absorbing pigments, c, as well as by 

 alterations in the intensity of illumination, I. In a homogeneous system, 

 the relation between these two factors is determined by Beer's law: 



(32.1) A = 7(1 - 10-"'^'^) 



where a is the absorption coefficient of the material and d the thickness of 

 the absorbing layer. We note that, whereas / is a proportionality factor, 

 c stands in the exponent. Therefore, the rate of absorption increases 

 proportionately with light intensity, but slower than proportionately with 

 the concentration of the absorbent. Only in the limiting case, when acd <C 

 1, equation (32.1) can be replaced by the linear approximation: 



(32.2) A = lacd In 10 



in which both / and c are proportionality coefficients. In this case — 

 realized when the absorbing layer is very thin, or the concentration of the 

 absorbing material very low, or its absorption coefficient very small — the 

 concentration factor and the intensity factor are interchangeable, i. e., 

 an increase in intensity by the factor x and simultaneous reduction of the 

 concentration by the factor 1/x leaves the absorption unchanged. 



This situation practically never occurs in plants, since even a single 

 cell, or isolated chloroplast, absorbs a considerable fraction of incident 

 light. Therefore the use of the linear approximation (32.2) is out of ques- 

 tion, except for extremely chlorotic cehs, or etiolated plantules just begin- 

 ning to form the green pigment. Furthermore, the influence of changes 

 in the concentration of the pigment, [Chl],t on light absorption cannot be 



* Bibliograph)', page 1310. 



t For the sake of simplicity, we proceed here as if chlorophyll were the only pigment 

 that matters. The role of other pigments will be discussed later in this chapter; it 

 seems that their presence has an effect equivalent to that of an increase in the absorption 

 coefficient of chlorophyll in certain spectral regions. 



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