1020 THE LIGHT FACTOR. I. INTENSITY CHAP. 2S 



(d) A nalytical Formulation : Effect of Processes in the Photosensitive Complex 



Leaving aside the effects of hypothetical preparatory reactions "on the 

 oxidation end," we return to equation (27.6), P = nA'*[AC02], for closer 

 consideration from the point of view of the Ukely mechanism of light par- 

 ticipation in photosynthesis. As mentioned before, the assumption, more 

 or less implicit in the derivations of chapter 27, was that k* is proportional 

 to I, the intensity of incident light : 



(28.13) K = ^*I 

 and consequently: 



(28.14) P = nl:*l.\C(),] = 'nk*ri\C(h] 



One remark, limiting the practical applicability of the analytical ex- 

 pressions derived in this section, must be made immediately. Kinetic 

 equations are based on the law of mass action ; they presume homogeneity 

 of the reacting system. The light intensity, /, is, however, not uniform 

 throughout a leaf or cell suspension; it varies even within a single cell or a 

 single chloroplast. This complication has been repeatedly mentioned be- 

 fore, and we shall return to it again on page 1044. In the meantime, we 

 will proceed as if light absorption were uniform throughout the region 

 under consideration. This means that our equations will be strictly valid 

 only for optically thin layers. In the following equations, then, I must be 

 taken as meaning the light flux actually reaching a chlorophyll layer, and 

 not the light flux falling on the outer surface of the system. (These two 

 fluxes are proportional to each other, but the proportionality factor varies 

 with depth, as well as with the wave length of the incident light.) Practi- 

 cally, most if not all kinetic measurements have been made, not with opti- 

 cally thin pigment layers but with leaves, thalli or suspensions absorbing a 

 large proportion (sometimes up to 100%) of incident light. We will con- 

 sider on page 1044 to what extent kinetic relationships derived for optically 

 thin layers are changed through inteji,iation over the path of the light in the 

 system (and also over the differently absorbed components of non-mono- 

 chromatic light). The treatment of this problem is further complicated 

 by the structural effects discussed in chapter 22 (scattering and "sieve ef- 

 fect"). Still another complication arises in the treatment of cell suspen- 

 sions rapidly agitated during the measurement, thus bringing the indi- 

 vidual cells more or less periodically into light fields of different intensity. 

 If stirring were so intense as to cause each cell to slip through all the various 

 light fields in a time which is short compared to the "Emerson- Arnold 

 period" (about 10 -^ sec, at room temperature, cf. chapter 34), it would 



