INHOMOGENEITY OF LIGHT ABSORPTION 1011 



front wall of the vessel). The conversion is made by reducing the abscissae 

 in the ratio a/c, where c is the concentration and a the per cent absorption, 

 {i. e., by 0.8/3 = 0.27 for the dense suspension, 0.6/1 = 0.6 for the medium 

 suspension, and 0.3/0.3 = 1 for the dilute one). This treatment causes 

 the curves for c = 0.3 and c == 1 to coincide almost exactly; but the last 

 point on the c = 3 curve still shows considerable deviation. 



There are two obvious reasons why one cannot expect the reduction method used to 

 be completely successful. In the first place, the averaging cannot be quite correct, be- 

 cause the cells are not actually exposed to the "average" light intensity, but some are 

 illuminated with stronger, and some with weaker light. This would not matter if the 

 yield were proportional to intensity; but, if the yield declines with increasing intensity 

 (as it does in the saturation region), the yield that corresponds to a given average intens- 

 ity will be lower whea the spread of actual intensities is wider, i. e., in the more concen- 

 trated suspension. 



A second complication arises from the stirring of the reaction vessel, which causes 

 the cells to come successively into light of different intensity. The effect of this varia- 

 tion is complex; it belongs to the group of phenomena (induction; photosynthesis in 

 alternating light) which will be treated in chapters 33 and 34. Only if the illumination 

 cycles are much shorter than the periods required for the completion of all dark processes 

 of photosynthesis can one expect the cells to work, in alternating light, with the same 

 efficiency as in steady light with the same average intensity. The known periods of 

 dark reactions, associated with photosynthesis, include at least one with a period as short 

 as T = 0.01 sec. at room temperature; stirring is not usually rapid enough to send each 

 cell through the whole cycle of intensities within 0.01 sec. (c/. chapter 29, page 1106). 

 Consequently, the cells in the stirred vessel are illuminated with an alternating light the 

 average frequency of which is smaller than 1/r. While the frequency of intensity 

 variations is identical for all three suspensions, their amplitude is the larger the denser 

 the suspension. Because of induction phenomena, the highest yield at a given average 

 illumination is obtained in continuous light (cf. fig. 34.5); consequently, the efficiency 

 losses caused by intermittency will be highest in the densest suspension. We have 

 thus found two reasons, each of which may explain the deviation from the average of the 

 last point in the c = 3 curve in figure 28.22C. 



It may be useful to note that the changes in the illumination of individ- 

 ual cells, caused by stirring, may be discontinuous. The absorption by a 

 single chloroplast {i. e., in the case of Chlorella, a single cell) is so strong 

 that the only significant intensities of illumination to which a cell is ex- 

 posed may be those with no cells or with a very small number of cells (1 or 

 2) between it and the light source. The scattering of light in the suspen- 

 sions tends to smooth over these discontinuities. 



With respect to the inhomogeneity of light absorption, two cell suspen- 

 sions with the same number of cells per square centimeter, but with a dif- 

 ferent concentration of chlorophyll within each cell, offer a case similar to 

 that of two suspensions of identical cells, but different dilution. Whether 

 the light curves of such two specimens will present a picture similar to that 

 sho-wn in figure 28.20, depends on their content of the catalyst that limit 



