466 



LIGHT AND LIFE 



The reason for greater effectiveness of a650 than of a680 at high 

 intensity seems to be that at 680 ni/x the fraction of light absorbed 

 by Cij650 and Ca695, is neghgibly small. Therefore the maximum 

 attainable rate at 680 mp. is essentially that of unenhanced photosyn- 

 thesis. At 650 n\fx, however, the enhancement appears to be added to 

 the basic rate. This happens because there is significant absorption by 

 C,,695 at 650 mix, but very little absorption by C,,650 at 680 ni/x. 



The Variation of the Course of Photosynthesis xcith Wavelengths 



An effect underlying the chromatic transients, and probably itself 

 also a consequence of the enhancement effect, is the variation of 

 photosynthesis rate during the induction period with wavelength. 

 This effect is shown in Fig. 17 for Chlorella, a repetition of the ex- 

 periment of Fig. 1 in Blinks (6) done with Porphyra. With appro- 

 priate choice of conditions, such curves have been used to calculate 

 the chromatic transient curves (49). 



Dark TOOmy 



Dark 



650injj 



Dark 



/ 



_L 



10 Rate units 

 o.iou o 



t 



J L 



Time 

 Ii<4. 17. riic time course of 0„ excliange rate ol Chlorella l>y)ciioid()sii for two 

 dilferent wavelengths. (From 48). 



Discussion 



The history of research on photosynthesis covers a fascinating maze 

 of well-founded experimental facts of general significance, mixed 

 with quantities of trivial observations often of doubtful validity. 

 There is a pressing need for clear underlying concepts, expressed in 

 terms of chemical kinetics, and compatible with the physical and 

 photochemical knowledge about the behavior of colored molecules 

 simpler than chlorophyll. Ideas from fields such as classical organic 

 chemistry, dye photochemistry, enzyme biochemistry, and solid state 



