374 



Photosynthesis /20 : 6 



Fucoxanthol 



photons are absorbed or, in terms of electronic structure, an electron is 

 raised from its lowest energy state to an excited one. Thereafter, this 

 energy is used to drive the phosphorylation chain and to reduce pyridine 

 nucleotide. Considerable evidence supports the existence of intermediate 



states associated with the light reaction. The 

 lifetime of these intermediates is long compared 

 to that normally expected for excited electronic 

 states. 



One very direct line of evidence for a 

 comparatively stable intermediate comes from 

 a study of the Hill reaction. In the presence 

 of an excess of electron acceptors, the reaction 

 follows the rate equation, relating 2 production 

 to light intensity / 



380 420 460 500 



Wavelength (m|i) 



540 



d[Q 2 ] 

 dt 



K d I 

 K d + I 



(7) 



Figure 9. Absorption 

 spectra of two caroten- 

 oids. These were both 

 dissolved in hexane. 

 The solvent alters the 

 location of the maxima 

 as well as the height of 

 the curve. Different 

 carotenoids in the same 

 medium have different 

 peaks. The two curves 

 shown are not plotted 

 on the same scale along 

 the absorption axis. 

 The /3-carotene is after 

 F. P. Zscheille, J. W. 

 White, B. W. Beadle, 

 and J. R. Roach, Plant 

 Physiol. 17: 331 (1942). 

 The fucoxanthol is after 

 E. I. Rabinowitch, 

 Photosynthesis, II, 1 

 (New York: Interscience 

 Publishers, Inc., 1951), 

 from Wald, unpublished, 

 modified. 



which is similar to that for Michaelis-Menten 

 kinetics. As in the latter case, the simplest 

 interpretation of Equation 7 is that a stable 

 intermediate must exist within the chloroplast. 

 In a similar fashion, one may measure the 

 relationship of the rate of photosynthesis in 

 intact cells to light intensity. The resulting 

 curves flatten out as the light intensity increases. 

 This phenomenon also supports the concept 

 of light-induced stable intermediates. 



Another quite different indication of in- 

 termediates comes from observing the effects of 

 flashing lights. From the previous analysis of 

 photosynthesis into three parts, it seems almost 

 trivial that a part of the reaction could proceed 

 with the light off, but historically this was not 

 always understood. At high light intensities, 

 the photosynthetic yield of carbohydrates (or 

 oxygen) per incident photon is greater with a 

 flashing light than with a constant light. 

 Essentially, the light may be thought of as in- 

 ducing stable intermediates up to saturation 

 during the "on" period, which continue to 

 drive the phosphorylation chain and carbon 

 cycle during the "off" (or dark) period. 



