. Biophysical science. Biophysics. 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 fr
. Biophysical science. Biophysics. 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 02 production to light intensity /. 380 420 460 500 Wavelength (m|i) 540 d[Q2] dt KdI Kd + 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is after E. I. Rabinowitch, Photosynthesis, II, 1 (New York: Interscience Publishers, Inc., 1951), from Wald, unpublished, modified.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
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