. Biophysical science. Biophysics. 20 : 6/ Photosynthesis 375 If the flashes are shortened to msec, it is found that one flash does not result in the liberation of oxygen from whole cells. A flash of the. 480 560 640 Wavelength (m\i) 720 Figure 10. Action spectrum of chlorella. Note the extremely flat curve as compared with those of Figures 7 and 9. After R. Emerson and C. Lewis, "Dependence of the Quantum Yield of Photosynthesis on Wave Length of Light," American J. Botany 30: 165 (1943). same total number of photons but spread over a 25 msec interval does result in the liberati
. Biophysical science. Biophysics. 20 : 6/ Photosynthesis 375 If the flashes are shortened to msec, it is found that one flash does not result in the liberation of oxygen from whole cells. A flash of the. 480 560 640 Wavelength (m\i) 720 Figure 10. Action spectrum of chlorella. Note the extremely flat curve as compared with those of Figures 7 and 9. After R. Emerson and C. Lewis, "Dependence of the Quantum Yield of Photosynthesis on Wave Length of Light," American J. Botany 30: 165 (1943). same total number of photons but spread over a 25 msec interval does result in the liberation of oxygen. (In the Hill reaction, even the short flash results in the liberation of oxygen.) Two consecutive flashes lead to a greater evolution of oxygen than twice that produced by one flash. Thus, each flash leads to the production of stable intermediates. It is not clear in the flashing-light experiments whether this is merely con- verting all the ADP to ATP the first flash, or whether it is the same intermediate indicated by the light-saturation experiments. The most direct evidence for the existence of an intermediate com- pound in an excited state comes from studies of electron spin resonance. This technique is discussed in Chapter 31, "Magnetic ; Essentially, it is based on the inherent magnetic moment (spin) possessed by all electrons. In most organic compounds encountered in biological materials, the electrons exist as pairs with their magnetic moments opposing one another and thereby cancelling. There also exist certain excited forms of compounds normally expected to have paired electrons only, in which one electron is missing, or an extra electron is present. In either case, the compound has a net magnetic moment and is called a free radical. Magnetic studies with chloroplast materials indicate that such free radicals form when light falls on the chloroplast. The very nature of the magnetic studies makes it impossible to tell just what comp
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