Thermodynamics of light emission and free-energy storage in photosynthesis.

A Planck law relationship between absorption and emission spectra is used to compute the fluorescence spectra of some photosynthetic systems from their absorption spectra. Calculated luminescence spectra of purple bacteria agree well but not perfectly with published experimental spectra. Application of the Planck law relation to published activation spectra for Systems I and II of spinach chloroplasts permits independent calculation of the luminescence spectra of the two systems; if the luminescence yield of System I is taken to be one-third the yield of System II, then the combined luminescence spectrum closely fits published experimental measurement.Consideration of the entropy associated with the excited state of the absorbing molecules is used to compute the oxidation-reduction potentials and maximum free-energy storage resulting from light absorption. Spinach chloroplasts under an illumination of 1 klux of white light can produce at most a potential difference of 1.32 ev for System I, and 1.36 ev for System II. In the absence of nonradiative losses, the maximum amount of free energy stored is 1.19 ev and 1.23 ev per photon absorbed for Systems I and II, respectively. The bacterium Chromatium under an illumination of 1 mw/cm(2) of Na D radiation can produce at most a potential difference of 0.90 ev; the maximum amount of free energy stored is 0.79 ev per photon absorbed.The combined effect of partial thermodynamic reversibility and a finite trapping rate on the amount of luminescence is considered briefly.