The Biophysical Problems of Photosynthesis

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Abstract
A reasonable framework for thought about the physical problems of photosynthesis is given by a series formulation in conjunction with the idea that the photochemical reaction centers are served by aggregates of light-harvesting pigments. The main physical questions appear to be the following. How does energy, absorbed by the major pigments, become localized efficiently at the reaction centers? What molecular species are involved in the primary photochemistry? How do these molecules convert electronic excitation energy into a stable separation of oxidants and reductants? Some progress has been made toward isolating and describing a reaction center in photosynthetic bacteria. With better chemical characterization and with improved differential spectrophotometry (incorporating better time resolution), the roles of P870 and cytochrome in this system will, hopefully, be clarified. Interpretations of fluorescence and of delayed-light emission have played a large part in shaping theories of the photosynthetic mechanism. New evidence indicates that the emission traditionally regarded as fluorescence includes a significant component of delayed light. To make matters worse, the most interesting part of the "fluorescence, " the time-varying part, is the most likely to be contaminated with delayed-light emission. The experimental basis of theories drawn from the nature of the fluorescence thus needs to be reexamined. In photosynthetic bacteria, the fluorescence and delayed-light emission can be distinguished and observed separately. These bacteria therefore offer the best prospect for obtaining an understanding of the meaning of emitted light and knowledge of the details of energy flow from major pigments to photosynthetic reaction centers. There is a bibliography of 66 references.