title: Photosynthesis and negative entropy production.
abstract: The widely held view that the maximum efficiency of a photosynthetic pigment system is given by the Carnot cycle expression (1-T/Tr) for energy transfer from a hot bath (radiation at temperature Tr) to a cold bath (pigment system at temperature T) is critically examined and demonstrated to be inaccurate when the entropy changes associated with the microscopic process of photon absorption and photochemistry at the level of single photosystems are considered. This is because entropy losses due to excited state generation and relaxation are extremely small (DeltaS << T/Tr) and are essentially associated with the absorption-fluorescence Stokes shift. Total entropy changes associated with primary photochemistry for single photosystems are shown to depend critically on the thermodynamic efficiency of the process. This principle is applied to the case of primary photochemistry of the isolated core of higher plant photosystem I and photosystem II, which are demonstrated to have maximal thermodynamic efficiencies of xi > 0.98 and xi > 0.92 respectively, and which, in principle, function with negative entropy production. It is demonstrated that for the case of xi > (1-T/Tr) entropy production is always negative and only becomes positive when xi < (1-T/Tr).
authors.affiliation: Istituto di Biofisica del Consiglio Nazionale delle Ricerche-Sezione di Milano, Dipartimento di Biologia, Università degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy. robert.jennings@unimi.it
authors.forename: Robert C
authors.initials: RC
authors.lastname: Jennings
authors.forename: Enrico
authors.initials: E
authors.lastname: Engelmann
authors.forename: Flavio
authors.initials: F
authors.lastname: Garlaschi
authors.forename: Anna Paola
authors.initials: AP
authors.lastname: Casazza
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Zucchelli
mesh_terms: D004735:Energy Transfer / Q000502:physiology*; D019277:Entropy; D005453:Fluorescence; D010788:Photosynthesis / Q000502:physiology*; D045331:Photosystem I Protein Complex / Q000378:metabolism* / Q000502:physiology; D045332:Photosystem II Protein Complex / Q000378:metabolism* / Q000502:physiology; D013696:Temperature; D013816:Thermodynamics; D003313:Zea mays / Q000502:physiology*
chemical_list: D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex