In the beginning of the evolution of photosynthesis, the trap energy was determined by available molecular absorbers, donors and acceptors. Nowadays, it is determined by the requirement to use water as the source of reducing equivalents. This requirement Selleck NSC23766 has focused interest on the minimal trap energy required for the production of its complement, oxygen. The methodology of photoacoustics allows the direct measurement of trap energies

(Mielke et al. 2013). Our measurements on A. marina, which uses chlorophyll d absorbing some 40 nm to the red of chlorophyll a, indicate a similar efficiency of the photosystems (Mielke et al. 2011). Thus, the reduction of excitation energy in the case of A. marina has not reached the minimum energy required for using water as the primary donor. The complication of predicting this trap energy in photosynthesis is the Jekyll–Hyde effect of the protein. On the one hand, holding the redox molecules at the optimum distance and orientation to provide the ideal environment are what produce the observed unity quantum yields of charge separation via quantum mechanical tunneling of

electrons. On the other hand, the innate flexibility of proteins, and their ungodly number of degrees of freedom, almost ensure that the thermal relaxations will extend over a wide selleck range of time scales. All measurements seem to converge on this last point (see, e.g., Parson 1982; Woodbury and Allen 1995; Xu and Gunner 2000; de Winter and Boxer 2003). The result is that the system is not at thermal equilibrium during some stages of the reaction. Its free energy is therefore not well-defined,

and it can only be described by methods of irreversible thermodynamics. Note that the enthalpy and entropy changes are still meaningful; in heptaminol fact, the excess entropy change, i.e., an entropy more positive than the equilibrium value, can be used as the criterion of irreversibility. Summary Considerations of thermal machines are irrelevant to the efficiency of photosynthetic reactions since these are essentially isothermal photochemical processes. The efficiency of converting the energy of the absorbed photon to free energy of products is limited only by kinetics: the ratio of loss channels to the product Doramapimod channel as stated by Parson (1978). If the losses were negligible, the efficiency could be >98 %. With a realistic estimate of the kinetically required loss reactions, the efficiency from the trap energy could be 54 %. The efficiency of forming oxygen and glucose from water and carbon dioxide, assuming eight photons at 680 nm are required, is close to the observed efficiency, 35 %, so it may be difficult to improve on evolution.