Reaction networks in thermodynamic equilibrium under isothermal and isobaric conditions minimize the Gibbs free energy, but chemical reactions in living organisms operate typically far from equilibrium. Currently, there is no general optimization principle for nonequilibrium systems which can be used in the analysis of biochemical networks. Motivated by the avalailabity of whole genome reconstructions of metabolic reactions, the thermodynamics of biochemical stoichiometric networks has made significant progress in the last decade. These include the consistent formulation of conservation conditions resembling Kirchhoff’s law for electrical networks. In addition, Beard and Qian suggested that the flow force relationship Δμ = RT log(_(J~_)~(J~+))between the forward and backward fluxes J~+ and J~_ and the chemical potential difference of a chemical reaction can be extended from mass action kinetics to more general reactions schemes. In this tutorial review we summarize the recent literature on reaction network thermodynamics and discuss its implications to the analysis of large biochemical systems. In addition, we discuss some recent work on flow-force relationships and global variational principles characterizing nonequilibrium steady states of reaction networks.
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