This small book is a simplified, abbreviated, and updated version of the author's Free Energy Transduction in Biology, published in 1977 (Academic Press, New York). The present book is meant to be a textbook for a class or for self-study. The first chapter gives a self-contained and elementary discussion of the principles of free energy transduction in biology. Section 5 includes new material on the Onsager coefficients Lij (for systems near equilibrium) not avail- able in 1977. Some readers may wish to study the first chapter only. The second chapteris a little more sophisticated, and deals with the so-called diagram method for calculating steady-state probabili- ties and cycle fluxes. Although these concepts are useful in the analysis of free energy transduction systems, they have an intrinsic importance and interest. Section 8 summarizes quite recent new results not included in the 1977 book. The third chapter is again a step more sophisticated. Some readers may wish to omit it. Free energy levels of the states in a kinetic diagram are introduced. This topic is primarily of conceptual inter- est for ordinary kinetic diagrams but it is essential in understanding muscle contraction (and related systems) at the molecular level. Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii . . . . . . . . . . . . . . . . Chapter 1 Survey of the Elements of Free Energy Transduction. . . . . . . . . . . . . .. . . . . . . . . . 1 1. States, Diagrams, Cycles, and Free Energy Transduction ...................... 2 2. Thermodynamic Forces. . . . . . . . . . . . . . . . 12 . . . . 3. Operational, Cycle, and Transition Fluxes. . . . . 20 4. Efficiency and the Rate of Free Energy Dissipation . . . . . . . . . . . . . . . . . . 24 . . . . . .