arXiv:0809.3798v2 [physics.bio-ph] 3 Dec 2008 EPJ manuscript No. (will be inserted by the editor) Kinesin and the Crooks theorem Esteban Calzetta 1 1 Depto. de F´ ısica, F. C. E. y N. - UBA 2 CONICET Received: date / Revised version: date Abstract. The thermal efficiency of the kinesin cycle at stalling is presently a matter of some debate, with published predictions ranging from 0 (A. W. C. Lau, D. Lacoste and K. Mallick, Phys. Rev. Lett. 99, 158102 (2007); D. Lacoste, A. W. C. Lau and K. Mallick, Phys. Rev. E78, 011915 (2008)) to 100% (G. Oster and H. Wang, in Molecular Motors, edited by M. Schliwa (Wiley-VCH Verlag GmbH, Weinheim (2003), p. 207). In this note we attemp to clarify the issues involved. We also find an upper bound on the kinesin efficieny by constructing an ideal kinesin cycle to which the real cycle may be compared. The ideal cycle has a thermal efficiency of less than one, and the real one is less efficient than the ideal one always, in compliance with Carnot’s theorem. PACS. 87.16.Nn Motor proteins – 05.40.-a Fluctuation phenomena, random processes, noise, and Brow- nian motion The application of physics thinking to life is one of the great breakthroughs of modern science [1]. The big questions (what is the fundamental difference between liv- ing and nonliving matter? is there life anywhere else in the Universe? [2]) are conspicuously evading us, but the progress in the analysis of concrete phenomena is aston- ishing [3,4]. Since this generally involves applying physical theory is contexts quite removed from the original ones, we physicists are learning quite a lot in the process. The above said is particularly true of the application of thermodynamics to biophysical problems [5]. If a physical description of a living system is possible at all, it must in- clude thermodynamics at some point. But when we focus on phenomena at the molecular level, it is the thermo- dynamics of small systems we are talking about, not the familiar macroscopic thermodynamics of heat engines and stars [6].