{"id":"d7603554-f26c-4590-a07f-0d2f7466aa65","arxiv_id":"2607.18905","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":7,"one_line_summary":"Within a unified thermodynamically consistent kinesin–cargo model, four thermodynamic efficiencies—including the information-thermodynamic efficiency—are all low, indicating kinesin is not optimized for thermodynamic efficiency.","lead":"Kinesin, the molecular motor that carries cargo inside cells, was tested in one consistent physics model against four different thermodynamic efficiency measures; all come out low. This matters because earlier work claimed kinesin-cargo information handling can be 70–90% efficient, and this paper argues that estimate came from parameter choices, not biology.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 'not optimized' conclusion rests on the non-unique transition rates (Eqs. 13–16); other thermodynamically consistent rates could change the efficiencies.","rationale":"The reader's weakest assumption identified the transition-rate functional forms (13)–(16) as the key structural choice, and the paper itself flags this non-uniqueness. This is indeed the most load-bearing concern: the central 'not optimized' claim is a quantitative statement about biological design, and it is only as strong as the rate model used to compute the efficiencies. A different thermodynamically consistent rate family, still fitted to the same force–velocity data, could plausibly change the efficiency values and possibly reverse the qualitative conclusion. The proposed test would settle this by scanning the allowed rate functional forms and checking whether the efficiency maxima remain low. I do not see an internal inconsistency in the derivations; the conditional verdict is appropriate, and no revision beyond the already-identified caveats is required.","tokens_in":84016,"tokens_out":4498,"duration_ms":61305,"concrete_test":"Replace Eqs. (13)–(16) by the more general thermodynamically consistent form k_α(m,x) = (1/τ_α) exp[φ_α (U(m,x)-U(m+δ,x)+Δμ_mech)/k_B T] for α=f,b, with load-sharing factors φ_α ∈ [0,1] (and analogous φ_b), refit τ_f, τ_b, φ_f, φ_b, k_c, Δμ_chem to the same force–velocity data of Ref. [16] (or scan φ_f, φ_b at fixed fitted τ's), and recompute the peak thermodynamic, information-thermodynamic, and TUR efficiencies. If any thermodynamically consistent choice yields η_info or η_TUR above 0.5 at F = −2 pN or at F = 0 with γ_vivo, the 'not optimized' conclusion is model-dependent rather than a robust property of kinesin.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central claim—that kinesin is not optimized for the thermodynamic efficiencies considered—depends quantitatively on the specific transition-rate functions (13)–(16) adopted from Ref. [27]. Local detailed balance fixes only the ratios (10)–(12), not the absolute rates; the paper itself acknowledges after Eq. (12) that 'the specific choice of transition rates may significantly affect physics in the nonequilibrium case.' The efficiencies are not invariant under this choice: θ_f and θ_b control how the load is split between forward and backward barriers, and τ_f, τ_b set absolute kinetic scales that directly enter the TUR and bipartite TUR efficiencies. The fit to the force–velocity relation constrains combinations of these parameters but does not uniquely determine the efficiency landscape. Furthermore, Table II reports very large uncertainties on some fitted parameters (τ_b = 0.194 ± 1.476 s, θ_b = 0.000 ± 0.312), and no uncertainty propagation is performed in the subsequent efficiency calculations. Thus the numerical values 0.18–0.26, and the qualitative 'low' verdict, could shift under an equally thermodynamically consistent rate model that still reproduces the same force–velocity data.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper constructs a thermodynamically consistent two-state kinesin–cargo model that retains discrete kinesin stepping and elastic coupling to a diffusing cargo. Assuming a separation of time scales between motor and cargo, the authors derive perturbative analytical expressions for four efficiencies—thermodynamic, information-thermodynamic, TUR, and bipartite TUR—and compare them with Gillespie simulations that discretize the cargo Fokker–Planck dynamics onto a lattice. Parameters are fitted to the force–velocity data of Ref. [16], and the model is validated against measured energetic quantities at F = −2 pN. The central quantitative claim is that all four efficiencies remain low (thermodynamic efficiency ≈ 0.18–0.26; information-thermodynamic efficiency at most ≈ 0.24 under an in vivo-like condition; TUR efficiencies below ≈ 0.4 for the currents considered), so kinesin does not appear to be optimized for these thermodynamic efficiencies. A secondary key result is that, under time-scale separation, the information-thermodynamic efficiency reduces to the standard thermodynamic efficiency (Eq. (98)), contradicting the high 0.7–0.9 estimate of Ref. [20].","tokens_in":84374,"tokens_out":4753,"duration_ms":55618,"significance":"If the quantitative conclusions are robust, the paper makes a valuable contribution: it provides a unified framework for comparing several thermodynamic efficiencies in a single molecular-motor model and offers a concrete explanation for the discrepancy with the higher information-thermodynamic efficiency of Leighton and Sivak (Ref. [20]). The analytic derivations are detailed and transparent, the simulation method is carefully constructed (lattice-discretized cargo dynamics with a continuum-matching condition), and the model is validated against experimental force–velocity and heat/power data. The paper also explicitly enumerates limitations, including the breakdown of time-scale separation away from the stall force and in the in vivo-like regime. However, the significance of the central claim—that kinesin itself is not thermodynamically optimized—depends on the model's representativeness and on the fitted parameter uncertainties, both of which need sharper treatment before the strong abstract-level conclusion can be accepted.","major_comments":[{"comment":"The central quantitative conclusion depends on the specific exponential transition-rate functional forms, which are not fixed by local detailed balance. The paper itself notes after Eq. (12) that 'the specific choice of transition rates may significantly affect physics in the nonequilibrium case.' Since θ_f, θ_b, τ_f, and τ_b enter directly into the analytic efficiency expressions (Appendix D, Eqs. (D2)–(D11)), an equally thermodynamically consistent rate model that reproduces the same force–velocity data could yield different efficiency values and potentially a different qualitative verdict. The authors should either justify Eqs. (13)–(16) as the unique physically appropriate choice or demonstrate that the 'low efficiency' conclusion is invariant under a physically motivated class of rate models (e.g., varying the load-sharing parameters θ_f, θ_b while maintaining the fit). Without such","section":"§II, Eqs. (13)–(16); Appendix B"},{"comment":"The fitted parameters carry very large uncertainties (e.g., τ_b = 0.194 ± 1.476 s, θ_b = 0.000 ± 0.312), yet all efficiency calculations in Section V use point estimates and no uncertainty propagation is performed. The reported maxima (η ≈ 0.18–0.26, η_TUR ≈ 0.25–0.38, etc.) are therefore single-point estimates whose statistical support is unknown. A bootstrap or covariance-based propagation of the fitting uncertainties should be added, or at least a sensitivity analysis over the plausible parameter ranges, to show that the efficiencies remain 'low' (e.g., below 0.5) across the uncertainty region. Without this, the quantitative claims are not established by the fitted model.","section":"Table II; §V"},{"comment":"The in vivo-like condition is implemented solely by increasing the friction coefficient to γ_vivo = 1.0×10⁻² pN s/nm, which the authors state gives ε ∼ 10, i.e., time-scale separation is no longer valid. The analytical perturbative results are therefore not applicable in this regime, and the numerical results for γ_vivo are used to support the conclusion that η_info remains low under in vivo-like conditions. This is load-bearing because the contrast with Ref. [20] (which reported η_info = 0.7–0.9 under in vivo conditions) depends precisely on this regime. The paper acknowledges this limitation in Section VI, but the abstract and conclusions do not carry the caveat. The authors should either qualify the in vivo-like conclusions as model-specific estimates for a single-parameter proxy, or add a validation/discussion showing that the qualitative low-efficiency result is insensitive to other","section":"§V, γ_vivo; §VI"}],"minor_comments":[{"comment":"The sentence 'the assumption of time-scale separation is strictly violated away from the stall force, where the kinesin velocity vanishes' is self-contradictory: the velocity vanishes at the stall force, not away from it. The intended meaning is presumably 'away from the stall force, where the velocity does not vanish' or 'except near the stall force.' Please fix.","section":"§IV A, near Eq. (75)"},{"comment":"The notation '(Fixed) 42.4' and '(Fixed) 38.3' in Table II is unclear—these are not fixed during the fit but selected via the stall-force argument in Appendix B. Consider using a different marker (e.g., 'Set by Eq. (…)' or a footnote).","section":"Table II and Table III"},{"comment":"The text says η_TUR and η_BTUR satisfy 0 ≤ η ≤ 1, but this is only true when the corresponding TUR bound is valid. For finite-time or approximate calculations, the efficiency can exceed 1; the authors should state that the inequalities hold in the exact long-time limit and that their approximate expressions may not strictly respect the bound.","section":"§III C, Eqs. (68) and (70)"},{"comment":"The information flow sign convention is described clearly, but the statement in the text 'information propagates from the kinesin to the cargo' could be confused with the definition of ˙I^(S,M)→X. Consider adding a parenthetical relating the sign convention to Eq. (34)–(35).","section":"§V A and Fig. 5"},{"comment":"The lattice spacing Δx = 0.05 nm is smaller than the thermal width ~7.4 nm, but the computational cost scaling with Δx⁻² is not discussed. The authors correctly state that convergence was checked, but a brief statement of the runtimes or of the error as a function of Δx would strengthen the reproducibility of the numerics.","section":"Appendix C"},{"comment":"Ref. [26] is cited as 'arXiv preprint cond-mat/0407262'; if this paper has been published in a refereed venue, please update the reference. The same applies to any other preprint-only citations.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The paper is technically sound in its derivations and the simulation method is carefully executed. The main weakness is the gap between the model-specific results and the general biological conclusion in the abstract. The authors may be able to address this with a robustness analysis over transition-rate functional forms and parameter uncertainties; without such analysis, the 'kinesin is not optimized' claim remains conditional on a particular model choice. The paper is suitable in scope for this journal and is likely to be of interest to the stochastic thermodynamics and molecular motor communities."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear colleague,\n\nThe paper is worth a serious referee. It builds a thermodynamically consistent two-state kinesin–cargo model, compares four efficiency measures on equal footing, and shows analytically that under time-scale separation the information-thermodynamic efficiency collapses to the ordinary thermodynamic efficiency. That result, plus the numerical verification and the consistency with the Ariga et al. energetics data, is a real step forward. The reported maxima (η around 0.18–0.26, TUR efficiencies below ~0.4) are credible within the model.\n\nThe main soft spot is exactly the one the authors flag themselves: the transition-rate functional forms in Eqs. (13)–(16) are not fixed by local detailed balance. The efficiencies are computed with those specific choices, and they are not invariant. The fit to the force–velocity curve constrains parameter combinations but does not remove the ambiguity; the large error bars in Table II (τ_b = 0.194 ± 1.476 s, θ_b = 0.000 ± 0.312) are not propagated into the efficiency curves. So the quantitative 'not optimized' verdict is model-dependent, not a statement about kinesin itself. The paper is candid about this, which I respect, but the abstract and conclusion lean harder on the verdict than the rate-model uncertainty allows.\n\nThe in vivo-like condition is only a larger friction coefficient, and the authors say so. That is a minor issue for a within-model comparison, not a fatal one. The absence of a code repository is a nuisance; the simulations are described well enough that a motivated reader could reproduce them, but 'available upon request' is weaker than shipping the source.\n\nWho is this for? People working on stochastic thermodynamics of molecular motors, and anyone arguing about information-thermodynamic efficiencies. The comparison framework and the η_info ≈ η result are the real contributions; the 'kinesin is not optimized' claim should be treated as an interesting hypothesis, not the take-home fact. I would send it to a good referee, and I would not desk-reject it. The authors have done the work carefully; the remaining ambiguity is in the model, and they acknowledge it.\n\nMy recommendation: engage with it, but push on the rate-model dependence. Ask for a sensitivity analysis over alternative thermodynamically consistent rate functions and over the parameter uncertainties. That would turn a conditional paper into a much stronger one.","headline":"A careful, useful unified efficiency analysis; the 'not optimized' claim is real but rides on non-unique transition-rate choices.","tokens_in":84768,"tokens_out":1925,"would_cite":true,"duration_ms":21212,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["05.70.Ln","87.16.Nn"],"model":"deepseek-v4-flash","headline":"In one thermodynamically consistent kinesin–cargo model, the thermodynamic, information-thermodynamic, TUR, and bipartite TUR efficiencies all come out low, pointing to kinetic rather than thermodynamic optimization.","keywords":["kinesin","stochastic thermodynamics","thermodynamic efficiency","information-thermodynamic efficiency","thermodynamic uncertainty relation","bipartite systems","molecular motors","time-scale separation"],"falsifier":"Evaluate the same four efficiencies with a different thermodynamically consistent transition-rate scheme (same local detailed balance ratios, different load-sharing or rate-shape choices); if the zero-load information-thermodynamic efficiency or the maximum thermodynamic efficiency rises above about 0.4, the low-efficiency conclusion is not robust. Alternatively, measure the zero-load quantity γv²/Ẇ(S,M) in a single-motor high-viscosity assay at controlled temperature; if it exceeds about 0.5, the model's calibration is inconsistent with the earlier high estimate.","tokens_in":83952,"feed_emoji":"⚡","tokens_out":6208,"duration_ms":58688,"temperature":0.7,"pith_summary":"The paper tests whether kinesin is thermodynamically optimized by computing four efficiency measures inside a single model that keeps the motor's discrete 8 nm steps and its elastic coupling to the cargo, so the measures are directly comparable. It finds that all four stay low: work conversion peaks around 0.26 in vitro and 0.18 in an in-vivo-like condition; the information-thermodynamic efficiency collapses to the ordinary thermodynamic efficiency when the cargo relaxes much faster than the motor, and reaches at most about 0.24 at zero load in the in-vivo-like case; the uncertainty-relation efficiencies remain below roughly 0.4. The paper therefore concludes that kinesin may not be optimized for any of these thermodynamic efficiency metrics, and that thermodynamic efficiency alone is an incomplete descriptor of motor design. This matters because a previous study reported information-thermodynamic efficiencies of 0.7–0.9, and the paper shows that discrepancy follows from parameter choice rather than from a different efficiency concept.","feed_headline":"Kinesin's thermodynamic efficiencies all stay below 0.4","feed_subtitle":"Work, information, and fluctuation efficiencies are all modest; earlier high estimate traces to parameter choices.","key_machinery":"The central object is a two-state kinesin–cargo model: an internal kinesin state, a discrete motor position with 8 nm steps, and a continuum cargo coordinate connected by a spring, all constrained by local detailed balance. The analytical engine is a perturbative expansion in the ratio of cargo relaxation time to motor transition time, which yields effective force-dependent exponential rates for the slow motor dynamics. In the fast-cargo limit this expansion produces the paper's key identity, Eq. (98): the information-thermodynamic efficiency equals the standard thermodynamic efficiency. For the fluctuation efficiencies, the machinery is the tilted generator of the effective slow Markov chai","core_discovery":"The central claim is that once four efficiency measures are computed within one model—thermodynamic efficiency, information-thermodynamic efficiency, TUR efficiency, and bipartite TUR efficiency—kinesin does not appear optimized for any of them. Under the in vitro high-ATP condition, the thermodynamic efficiency reaches about 0.26; under in-vivo-like friction, about 0.18. The information-thermodynamic efficiency, under time-scale separation, is analytically equal to the thermodynamic efficiency (Eq. 98): at zero external force it is near zero in vitro and at most about 0.24 in the in-vivo-like condition, contradicting earlier estimates of 0.7–0.9, with the discrepancy traced to assumptions a","pith_inferences":["If the conclusion extends beyond this model, kinesin's reliability—high processivity, directionality, and low backstepping—is likely achieved through kinetic design rather than thermodynamic efficiency, so future optimization studies should focus on rate asymmetries or other time-symmetric (frenetic) costs.","The equivalence of information-thermodynamic and standard thermodynamic efficiency under time-scale separation may hold for any bipartite motor with a fast relaxation coordinate; it would be worth testing in six-state or more detailed motor models.","Because the in-vivo-like condition here is modeled only by larger friction, the low-efficiency verdict may not survive a realistic cytoplasm with active nonthermal fluctuations; extending the model to a non-equilibrium bath is a testable next step.","A direct experimental test: measure the zero-load ratio γv²/Ẇ(S,M) in a controlled single-motor intracellular or high-viscosity assay; values above roughly 0.5 would falsify the parameter-based explanation for the earlier high estimate."],"forward_implications":["Kinesin's design target is probably not maximum work output, information transduction, or fluctuation suppression: all four measures top out well below unity.","The earlier result that kinesin has 70–90% information-thermodynamic efficiency is not reproduced; at zero load the model gives near zero in vitro and at most about 0.24 in the in-vivo-like condition.","Under fast cargo relaxation, information flow between motor and cargo does not create a separate high-efficiency channel; it is thermodynamically equivalent to standard work-conversion efficiency.","The TUR efficiency under the in-vivo-like load (about 0.28 at -1 pN) matches a prior six-state model, suggesting the low fluctuation-suppression verdict is not an artifact of the two-state coarse graining.","The low-efficiency conclusion is qualitatively insensitive to ATP concentration: high- and low-ATP conditions give the same overall picture."],"fun_headline_variants":["Kinesin's thermodynamic efficiency maxes out at 0.26","Kinesin not optimized for energy, info, or fluctuation efficiency","Earlier kinesin efficiency claims of 70-90% now in doubt","One model shows kinesin's efficiencies are all modest","Kinesin's real efficiency under 0.3, contradicting past estimates"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The numerical verdict rests on the specific exponential transition-rate functions chosen for the mechanical steps; local detailed balance fixes only the ratios of these rates, and the paper itself notes that a different thermodynamically consistent choice could significantly change the nonequilibrium physics.","fun_headline_variants_meta":{"raw":{"variants":["Kinesin's thermodynamic efficiency maxes out at 0.26","Kinesin not optimized for energy, info, or fluctuation efficiency","Earlier kinesin efficiency claims of 70-90% now in doubt","One model shows kinesin's efficiencies are all modest","Kinesin's real efficiency under 0.3, contradicting past estimates"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000321,"raw_usage":{"total_tokens":1651,"prompt_tokens":758,"completion_tokens":893,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":502,"completion_tokens_details":{"reasoning_tokens":812}},"tokens_in":502,"tokens_out":893,"duration_ms":7208,"temperature":1.0,"reasoning_tokens":812,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-01T13:58:25.273094+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Evaluate the same four efficiencies with a different thermodynamically consistent transition-rate scheme (same local detailed balance ratios, different load-sharing or rate-shape choices); if the zero-load information-thermodynamic efficiency or the maximum thermodynamic efficiency rises above about 0.4, the low-efficiency conclusion is not robust. Alternatively, measure the zero-load quantity γv²/Ẇ(S,M) in a single-motor high-viscosity assay at controlled temperature; if it exceeds about 0.5, the model's calibration is inconsistent with the earlier high estimate.","supporting_citations":[],"review_version":1}