Molecular machines from biology and nanotechnology mostly depend on soft structures to perform mechanical functions,with the underlying mechanisms not fully understood yet.We report here a rigorous study of mechanical transduction along a single soft polymer based on exact solutions to the realistic worm-like-chain model and augmented with analytical relations derived from simpler polymer models.The results reveal surprisingly that a soft polymer with vanishingly small persistence length below a single chemical bond still transduces biased displacement and mechanical work up to practically significant amounts,but by an entropy-based mechanism distinctly different from the rigidity-based transduction via solid components in macroscopic machines.The inherent entropy capacity of the polymer imposes a finite limit to the amount of transduced work; but this limit becomes infinite for rigid polymers.The effective soft-polymer transduction explains many experimental findings of biomotors and artificial nanomotors.The entropy limit presents a new dissipation mechanism that restricts the energy efficiency of molecular motors.This mechanism of dissipation possesses unique features,which match experimental findings on biomotor kinesin and suggest a possibility of observing a higher efficiency for this motor than currently reported.The surprising effectiveness of soft-structure-based transduction and the entropy limit to work/efficiency have wide implications to molecular motors and mechanical devices in biology and nanotechnology.