The synthesis of ATP from ADP and inorganic phosphate by F1F0-ATP synthase, the universal enzyme in biological energy conversion, using the energy of a transmembrane gradient of ions, and the use of ATP by the myosin-actin system to cause muscular contraction are among the most fundamental processes in biology. Both the ATP synthase and the myosin-actin may be looked upon as molecular machines. A detailed analysis of the molecular mechanisms of energy transduction by these molecular machines has been carried out in order to understand the means by which living cells produce and consume energy. These mechanisms have been compared with each other and their biological implications have been discussed. The thermodynamics of energy coupling in the oxidative phosphorylation process has been developed and the consistency of the mechanisms with the thermodynamics has been explored. Novel engineering applications that can result have been discussed in detail and several directions for future work have been pointed out.