The catalytically competent ensemble model can also be expanded to understand the operating principles of large macromolecular machines. For example, the ribosome undergoes many large structural rearrangements during the catalytic steps involved from initiation to elongation. These rearrangements involve the making and breaking of many interactions that are relatively straightforward to interpret enthalpically. An elegant series of single-molecule fluorescence resonance energy transfer (smFRET) experiments leveraged temperature to probe the relative importance of enthalpy and entropy when two different charged tRNAs are bound to the ribosome. They revealed that both the equilibrium and rate of interconversion between the two conformational states of the ribosome, GS1 (associated with initiation) and GS2 (associated with elongation), have distinct entropic contributions depending on the identity of the tRNA present. The model that emerges is that the tRNA Met , which is critically important for initiation, and tRNA Phe , which only plays a role in elongation, differ in how they affect ribosome conformational dynamics. The energetic basis of this difference is driven by entropy favoring the corresponding states and barriers, indicating that the ribosome has evolved to manipulate entropy in response to specific tRNA-binding events that bias towards functionally relevant states (Figure ). How exactly this entropic difference is partitioned between broadening the transition state itself or an increase in the conformational entropy of the scaffold (including bound metals and water) remains an open question. This, and other studies demonstrating how the ribosome harnesses entropy during peptide bond formation 66 suggests a complex interplay of different entropic forces during its conformational and catalytic cycles of translation .