"Catalysis". It is sometimes said that the molecular motor or self-assembled material "catalyzes" the coupled reaction. Although it is true that these species mediate the coupled reaction without being consumed, this does not necessarily mean that they correspond to what most chemists would consider a catalyst. Chemists usually understand catalysis to mean that the overall energy barrier of a transformation has been lowered relative to the uncatalyzed pathway. In some cases (e.g., 6), the rate of the coupled reaction is higher outside of the reaction network. Thus, we prefer "mediates" to "catalyzes" as it covers all eventualities. That said, catalyze is obviously correct in analogous biological systems, which are highly optimized. But my reaction is irreversible… Throughout we have treated all reactions as fully reversible. This is necessary and correct because all reactions are technically reversible with the forward and reverse rate constants related by the corresponding equilibrium constant. Thus, it is never accurate to set a reverse rate constant arbitrarily to 0this would correspond to the reaction free energy being infinite and negative. However, it is true that many reactions are practically irreversible if one of the products is lost from the system. For example, in the case of catenane 6, the loss of CO2 from the reaction mixture during the operation prevents the cleavage reaction from reversing at an appreciable rate. This observation allows us to square the circle; the problem can be resolved by setting [CO2] to a low but realistic value once an equation for r0 has been constructed (Box 2). More generally, it is obviously appropriate to take mathematical limits once equations describing a network have been constructed. However, arbitrarily ignoring processes enroute to these equations can lead to physically inconsistent results.