ABSTRACT
The rate at which things happen in life is important. Too fast risks losing control, and lost control means lost coordination in a complex world in which any one process depends on others. Panic replaces coordinated ight and as a result you get eaten. Too slow — and you just get eaten. Yet the requirement of rate control would appear unrealisable within the nanoscale world of biomolecular machines and cell membranes that control metabolism. Here, Brownian motion is always a large component of dynamics. Every molecule is buffeted continuously by the stochastic and unpredictable thermal forces from its environment. What scope is there here for clocks? The answer is a deep and interesting one: if care is taken to design an ‘energy landscape’ in such a way that the ensemble of states that dene the start and end points of a biochemical process are separated by a third ensemble that must be visited on the way but requires signicantly higher internal energy (on a scale of kBT ) than the initial state, then the mean transition rate over this ‘energy barrier’ is controlled and relatively narrowly distributed.
