Concentration dependent cellular processes
There are several, but I guess the most well known is the neuron action potential, where the concentrations of Na+, K+, and Cl- ions determine the membrane potential of the cell. When the potential reaches a certain level, it triggers the action potential. You can read about this in much more detail on wikipedia, https://en.wikipedia.org/wiki/Action_potential .

But to be pedantic about it, almost every cellular process is concentration dependent.