Counting time in computing with cells

We consider models of P systems using time either as the output of a computation or as a means of synchronizing the hugely complex processes that take place in a cell. In the first part of the paper, we introduce and study the properties of timed symport/antiport systems. In the second part we introduce several new features for P systems: the association/deassociation of molecules (modeling for example the protein-protein interactions), ion channel rules and gene activation rules. We show that such timed systems are universal. We also prove several properties concerning these systems.