The molecular pathway considered in this paper incorporates the GTPases Rac, Rho and Cdc42, and the phosphatidylinositols PIP, PIP2 and PIP3. The subscripts, a, mi and ci denote active, membrane-bound inactive and cytosolic inactive GTPases, whereas the PIPs are all membrane-bound. We consider the interactions of active GTPases and PIPs, the activation and deactivation of membrane-bound GTPases, and the binding and unbinding from the membrane of the inactive GTPases.