In this paper we consider three restricted variants of P systems with active
membranes: (1) P systems using out communication rules only, (2) P systems using elementary
membrane division and dissolution rules only, and (3) polarizationless P systems
using dissolution and restricted evolution rules only. We show that every problem in P
can be solved with uniform families of any of these variants. This, using known results on
the upper bound of the computational power of variants (1) and (3) yields new characterizations
of the class P. In the case of variant (2) we provide a further characterization
of P by giving a semantic restriction on the computations of P systems of this varian
