Editing P system (section)

===Membranes===
Membranes are the main “structures” within a P system. A membrane is a discrete unit which can contain a set of objects (symbols/catalysts), a set of rules, and a set of other membranes contained within. The outermost membrane, held within the environment, is often referred to as the 'container membrane' or 'skin membrane'. As implied to by their namesake, membranes are [[Semipermeable membrane|permeable]] and symbols resulting from a rule may cross them. A membrane (but not the container membrane) may also “dissolve”, in which case its content, except for rules (which are lost), migrate into the membrane in which it was contained.<ref name=paun2002 />

Some P system variants allow for a membrane to divide, possess a [[Electric charge|charge]] or have varying [[Semipermeable membrane|permeability]] by changing membrane thickness.<ref name=paun2002 />