Dynamics of membranes: From passive to active systems

Fluid lipid membranes, which, together with filamentous proteins like actin, form the basic structural component of cells, have been a central topic of biological physics for the last twenty years. Traditionally, soft matter research has mainly focused on equilibrium or near equilibrium properties. In this respect, we review our work on the dynamics of simple model membranes, which are mechanically or chemically perturbed. We consider both lipid as well as diblock-copolymer membranes. Future opportunities for physics are opening up in the study of active biological systems using a modular approach. We summarize our recent work on the spreading dynamics of mouse embryonic fibroblast cells on adhesive substrates. In a somewhat reductionistic approach such an advancing cell can be considered to consist of a few main modules. An active motile gel wrapped by a membrane relaying environmental information is coupled to the cellular signaling network and the reproductive machinery. We monitored the dynamics of the leading membrane edge reflecting dynamic phase transitions of the enclosed active gel. We argue that observing cellular modules and phases will provide a powerful tool for quantitative cell biology. Physics must formulate new general concepts for interacting active modules and protein networks.