Nonequilibrium noise and instabilities in membranes with active pumps

We study the statistical properties and dynamics of flexible fluid membrane s containing active transmembrane proteins and find that such active membra nes can be either stable or unstable to small disturbances, depending on th e signs of certain crucial parameters characterising the protein-membrane i nteraction. A major finding is that the shape-fluctuation spectrum in the s table case for a zero-tension active membrane has the same form over a larg e range of length scales as at equilibrium, but with a nonequilibrium "nois e temperature" determined by the activity.