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Abstract

Whether live cellmembranes show miscibility phase transitions (MPTs), andif so, how they fluctuate near the transitions remain outstanding unresolved issues in physics and biology alike. Motivated by these questions we construct a generic hydrodynamic theory for lipidmembranes that are active, due for instance, to the molecular motors in the surrounding cytoskeleton, or active protein components in the membrane itself. We use this to uncover a direct correspondence between membrane fluctuations and MPTs. Several testable predictions aremade: (i) generic active stiffening with orientational long range order (flat membrane) or softening with crumpling of the membrane, controlled by the active tension and (ii) formixed lipid membranes, capturing the nature of putative MPTs by measuring the membrane conformation fluctuations. Possibilities of both first and second order MPTs in mixed active membranes are argued for. Near second orderMPTs, active stiffening (softening) manifests as a super-stiff (super-soft) membrane. Our predictions are testable in a variety of in vitro systems, e.g. live cytoskeletal extracts deposited on liposomes and lipid membranes containing active proteins embedded in a passive fluid.