Active processes make mixed lipid membranes either flat or crumpled

TL;DR

This paper develops a hydrodynamic theory linking active processes in lipid membranes to their phase transitions and fluctuations, predicting membrane stiffening or softening depending on activity and order.

Contribution

It introduces a generic active membrane model that connects active tension to membrane flatness or crumpling, and predicts observable fluctuation behaviors near phase transitions.

Findings

Active stiffening with orientational order leads to flat membranes.

Active softening causes membrane crumpling.

Membrane fluctuations can indicate the nature of phase transitions.

Abstract

Whether live cell membranes show miscibility phase transitions (MPTs), and if 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 lipid membranes {{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 are made: (i) generic {\em active stiffening} with orientational long range order (flat membrane) or {\em softening} with crumpling of the membrane, controlled by the {\em active tension} and (ii) for mixed 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 order MPTs, active stiffening (softening) manifests as a {\em super-stiff (super-soft) membrane}. Our predictions are testable in a variety of {\em in-vitro} systems, e.g., live cytoskeletal extracts deposited on liposomes and lipid membranes containing active proteins embedded in a passive fluid.