This review summarizes theoretical progress in the field of active matter, placing it in the context of recent experiments. This approach offers a unified framework for the mechanical and statistical properties of living matter: biofilaments and molecular motors in vitro or in vivo, collections of motile microorganisms, animal flocks, and chemical or mechanical imitations. A major goal of this review is to integrate several approaches proposed in the literature, from semimicroscopic to phenomenological. In particular, first considered are ''dry'' systems, defined as those where momentum is not conserved due to friction with a substrate or an embedding porous medium. The differences and similarities between two types of orientationally ordered states, the nematic and the polar, are clarified. Next, the active hydrodynamics of suspensions or ''wet'' systems is discussed and the relation with and difference from the dry case, as well as various large-scale instabilities of these nonequilibrium states of matter, are highlighted. Further highlighted are various large-scale instabilities of these nonequilibrium states of matter. Various semimicroscopic derivations of the continuum theory are discussed and connected, highlighting the unifying and generic nature of the continuum model. Throughout the review, the experimental relevance of these theories for describing bacterial swarms and suspensions, the cytoskeleton of living cells, and vibrated granular material is discussed. Promising extensions toward greater realism in specific contexts from cell biology to animal behavior are suggested, and remarks are given on some exotic active-matter analogs. Last, the outlook for a quantitative understanding of active matter, through the interplay of detailed theory with controlled experiments on simplified systems, with living or artificial constituents, is summarized.

Cholesterol and sphingolipid-enriched "rafts" have long been proposed as platforms for the sorting of specific membrane components including glycosyl-phosphatidylinositol-anchored proteins (GPI-APs), however, their existence and physical properties have been controversial. Here, we investigate the size of lipid-dependent organization of GPI-APs in live cells, using homo and hetero-FRET-based experiments, combined with theoretical modeling. These studies reveal an unexpected organization wherein cell surface GPI-APs are present as monomers and a smaller fraction (20%-40%) as nanoscale (<5 nm) cholesterol-sensitive clusters. These clusters are composed of at most four molecules and accommodate diverse GPI-AP species; crosslinking GPI-APs segregates them from preexisting GPI-AP clusters and prevents endocytosis of the crosslinked species via a GPI-AP-selective pinocytic pathway. In conjunction with an analysis of the statistical distribution of the clusters, these observations suggest a mechanism for functional lipid-dependent clustering of GPI-APs.

We study the interplay of activity, order and flow through a set of coarse-grained equations governing the hydrodynamic velocity, concentration and stress fields in a suspension of active, energydissipating particles. We make several predictions for the rheology of such systems, which can be tested on bacterial suspensions, cell extracts with motors and filaments, or artificial machines in a fluid. The phenomena of cytoplasmic streaming, elastotaxis and active mechanosensing find natural explanations within our model. PACS numbers: 87.16.Ac, 87.15.Ya, 87.10.+e An active particle [1, 2] absorbs energy from its surroundings or from an internal fuel tank and dissipates it in the process of carrying out internal movements usually resulting in translatory or rotary motion. This broad definition includes macroscopic machines and organisms, living cells, and their components such as actin-myosin and ion pumps [3]. In this paper, we consider the interplay of activity, order and flow via coarse-grained equations governing the hydrodynamic velocity, concentration and stress fields in a suspension containing active particles of linear size ℓ, at concentration φ, each particle exerting a typical force f on the ambient fluid, with the activity of an individual particle correlated over a time τ 0 (say the 'run' time of a bacterium), and collective fluctuations in the activity correlated over length scales ξ and timescales τ . Rather than focussing on ordered phases [4], instabilities [4,5], or patterns (asters, vortices, spirals) formed in such assemblies [6] which our equations are of course capable of predicting, we apply them in the isotropic phase, with a view to understanding how a system such as a biological cell, composed of active elements, responds to deformation or mechanical stress. In addition to throwing light on full-cell rheometry [7,8], our equations form the framework for an analysis of any experiment probing the mechanical consequences of biological activity.Our simple model makes rather interesting predictions: An orientationally ordered state of active particles has a nonzero, macroscopic, anisotropic stress in contrast with thermal equilibrium nematics. Activity contributes an amount δη ∼ f ℓc 0 τ to the viscosity, with a sign determined by the type of active particle, and always enhances the apparent (noise) temperature. The latter greatly enhances the amplitude of the t −d/2 long-time tails [9] in the velocity autocorrelation. On approaching an orientationally ordered state, active suspensions with δη > 0 behave like passive systems near translational freezing, showing strong shear thickening and Maxwelllike viscoelasticity. Nonlinear fluctuation corrections give a dynamic modulusobservable over a large dynamic range, since τ is large. Cytoplasmic streaming [10], in which material flows from the depolymerising trailing edge to the polymerising leading edge of a crawling amoeboid cell, finds a natural explanation in our model, as do elastotaxis [11] and active mechanosensing [12], where cells orient t...