  Lateral organization in the plane of the plasma membrane is an important
driver of biological processes. The past dozen years have seen increasing
experimental support for the notion that lipid organization plays an important
role in modulating this heterogeneity. Various biophysical mechanisms rooted in
the concept of liquid-liquid phase separation have been proposed to explain
diverse experimental observations of heterogeneity in model and cell membranes,
with distinct but overlapping applicability. In this review, we focus on
evidence for and consequences of the hypothesis that the plasma membrane is
poised near an equilibrium miscibility critical point. Critical phenomena
explain certain features of the heterogeneity observed in cells and model
systems, but also go beyond heterogeneity to predict other interesting
phenomena, including responses to composition perturbations.
