Phase transitions in materials are accompanied by drastic changes in their properties. Systems suddenly become more flexible, more conductive, have better heat storage or withstand chemical reactions more efficiently. Since the changes occur on small variations in external conditions (voltage, temperature, pH, calcium), they appear as an on/off switch. Here, we provide experimental evidence that single live cell membrane patches can go through a reversible phase transition. It is extremely “sharp” (highly non-linear), and from a thermodynamic point of view we conclude that it cannot only be triggered by temperature but also by changes in pH (produced by enzymes). The results strongly support the idea that phase transitions can be a tool for living systems to control their functions, even in specific ways.


The origin of nonlinear responses in cells has been suggested to be crucial for various cellular functions, including the propagation of nerve impulses. In physics, nonlinear behavior often arises from phase transitions. However, no evidence for such transitions at the single cell level has so far been provided, leaving the field unattended by the biological community. Here we demonstrate that single cells of a human neuronal cell line display all the optical characteristics of a sharp and highly nonlinear phase transition within their membrane. The transition is reversible and does not arise from protein denaturation. Triggered by temperature and modified by pH here, a thermodynamic approach strongly suggests that similar nonlinear state changes can be induced by other variables such as calcium or mechanical stress. At least in lipid membranes, such state changes are accompanied by significant changes in permeability, enzyme activity, elastic and electrical properties.