Physical Approach of Biological Problems

Cell biology and soft matter physics both share similar orders of magnitude with two important differences: biological systems are clearly out of equilibrium and molecular specificity can be strongly relevant. We are using the standard tools of soft matter physics to provide a quantitative description of cellular systems. Drawing inspiration from living systems, we are raising new and challenging physical questions. This project only makes sense with strong interactions with biologists.

Cells contain a large number of components, but if we focus on mechanical properties, only a few classes of these components are relevant, e.g. the cytoskeletal network, molecular motors, phospholipid membranes and a large class of adhesion molecules such as integrins or cadherins. We therefore study each of these components, keeping in mind the importance of the non-equilibrium behavior. In some cases, this requires the introduction of new physical concepts such as “active” membranes, or “active” gels.

To optimize our physical understanding, we systematically carry out quantitative comparison between theoretical and experimental data obtained by varying controlled parameters. For instance, we are contributing to the theoretical understanding of the formation and size of the nucleoid in bacteria. We are also trying to understand what fixes the shape of active soft surfaces like some epithelial tissues.

We have now reached a reasonable physical understanding of single component systems and are consequently extending our analysis to the collective behavior of many components. We can already describe certain aspects of cell behavior such morphological transformations of biological membranes, cell motility, cell division and mechano-transduction. A large part of our work is now devoted to the effect of mechanical stresses on the growth of tissues. We have for example shown the existence of an instability of the shape of cellular aggregates with spontaneous symmetry breaking.