Physical Chemistry

 Read the scientific activity report. (pdf 31Ko, last update 26th, march 2010)

A rapid inspection of orders of magnitude involved in cell components show that they are very similar to those relevant to “Soft Matter Physics”. There are however two important differences: biological systems are clearly out of equilibrium and molecular specificity can be strongly relevant. These simple remarks convince us that on the one hand Soft Matter Physics can provide a quantitative description of cellular systems, and that on the other hand biological systems raise an interesting number of new and challenging physical questions. For these reasons we concentrate our efforts towards understanding physical features of cell morphology and dynamics. This project is meaningful only with strong interactions with biologists.

Cells contain a very large number of components, but if we focus on mechanical properties, only a few classes of component are relevant: the cytoskeletal networks, molecular motors, phospholipid membranes and the large class of adhesion molecules such as integrins or cadherins. Therefore we 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, “active” gels or  “isothermal ratchet”, which is a model to describe molecular motors by the Brownian motion of a particle switching between two different states.

A good physical understanding requires quantitative comparison between theory and experiments by systematically varying controlled parameters: for that reason, we work in close collaboration with the experimental groups both in our laboratory and in the Curie Subcellular structure and cellular dynamics Unit (UMR 144). We contribute to the theoretical description of polymerization-based motion using biological models such as the bacteria Listeria and the keratocytes type of cells, but also biomimetic systems such as plastic beads and oil drops properly treated. We also describe  certain  aspects  of  cell behavior such as phospholipidic nanotube pulling by  molecular  motors,  cell motility, cell division and mechano-transduction. As an additional example, we show on Figure 1 the results of calculations on spontaneous oscillations  of assemblies of molecular motors

Fig. 1 Motor oscillations: Oscillations of assemblies of molecular motors obtained from stochastic simulations. The model is sketched in (a) which shows the potentials seen by the motors. The oscillations in the position of the filament interacting with the motors, the power spectrum and the histogram of the positions of the filament are then shown for two sets of parameters.

We  have  now  reached  a  reasonable  physical  understanding   of   single components and are consequently extending our analysis  to  the  interaction between  components, that is to  the  multi-cellular  level  and  the  mechanical  properties  of tissues. We have for example  performed  numerical  simulations  of  tissues competing for space (see Figure 2 below).

Fig. 2 Tissue competition: Numerical simulation of the competition between two tissues.

At last, we keep close contact with the evolution  of  statistical  physics. In particular  we  have  derived  a  general  relation  between  the  linear response of a system to any  external  perturbation  and  its  fluctuations. This relation is valid as soon as the system is  described  by  a  Markovian dynamics. It will  be  useful  for  discussing  fluctuations  of  biological systems.