Abstract:  Walled cells have the ability to remodel their shape while sustaining an internal turgor pressure that can reach values up to 10 atmospheres. This requires a tight and simultaneous regulation of cell wall assembly and mechanochemistry, but the underlying mechanisms by which this is achieved remain unclear. In this talk I will discuss the interplay between growth and mechanics in shaping a walled cell, in the particularly simple geometry of tip-growing cells, which elongate via the assembly and expansion of cell wall in the apical region of the cell. Using only conservation laws and describing the observed irreversible expansion of the cell wall during growth as the extension of an inhomogeneous viscous fluid shell under the action of turgor pressure, we determine theoretically the radius of the cell and its growth velocity in terms of the turgor pressure and the secretion rate and rheology of the cell wall material. Moreover, we derive simple scaling laws for the geometry of the cell and find that a single dimensionless parameter, which characterizes the relative roles of cell wall assembly and expansion, is sufficient to explain the observed variation in shapes of tip-growing cells. Our work shows that the physics of cell wall expansion tightly constrains cell shape, providing a unified explanation of the characteristic morphologies of tip-growing cells across species that span several kingdoms, even though their underlying molecular mechanisms of cell morphogenesis are very different. More generally, our description provides a general framework to understand cell growth and remodeling in plants (pollen tubes, root hairs, etc.), fungi (hyphal growth and fission and budding yeast) and some bacteria. Bio:  Otger Campas is currently a postdoctoral fellow at Harvard Medical School and the Children's Hospital of Boston, working with Professors Mahadevan, Brenner and Ingber measuring the forces generated during embryogenesis. His current and future research is broadly focused on the fields of morphogenesis and morphological variation, using both theoretical and experimental methods to address his topics of interest.   Campas received his B.S.  in Physics from the University of Barcelona, and completed his Ph.D. in Biophysics in a joint program between the University of Barcelona and the Institut Curie (Paris), working under Jacques Prost, Jean-Francois Joanny, and Jaume Casademunt studying how cellular movements and cellular organization arise from the molecular forces generated by motor proteins and polymerization of cytoskeletal filaments. Campas was the recipient of an NIH interdisciplinary research fellowship in 2009. In 2008 Campas spearheaded a highly popular event titled “Cooking and Science with Ferran Adria”, a sold out event hosted at Harvard which was live broadcast on the internet and covered by worldwide media outlets, including Time Magazine. His website is http://people.seas.harvard.edu/~ocampas/Intro/Intro.html Host: Prof. Megan Valentine