Triangle Seminars

Abstract:
Deformations of cell sheets are ubiquitous in early animal development, yet they arise from an intricate interplay of cell shape changes, cell migration, cell intercalation, and cell division. We combine theory and experiment to explore what is perhaps the simplest instance of cell sheet folding, the "inversion" process in the green alga Volvox: at the end of cell division, a Volvox embryo consists of several thousand cells arrayed to form a thin spherical sheet, but those cell poles whence will emanate the flagella point into the sphere. In a process hypothesised to arise from cell shape changes alone, the embryos therefore turn themselves inside out to acquire the ability to swim. We have recently acquired the first three-dimensional time-lapse visualisations of this inversion, using light sheet microscopy to reveal the intriguing dynamics of the process. A theoretical model, in which cell shape changes correspond to local variations of intrinsic curvature and stretches of an elastic shell, sheds light on the underlying mechanics of inversion and reproduces the shapes and dynamics of inversion qualitatively.

This is joint work with Stephanie Höhn, Aurelia Honerkamp-Smith, Philipp Khuc Trong and Ray Goldstein.

Abstract:
Neurotransmitter-gated ion channels are complex neuroreceptors located in the membrane of nerve cells that control the rapid transmission of nerve impulses. Their malfunction is linked to serious neurological disorders, including Alzheimer’s disease, and they are major therapeutic targets; in invertebrates they are involved in insecticide resistance. However, we have little idea of how they function at the molecular level due to their complexity and limited experimental information. In particular it is not clear how the binding of a small molecule (the neurotransmitter) triggers a series of events culminating into the opening (gating) of the transmembrane channel: ions can then flood across the cell membrane modifying the cell activity. State-of-the-art and novel computational techniques are therefore crucial to build an accurate picture at the atomic level of the mechanisms that drive the activation of these ion channels, complementing the available experimental data. We have used a range of simulation techniques, including metadynamics (a method for accelerating rare events and sample free energy landscapes), to explore the mechanisms of neurotransmitter binding and a potential molecular switch for channel gating. As prototypical examples, we have focussed on the insect GABA-activated RDL receptor and on the serotonin-activated 5-HT3 receptor.

Abstract:
In this talk, I will summarize recent progress in the description of thermoelectric transport using gauge/gravity duality. I will first review thermoelectric transport in hydrodynamics, where momentum conservation implies infinite zero-frequency conductivities. By a change of basis of the conserved currents, a universal, finite conductivity can be extracted. It can be computed holographically. I will discuss its low-temperature scaling in terms of critical exponents characterizing time and space anisotropy and anomalous dimensions for the free energy and conserved current. When momentum is almost conserved, the zero-frequency delta functions broaden into Drude-like peaks. A holographic computation precisely identifies the redistribution of the low-frequency spectral weight between two contributions originating from the non-conservation of momentum and intrinsic dissipation respectively. It also sheds some light on how to construct effective theories of thermoelectric transport when momentum is not conserved.