Parking garage structures in nuclear astrophysics and cellular biophysics
Chuck Horowitz and Greg Huber
Indiana University and KITP/Department of Physics UCSB
A striking shape was recently observed in the endoplasmic reticulum (or "ER"), a cellular organelle of eukaryotes consisting of stacks of flat sheets connected by helical ramps - the Terasaki ramps. This shape is interesting both for its biological function (the ER is the site of protein synthesis through a large proportion of the cell's ribosomes), and its geometrical properties. Surprisingly, very similar shapes are found in simulations of the nuclear-pasta phases of dense nuclear matter expected to occur in the crusts of neutron stars - this despite the fact that nuclear pasta is 14 orders of magnitude denser than the aqueous environs of the cell nucleus, and that it is governed by the strong interactions between protons and neutrons, while cellular-scale biology is dominated by the entropy of water and complex assemblies of biomolecules. Nonetheless, the very similar geometries suggest that both systems may have similar coarse-grained dynamics and that the shapes are indeed determined by geometrical considerations, independent of microscopic details. These analogs of Terasaki ramps may impact the thermal and electrical conductivities, viscosity, shear modulus, and breaking strain of neutron-star crust, and, as such, be observable. Even more surprisingly, the interactions and methodology used for nuclear matter may provide a way to understand other biologically important intracellular shapes.