Parking-garage structures in astrophysics and biophysics

TL;DR

This paper reveals striking geometric similarities between cellular endoplasmic reticulum structures and nuclear pasta phases in neutron star crusts, suggesting shape formation is driven by geometry rather than microscopic details.

Contribution

It demonstrates that similar complex shapes occur in biological and astrophysical systems despite vastly different scales and interactions, highlighting the role of geometry in shape formation.

Findings

Cellular and nuclear pasta structures share similar geometries.

Simulations show self-assembly into sheets with helical ramps.

Shapes may influence physical properties of neutron star crusts.

Abstract

A striking shape was recently observed for the cellular organelle endoplasmic reticulum consisting of stacked sheets connected by helical ramps. This shape is interesting both for its biological function, to synthesize proteins using an increased surface area for ribosome factories, and its geometric properties that may be insensitive to details of the microscopic interactions. In the present work, we find very similar shapes in our molecular dynamics simulations of the nuclear pasta phases of dense nuclear matter that are expected deep in the crust of neutron stars. There are dramatic differences between nuclear pasta and terrestrial cell biology. Nuclear pasta is 14 orders of magnitude denser than the aqueous environs of the cell nucleus and involves 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 geometry suggests both systems may have similar coarse-grained dynamics and that the shapes are indeed determined by geometrical considerations, independent of microscopic details. Many of our simulations self-assemble into flat sheets connected by helical ramps. These ramps may impact the thermal and electrical conductivities, viscosity, shear modulus, and breaking strain of neutron star crust. The interaction we use, with Coulomb frustration, may provide a simple model system that reproduces many biologically important shapes.