Is the endoplasmic reticulum an obstacle to intracellular diffusion?

Intracellular spaces are tightly packed and the biggest potential obstacle in the crowded interior of a eukaryotic cell is the endoplasmic reticulum (ER), whose total membrane area can be 20-30 times that of the cell's plasma membrane. Much of this area is in parallel sheets, however the sheet-like ER has also been found to be riddled with spiral dislocations, known as 'Terasaki ramps', in the vicinity of which the sheets’ doubled bilayer membranes can be approximately modeled by helicoids. We analyze diffusion on a surface with locally helicoidal topological dislocations, and use the results to argue that the Terasaki ramps facilitate an efficient transport of water-soluble molecules both within the lumen of the endoplasmic reticulum, and in the adjacent cytoplasmic space. (This is joint work with Michael Wilkinson.)