The carboxysome, and microcompartments generally, are 15 bounded by protein components that oligomerize to form 16 sub-assemblies with hexagonal or pentagonal symmetry that 17 further assemble into complete shells with icosahedral symme-18 try , similar in scale to some viruses. Since rubisco D R A F T and assumptions . Missing quantitative permeation coefficients hinder efforts to better understand the limits to 32 photosynthesis and develop strategies to further improve its 33 efficiency or incorporate it into new contexts . Shell permeability coefficients are a key input to existing systems biology models that guide our understanding 36 of cyanobacterial CCM function and engineering approaches 37 . Permeability is difficult to measure based on the rapid 38 turnover within photosynthesis and the difficulty of probing 39 the interior of the carboxysome, and so for simplicity, systems