439 Rubisco packing considerations. In the in-silico carboxysome 440 shell model we developed (Fig. ) used PACKMOL The primary finding from our simulation results is that the 500 carboxysome shells are not adept at excluding gas molecules 501 from transiting the shell. If anything, gases permeate more readily at an equal concentration gradient than photosynthetic metabolites, particularly RuBP, which has the slowest permeability of the compounds tested. However, by concentrating rubisco and carbonic anhydrase, and substantially increasing the path length that gases must travel to leave or enter the shell, the carboxysome creates an efficient CCM from nominally leaky components. Indeed, the greater than 2600:1 ratio between fixed and lost CO2 in a typical carboxysome is an astounding feat of engineering by evolution.