192 Assessing CO2 leakiness in a crowded carboxysome context. The surprising corollary to the high CO2 permeability in Table is that once HCO3 -is converted by carbonic anhydrase into CO2 (Fig. ), the CO2 may leak from the carboxysome prior to CO2 capture and fixation by rubisco. To estimate carboxysome leakiness for CO2, we developed a model for a full carboxysome packed with rubisco (Fig. ) based on recent cryo-electron tomography (cryo-ET) experiments for marine cyanobacteria , α- and β-carboxysomes . Our intent is to track the statistics of how often CO2 will hit the carboxysome shell first or will instead come close to the rubisco active site. Since rubisco has multiple subunits, and over a hundred rubisco are densely packed within any real carboxysome, treating this system with atomistic MD as we did for the synthetic system is infeasible. Thus, we use atomic resolution brownian dynamics (ARBD) methodology to represent each of the 160 rubiscos as an interacting grid that can exert force on nearby CO2 particles. The rubisco structure used in ARBD simulations should be in the apo or substrate-free form to allow CO2 to access the well-conserved active site, and hence the PDB:7JN4 (36) structure from Chlamydomonas reinhardtii was used in place of similar models from other organisms (SI Fig. ). The apo rubisco model is then packed using inter rubisco distance observed in cryo-electron tomograms obtained in marine cyanobacteria to approximate the densely packed carboxysome interior.