from systems model , and two orders of magnitude higher 155 than the optimal permeability predicted by a revised pH-156 aware CCM model . However, even the pH aware model 157 supports CCM activity over a range of permeabilities, including 158 those on the order of 10 -2 cm/s . The exceptions to 159 this are the roughly 10-100 fold lower permeability predicted 160 for RuBP, in line with assumed permeabilities between 10 -3 161 cm/s and 10 - cm/s in other models , and the higher 162 permeability for water. Since water forms continuous channels 163 through the CcmK pore (SI Fig. ), and protons are known 164 to permeate across carboxysomes , a high permeability for 165 water (Table ) is natural, and suggests that there can be no 166 osmotic gradient across the shell. 167 However, the high permeability for CO2 and O2 are a 168 surprise, as prior simulations based on enhanced sampling 169 methods predicted that there should be a barrier for CO2 and 170 O2 to cross through the hexameric pore compared with larger 171 photosynthetic metabolites . Having hints from Fig. 172 that the pathways the different metabolites follow are distinct, The picture that emerges from Fig. is that the primary pore 190 within the hexamer is selective to larger metabolites, but that 191 gases transit wherever there is a momentary pocket.