To whom correspondence should be addressed. E-mail: vermaasj@msu.edu diffusion within a crowded carboxysome. Based on geometry considerations alone, we find that a CO2 molecule is likely to find a rubisco active site prior to hitting the edge of a real carboxysome. When combining these results with estimates for how often CO2 is reflected back into the carboxysome interior by shell proteins, we estimate that 2650 CO2 molecules can be fixed by rubisco for every 1 molecule that escapes the carboxysome. Thus, while our high CO2 permeability estimates suggest that a CO2 gradient cannot be maintained long term in the absence of enzymatic activity, replenishing CO2 within the carboxysome from soluble bicarbonate via carbonic anhydrase activity is sufficient to keep the local CO2 concentration around rubisco high within the carboxysome.