However, since CO2 will encounter a rubisco active site within a microsecond (Fig. ), and the diffusion coefficient for CO2 is 1670 nm 2 /µs (52), the average pathlength a CO2 can travel is only about 40 nm or so, with substantial variation due to stochastic motion. This means that there are diminishing returns if the distance between a carbonic anhydrase and a rubisco active site substantially exceeds that dimension, since distant rubiscos would not reliably be supplied with CO2, and would effectively be a waste of resources to produce. Typical α-carboxysomes are measured to be about 100-150 nm in diameter , which would seem to be sized appropriately if carbonic anhydrases were localized to the carboxysome periphery and CO2 were to diffuse into the interior. However, β-carboxysomes are much larger, with diameters up to 600 nm, suggesting that if carbonic anhydrases were only on the periphery, interior rubiscos would be starved of CO2. Structural data from cryo-electron and super-resolution microscopy in α and β-carboxysomes respectively, suggests that CA enzymes inside the shell closely interact with rubisco, to form a supercomplex . Experimentally in the case of β-carboxysomes the rubisco-CA localization is measured to a sub-shell layer of ∼ 5 -6 nm beneath the outer shell surface , consistent with a more uniform distribution of carbonic anhydrases. Integrating these details into the ARBD model (Fig. ) requires detailed knowledge for the number of CA enzymes inside the carboxysome shell and their distribution within an assembled microcompartment, which are areas of active research beyond our current scope.