There are two reasonable ways to create a larger gradient for RuBP. If the external concentrations were similar, a smaller concentration delta for CO2 would mean a high internal concentration of CO2 equivalents inside the carboxysome. This seems unlikely, since carbon fixation accelerates when cyanobacteria a grown in a CO2-rich environment , and a high internal CO2 concentration will inhibit carbonic anhydrase turnover. What we consider more plausible is that the cytosolic metabolite pool for RuBP is instead high, and that the intra-carboxysomal concentrations of both metabolites are similar in scale. In-vivo measurements report millimolar concentration of RuBP in α and β carboxysomes , which are consistent with estimated intra-carboxysome concentrations for CO2 in systems biology models .