RuBP concentration is high to match rubisco stoichometery. One of the big surprises in our results is how few permeation events we observe for RuBP compared with the other photosynthetic metabolites (Table ). Since RuBP and CO2 are used in a 1:1 stoichiometric ratio within the Calvin-cycle, that implies that at steady state the fluxes for RuBP and HCO3 -should be equal for continued catalysis. From the structure of Eq. 1, the smaller permeability for RuBP relative to CO2 or HCO3 -implies that the concentration gradient needed to drive transport across the shell must be larger.