Fig. 6. High PCCM performance requires low-pH thylakoids and a high-pH stroma.

a–f, pH values of the thylakoid lumen and the stroma are varied in a modelled chloroplast with an impermeable starch sheath employing passive CO₂ uptake under air-level CO₂ (a–c) (10 μM cytosolic; parameters as in Fig. 4d,e) or active HCO₃⁻ pumping under very low CO₂ (d–f) (1 μM cytosolic, parameters as in Supplementary Fig. 17c,d). Normalized CO₂ fixation flux (a,d) and ATP spent per CO₂ fixed (b,e) as functions of the pH values in the two compartments are shown. c,f, Schematics of inorganic carbon pools and fluxes for the pH values indicated in a, b, d and e. White stars indicate the baseline pH values used in all other simulations.